United States

Department

of Agriculture

Forest Service

Pacific Southwest

Research Station

General Technical

Report PSW-GTR-144-www

Handbook of Field Methods

for Monitoring Landbirds

C. John Ralph Geoffrey R. Geupel Peter Pyle

Thomas E. Martin David F. DeSante
Ralph, C. John; Geupel, Geoffrey R.; Pyle, Peter; Martin, Thomas E.; DeSante, David F. 1993. Handbook

of field methods for monitoring landbirds. Gen. Tech. Rep. PSW-GTR-144-www. Albany, CA:

Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture; 41 p.

This handbook is a compilation of methods that can be used to assay population size, demographics,

and status of many species of birds occurring in a wide variety of habitats. The handbook will prove

useful to field biologists, managers, and scientists anywhere in the New World from the arctic through

the tropics. The methods include four types of censuses for determining population size and trends,

mist-netting and nest searches to determine demographic parameters, and other methods that will be

useful in operating a monitoring station, including habitat and weather observations, and suggestions for

training personnel and possibilities for detailed studies. Suggestions of specific methods and data forms

are included.

Retrieval Terms: bird populations, census, mist-nets, monitoring, nesting birds

Acknowledgments:

The late L. Richard Mewaldt was the first among equals in setting high standards and maintenance

of accurate records. His contributions are detailed by Ralph (1992). We take great pleasure in dedicating

this handbook to him.

This handbook is a direct outgrowth of the landbird program started more than 25 years ago at the

Point Reyes Bird Observatory to monitor landbird populations in coastal California, and much of this

handbook is the result of the methods developed and adapted there. Over this period, many people

contributed to the development of this landbird monitoring program.

The handbook benefitted greatly from discussions and correspondence by Bruce Bingham, Grant

Ballard, Danny Bystrak, Barbara Carlson, Brenda Dale, Sam Droege, John Faaborg, Kevin J. Gutzwiller,

Denise Hardesty, Kimberly Hollinger, Bill Howe, David W. Johnston, Stephanie Jones, Cherry Keller,

Kathy Klimkiewicz, Rolf R. Koford, Karin Kozie, Borja Milá, Sherri Miller, Michael Morrison, Barry

R. Noon, Nadav Nur, Raymond J. O’Connor, Will Peach, Carol Pearson Ralph, Martin Raphael, Dan

Reinking, Christopher C. Rimmer, Sharon Ritter, John T. Rotenberry, John R. Sauer, Tom Sherry, Sue

Sniado, John Tautin, Jared Verner, Dennis Vroman, George E. Wallace, Dan Welsh, and Joseph M.

Wunderle, Jr.

The Authors:

C. John Ralph is a Research Biologist at the Station’s Redwood Sciences Laboratory, 1700 Bayview

Drive, Arcata, Calif. 95521. Geoffrey G. Geupel and Peter Pyle are Director of Terrestrial Research

Programs and Farallon Biologist, respectively, at the Point Reyes Bird Observatory in Stinson Beach,

Calif. Thomas E. Martin is Assistant Unit Leader—Wildlife at the U.S. Fish and Wildlife Service in

Fayetteville, Arkansas, and Associate Professor at the University of Arkansas, Fayetteville. David F.

DeSante is Executive Director at the Institute for Bird Populations in Point Reyes Station, Calif.

Publisher:

Pacific Southwest Research Station

Albany, California

(Mailing address: P.O. Box 245, Berkeley, CA 94701-0245

Telephone: 510-559-6300)

May 1993
Handbook of Field Methods

for Monitoring Landbirds

C. John Ralph Geoffrey R. Geupel Peter Pyle Thomas E. Martin David F. DeSante

Contents

In Brief .................................................................................................................................................... iii

Introduction .............................................................................................................................................. 1

Objectives of a Monitoring Program ............................................................................................... 1

Glossary ............................................................................................................................................ 2

Selecting Monitoring Methods and Locations of Monitoring Stations ............................................. 2

Selection of Methods........................................................................................................................ 2

Selection of a Station Location ........................................................................................................ 4

Permanent Stations ........................................................................................................................... 4

General Monitoring Procedures ............................................................................................................ 4

Species To Be Covered .................................................................................................................... 4

Monitoring Period ............................................................................................................................ 5

Maintenance of a Study Plot ............................................................................................................ 5

Journal Keeping ................................................................................................................................ 6

Training and Numbers of Personnel ................................................................................................ 6

Data To Be Taken ............................................................................................................................ 6

Constant-Effort Mist Nets and Banding ............................................................................................... 7

Scope................................................................................................................................................. 7

Net Placement................................................................................................................................... 7

Net Locations ................................................................................................................................... 8

Erecting and Operating Nets ............................................................................................................ 8

Net Specifications and Maintenance ............................................................................................... 9

Operation of Nets ............................................................................................................................. 9

Removing Birds from Nets ............................................................................................................ 10

Processing ....................................................................................................................................... 12

Special Problems ............................................................................................................................ 12

Data Collection ............................................................................................................................... 12

The Process of Determining the Extent of Skull Pneumatization ................................................ 14

Data Entry ....................................................................................................................................... 20

Sources of Equipment .................................................................................................................... 22

Bird Banding Laboratory and Office .............................................................................................22

Nest Searches .......................................................................................................................................... 22

Nest Sites ........................................................................................................................................ 23

Nest Monitoring ............................................................................................................................. 25

Filling Out the Forms ..................................................................................................................... 25

Predation Risk from Monitoring .................................................................................................... 28

Vegetation Measurement ............................................................................................................... 28

Censusing ................................................................................................................................................ 29

General Considerations .................................................................................................................. 29

Point Count ..................................................................................................................................... 31

Strip Transects ................................................................................................................................ 35

Area Search .................................................................................................................................... 35

Spot Mapping ................................................................................................................................. 35
2 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Contents

Other Considerations ............................................................................................................................ 37

Color banding Individuals.............................................................................................................. 37

Methods of Habitat Assessment .................................................................................................... 37

Weather Monitoring ....................................................................................................................... 39

References ............................................................................................................................................... 39

ii
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 3

In Brief . . .

Ralph, C. John; Geupel, Geoffrey R.; Pyle, Peter; Martin,

Thomas E.; DeSante, David F. 1993. Handbook of field

methods for monitoring landbirds. Gen. Tech. Rep. PSWGTR-144. Albany, CA: Pacific Southwest Research Station,

Forest Service, U.S. Department of Agriculture; 41 p.

Retrieval Terms: bird populations, census, mist-nets,

monitoring, nesting birds

The increased attention devoted to the status and possible

declines of populations of smaller species of terrestrial birds,

known collectively as “landbirds,” has resulted in an

immediate need for specific methodology for monitoring

their populations. This handbook is derived from several

sources and is based on the authors’ collective experiences

in operating monitoring stations. Presented here are a

compilation of methods that can be used to assay population

size, demographics, and status of virtually all species of

landbirds in a wide variety of habitats, from grassland and

tundra to temperate and tropical rain forests. Rare species, or

those with unusual habits, will require some modifications.

The handbook will prove useful to field biologists, managers,

and scientists anywhere in the New World. The handbook

first suggests priorities for selecting a monitoring method

and determining station locations. Then, general tasks that

determine which species can be monitored, and methods of

establishing and maintaining a study plot, journal keeping,

and training of personnel are presented. Two demographic

methods are described, one involving mist nets, and the

other finding nests during the breeding season. Detailed

suggestions are given for both methods which should allow

a trained person to successfully operate a station. Both methods

involve monitoring at a station at regular intervals during the

breeding season. The handbook also includes descriptions of

four types of censuses for determining population size and

trends: spot mapping of territories, area searches of specific

sites, strip transects along predetermined routes, and point

counts. The latter method has been accepted as the standard

method, is treated in most detail, and involves a person

standing in one spot for 3 to 10 minutes and recording all

birds seen or heard. In addition, methods are suggested for

measuring habitat, recording weather, and color-banding

individuals to determine specific demographic parameters.

Throughout the handbook, sources of materials are given

that are needed for each method, as well as specific references

to published works.

iii
Introduction

Throughout the New World attention is now being

focused on the status of populations of landbirds,

which are the many species of smaller birds, sometimes

referred to as “non-game” birds. Landbirds have not usually

been the focus of management activities except in a few cases

of threatened or endangered species, such as the Kirtland’s

Warbler. Recent evidence suggests that some landbird species

are declining in abundance, fueling much speculation upon

the causes of these declines, the species involved, and their

habitat preferences. Hypotheses about the causes of these

declines are varied, ranging from tropical deforestation to nest

parasitism by the cowbird. However, part of the difficulty in

determining the status of landbirds results from problems in

monitoring these small birds, as compared to larger, more

easily-studied species. To determine population changes, and

to hypothesize possible causes of these changes, more basic

information needs to be gathered.

Much of the evidence for these population declines in

the New World has come from the results of the Breeding

Bird Survey coordinated by the U.S. Fish and Wildlife

Service and the Canadian Wildlife Service (Robbins and

others 1986, 1989). These roadside counts provide excellent

baseline data. However, they do little to identify the factors

contributing to changes in landbird populations, and are

limited to areas with major roads.

The use of population size as a measure of health of a

species has been a common tool of biologists for many years

(Hutchinson 1978; Lack 1954, 1966). Methods for surveying

population size have been detailed by Ralph and Scott

(1981), in the excellent compendium by Cooperrider and

others (1986), and the manual by Koskimies and Vaisanen

(1991). Population size, however, is only a retrospective

tool. It tells only after the fact that a species has enjoyed an

increase or suffered a decline. To ponder causes of changes,

the biologist must couple information on population size

with data on the internal composition of a population—its

demographics (Temple and Wiens 1989). For example, data

on sex ratio, age distribution, nesting success, survivorship,

average weight, and population movements can all give

valuable cues to factors or events regulating a population.

Moreover, such primary population characters can provide

early warning signals of population problems prior to actual

declines. Many studies have used data such as these to

describe the dynamics of various populations (DeSante and

Geupel 1987, Hutchinson 1978).

Several other efforts have been under way to document

changes in adult populations and in productivity. For

example, in the late 1970’s, the Germans and Austrians

started the “Mettnau-Reit-Illmitz-Programm” (Berthold and

Scherner 1975). Since 1981, the British Trust for

Ornithology has conducted their Constant Effort Sites (CES)

Scheme (Baillie and others 1986; Baillie and Holden 1988;

Peach and Baillie 1991; Peach and others 1991). Martin has

started a program involving nest searches (Martin and

Geupel in press). DeSante (1991, 1992a,b) has started a

cooperative mist-netting program in North America known

as “Monitoring Avian Productivity and Survivorship”

(MAPS) along these same lines. The Point Reyes Bird

Observatory has been monitoring landbird populations in

coastal California for more than 25 years (Ralph 1967,

Geupel and DeSante 1990b).

In this handbook we outline the steps that might be

followed in monitoring many species of landbirds. We cover

methods used in monitoring of population size, productivity,

age and sex ratios, survivorship, habitat relationships, and

other parameters. We provide details of four methods that

estimate population size, two methods that measure

demographic factors, and two suggestions for conducting

habitat assessment. We have tried to give the land manager,

biologist, and others complete information on basic

requirements, tools, resources, and methods to carry out a

successful landbird monitoring program. Depending on

funding and staffing, any combination of the techniques we

describe is applicable to virtually any site and budget. This

handbook does not provide the objectives of each study that

might be conducted, or what analyses can be conducted.

These both must be examined carefully before monitoring

begins. We hope that this handbook will generate interest in

monitoring programs using methods that can give insight

into causes, as well as the facts, of population changes.

Objectives of a Monitoring Program

A monitoring program ideally should provide three types of

data. One is an estimate of the population size and trends for

various species of birds. The second is an estimate of the

demographic parameters for at least some of those

populations. The third is habitat data to link the density and

demographic parameters of bird populations to habitat

characteristics. Ideally a monitoring program should take a

community approach and monitor all avian species in the area.

We have recently seen a marked increase in interest in

monitoring, far outstripping available personnel, training,

and resources. Indeed, this increase is the impetus for this

handbook. While this has been gratifying, we think that it is

essential that people first determine why they might want to

establish a census, mist netting, or nest searching program.

Not everyone requires a monitoring program to meet their

goals. We have sometimes seen the establishment of a

monitoring program first, followed by an attempt to decide

what type of information can be obtained. We very strongly

suggest that, before a monitoring program is put in place, the

following steps be carried out: (1) decide the objectives and

goals desired; (2) determine whether monitoring is the way

to accomplish these; (3) with the goals firmly in mind, write
2 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

down the questions being asked, clearly and objectively; (4)

determine which monitoring methods most directly answer

the questions posed; (5) review the types of data that can be

obtained from these methods, and outline exactly how these

data will answer the questions; (6) outline the analytical

methods that can be employed; (7) determine the cost,

logistics, availability of personnel, and probable length of

commitment to the project; and (8) write a study plan and

have it reviewed by a person competent in research and

statistics. This procedure is vital, because accumulation of a

data base does not itself lead to meaningful analyses later.

Participants in a monitoring program can include private,

state, provincial, and federal groups. Our premise is that the

basic entity for this exercise is an administrative unit, such as

a Forest Service District or a State Park. Not all such units

will want or need such a program. Each unit should outline

its needs and goals before starting, suggest monitoring

programs to meet those needs, and have them reviewed by a

competent biostatistician. We do believe, however, that our

recommendations below have generality among many types

of administrative units. These units can be very

heterogeneous, and thus a variety of methods may be needed.

Glossary

Landbirds: the general term used for the generally smaller

birds (usually exclusive of raptors and upland game birds) not

usually associated with aquatic habitats. By contrast,

waterbirds include seabirds and other aquatic species.

Region: an area of several thousand acres, often including

several drainages, that the biologist wishes to sample. Here,

extensive point counts are conducted on roads to monitor

overall population sizes and their changes.

Administrative unit: the basic entity that conducts

monitoring. Examples are a Forest Service District or Forest,

a State Park, a National Wildlife Refuge, a private nature

center, or a commercial forest.

Monitoring station: an area of usually less than about 50 ha

(125 acres) within a region. Here, intensive censuses, nest

searching, and mist netting are conducted.

Capture array: the generally rectangular or circular

configuration of mist net locations at a station.

Nest search or census plot: an area of, preferably, a single

habitat type where spot mapping or area searches are

conducted.

Census grid: the arrangement of intensive point counts

overlaying a demographic mist net array or nest search plot.

Census point: the place where a single point count census is

taken.

Net location: the place where a single net is placed.

Nest site: the place where a single nest is found.

A 10-day time: interval is the basis for most monitoring and

analyses.

Selecting Monitoring Methods and

Location of Monitoring Stations

Before beginning work, careful attention should be given

to selecting the appropriate method for the questions being

asked, and great care should be given to selecting locations of

monitoring stations to best answer these questions.

Selection of Methods

The standardized set of methodologies described below

should be followed closely to ensure compatibility with those

of other monitoring stations. These methodologies are

integrated and hierarchic, so as to allow a region’s sampling

schemes to complement other programs and to allow

comparisons between monitoring stations in a region, and

between regions.

Methods recommended should be employed for a

minimum of three years, and preferably longer. However,

depending on individual objectives, some results may be

obtainable in a year or two.

What Will the Data from This Program Provide?

These data will be used at two geographic scales. At the

level of the managed forest, for instance a large National

Forest District, they will provide a local assessment of the

status and trends of landbirds. The scheme below samples

the landscape as a whole within the unit and will permit

statements such as: “Scarlet Tanagers have significantly

increased on the sampled units in the forest,” “Hermit

Thrushes have had high mortality during migration or the

winter in the past two years,” or “15 of 20 neotropical

migrants have increased over the past 3 years.” Such a local

scheme will permit some investigation of patterns of

population change (e.g., “are declining trends more prevalent

in units of the northern half of the forest?”, “do increasing

trends appear to be associated with certain forestry

practices?”). Their primary purpose, however, is to estimate

the status and trends of the population. Assessment of the

cause of population change, or associations with

environmental factors such as cutting practices, are more

efficiently studied by other research programs with more

appropriate techniques.

At the larger scale, perhaps a Forest Service Region, a

state, or a province, the program will permit evaluation of

geographic patterns of various attributes of landbirds. It is

important to realize that the program cannot evaluate the

population status of birds of the entire geographic area,

whether regional, state, federal, or continental. If, for

example, samples are only from forested environments, only

statements about birds using forested lands can be made.

Additionally, because sites are chosen by the unit, and are

not a random sample of all available units or sites, the

program can investigate only the patterns of population

change, rather than the population’s overall status. Questions
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 3

Table 1—Census and demographic monitoring methods

__________________________________________________________________________

Census Demographic

Variables and Point Spot Area Mist Nest

characters count map search nets search

Variables measured

Index to abundance yes yes yes yes partly

Density no yes no no partly

Survivorship (adult) no no no yes no

Survivorship (juvenile) no no no yes partly

Productivity no no no yes yes

Recruitment no no no yes no

Habitat relations yes yes yes little partly

Clutch size no no no no yes

Predation/parasitism no no no no yes

Individuals identified no no no yes yes

Breeding status known no yes no partly yes

General characters

Habitat types measured all some most some few

Habitat specificity good good good fair good

Rare species measured many few many some few

Canopy species measured all all all some few

Area sampled known partly yes yes partly yes

Size of area sampled moderate small small large small

Training necessary much much moderate much much

Observer error potential high high moderate moderate moderate

Use in non-breeding yes no yes yes no

Cost per data point low high low high very high

Applicable scale broad local broad broad local

this approach can answer, for example, are: “are population

increases or reproductive failures in a group of species more

prevalent in some regions or states?”; “what is the

association between forest management and population

status of a group of species?”; “do some forest types have

more neotropical migrants than others?”; or “are populations

increasing in some forest types, but not in others?”

Coverage

While it would be best to have complete coverage of

any state, province, or region, we do believe that it is

acceptable and inevitable that gaps will exist. These gaps

will occur within habitat types, forests, states, provinces, or

regions. At a minimum, we hope to have several units

involved in each state, province, or region. We strongly urge

that each unit have both population and demographic

methods in operation, and cover anything from a few

hundred to several thousand acres. Further, we suggest that

sampling within a unit should be stratified by at least general

habitat type, such as “mixed coniferous forests,” “tropical

thorn forest,” or “coastal chaparral.” Samples in an analysis,

in general, should not be pooled across habitat types. The

data from these units would be searched for large-scale

patterns, e.g., species showing consistent declines over the

entire region or within a given habitat type. Results from

these investigations will identify patterns that need further

research or greater intensity of monitoring to determine their

causes. The overall program could be considered a largescale hypothesis-generating mechanism.

Priorities

Methodologies are compared in table 1. At a minimum

we recommend that the following programs of demographic

and population monitoring be implemented in each unit, in

the following order. Although this handbook describes three

censusing methods, the point count method has been adopted

as the recommended standard, and its implementation is

suggested below. Each recommended method is in segments

of 10 person-days, other than the first which takes one

person-day. For example, if funding is available for 21

person-days of field work, then only Priorities I through III

(outlined below) would be implemented. These estimates of

time do not include set-up or training. These will vary

depending upon qualifications of personnel. The minimum

numbers of counts or netting sites, noted below, are derived

from our experiences with many such population monitoring

programs. We believe they are useful, but not restrictive,

minima for a unit’s effort.

Priority I. Breeding Bird Survey—If the unit is in North

America and has an unsurveyed Fish and Wildlife Service

Breeding Bird Survey route within or near it, we recommend

that the standard survey be conducted. This involves 50 3-

minute point counts along roads at 0.5-mile (1-km) intervals.

The effort takes one person-day at the height of the breeding

season, usually in early June. The surveyor must know all of

the vocalizations of species likely to be encountered. This

Survey will help detect regional trends in many species in the

unit, or its vicinity.

Priority II. On-Road Point Counts—As a second priority,

we recommend that the unit put in point-counting stations in

multiples of about 250 stations to monitor overall population

changes and responses to habitats. We suggest that the stations

be in habitats representative of the unit, stratified by these

major habitats, systematically placed, and placed primarily

along secondary roads. This level of effort will require about

10 person-days during the early breeding season, usually in

May or June. It is based on the assumption that in the 10-day

period, an average of about 25 stations can be censused in each

day. While we acknowledge the fact that an on-road monitoring

program is not without bias, the benefits are considered by

most workers to outweigh the disadvantages, and are at least

partly offset by Priority IV, below.

Priority III. Demographic Monitoring—We recommend

that the unit establish at least one site to measure demographic

parameters. Either constant-effort mist netting or nest searches

(both, if possible) should be conducted on usually about six

plots within each unit. These monitoring stations will estimate

demographic variables that influence the density estimates.

Constant-Effort Mist-Netting Sites—Operating mist nets

through the breeding season, at most North American stations,

will require about 10 person-days per site, beginning about

June and continuing through the end of August. In Latin

America, the season would be longer. The program will

provide information on productivity, survivorship, and

movement of many species. Mist netting involves capturing

birds, banding them, and taking data on age, sex, breeding
4 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

status, molt, and survivorship. At a minimum each monitoring

station should operate 8-12 nets at least once every 10 days

throughout the breeding season. It has become well established

that results from constant-effort mist netting provide excellent

indexes of productivity and recruitment for a variety of

species (e.g., DeSante and Geupel 1987, Peach and others

1990, Peach 1992). It is the only method that estimates

survivorship and recruitment using mark and recapture. Its

major weakness is that the recruitment data are not habitat

specific, especially late in the summer. The survivorship data

are excellent, and all data are most habitat specific for adults,

especially early in the breeding season. As the season

progresses, influx of peripheral birds and young from other

areas dilutes this specificity.

Nest Searching Sites—Nest searches involve intensively

finding nests in a plot. Typically, one plot can be done in about

20-40 person-days, beginning about May and continuing to

about August. Nest searching involves finding nests, monitoring

their outcome, and measuring associated vegetation. A study

plot needs to be visited at least once every four days to find and

check nests. Nest searches provide direct measures of

reproductive success (rather than an index) and can provide

direct data on influences of habitat on reproductive success

and the incidence of nest parasitism. Nest searching, however,

is quite labor intensive and is applicable to fewer species than

mist netting.

A drawback to both demographic methods is that they will

assay the population health of only certain species in an area.

As a general rule of thumb, usually about 10 species at any one

station will be monitored. However, when data from several

stations are combined over a larger geographical area,

meaningful insights may be gained on many species.

In addition, within each demographic plot, at least 9-16

intensive point count censuses should be conducted at least

twice during the peak of the breeding season. Other census

methods (i.e., spot mapping, area search) may also be employed,

depending on objectives, size of study area, and availability of

personnel. Vegetation measures should also be made at each

census point and within each demographic plot.

Priority IV. Off-Road Point Counts—As a fourth priority,

we recommend that the unit conduct point counts in segments

of approximately 100 points off-roads in habitats not covered

by the on-road point counts. Each segment of 100 points will

require up to 10 person-days during the same period as onroad counts, and assumes about 10 stations per day are

covered along trails or cross-country.

Priority V. Additional Work—When resources are available,

we recommend that the unit add programs, in increments of 10

person-days, of the three programs (II-IV) above. We do

encourage additions of programs in the order they are

recommended. However, local conditions, variety of habitat

types, length of sampling season, areas of management concern,

and consultation with biostatisticians will modify the order

and magnitude of additional work in each unit. Additionally,

at some point a unit will be best served collecting information

other than that outlined above.

Selection of a Station Location

A monitoring station should be located in representative

habitat for a given region, or in a habitat of concern. A station

may have a variety of habitat types, and some will have a

higher density of birds than others. Because the derived

population and demographic parameters are likely to be highly

sensitive to successional changes in the habitats sampled,

stations should not be placed in very young habitats. However,

young habitats are acceptable if they are held in a lower

successional stage by active management.

If the census methods involve extensive point counts, the

points can be spread out along a road or trail network, over a

fairly large area of the region. This makes for a robust data set,

because each point is at a location somewhat representative of

the habitats in the region. In spot mapping and nest searches, a

plot is usually established in a single habitat type, and is usually

square or rectangular. Plots in heterogeneous habitat are often

not as useful because they are more difficult to generalize about.

For constant-effort mist netting, we suggest the capture

array be placed where a high rate of capture can be achieved.

By contrast, extensive census points and the nest search plot

should be placed in the representative habitats of the region.

Permanent Stations

While the need for broad-scale monitoring is of vital

importance, in-depth studies in small, protected areas, such as

natural areas, nature reserves, and parks, can contribute greatly

to our knowledge of landbird populations. In-depth studies of

bird life histories (normally using individually color-banded

birds) can provide important insights into vulnerability and

management of species. Other biological studies concurrently

conducted at the station can add greatly to our knowledge of

the factors affecting local landbird populations. Monitoring

stations with active field programs or living quarters for

biologists are ideal for intensive programs in remote areas and

can often attract volunteers.

Obtaining institutional sponsorship of permanent stations

can provide long-term commitment over many years. A

monitoring program with such a commitment will continue

despite turnover in personnel and can provide some stability

in funding. Furthermore, by using local volunteers to collect

data in such a program, a community outreach and education

program can be established. Bird observatories and some

university field stations in North and Latin America have been

conducting programs similar to this for many years.

General Monitoring Procedures

Species To Be Covered

Although many species will be censused at a single station,

fewer will be captured, and still fewer species will have their

nests found. However, biologists at a single station should get

a good sample of the population size of perhaps 30 species and
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 5

some indication of demographics on about 10 species. In a

region with perhaps six stations, more species will be monitored.

Over a wide geographic area, these data can be combined to

produce patterns of the population sizes and demographics of

many species.

Monitoring Period

Breeding Season

The period of study for the breeding season differs,

depending upon the individual species, latitude, rainfall pattern,

temperature, elevation, or even year. Therefore, each region

should establish its own monitoring period on the basis of the

local breeding season and the criteria described below.

Demographic monitoring, by mist nets or nest searches,

should span the entire breeding season. Censusing, by contrast,

is usually conducted only during approximately the first half

of the breeding season, when birds are most active, paired, on

territories, and vocal.

For all monitoring, we recommend the use of the sampling

interval time period of 10 days, as used in the British CES

project, for several reasons. This interval allows at least one

weekend for making up for inclement weather, and divides the

month into three approximately equal portions. It also provides

a basis for direct comparison between stations.

Operation of the demographic monitoring station by mist

netting or nest searching should begin no sooner than the 10-

day interval when virtually all of the breeding birds have

established territories, but before many have begun laying

eggs. For most lower elevation areas in temperate North

America this will be about May 1 or May 11. The date,

however, should be adjusted to conform with the local situation.

For example, in the more northern parts of the United States,

the first period can begin May 21 or May 31. In Alaska or

northern Canada, or at high altitudes, the first period may

begin as late as June 10. In the southwestern United States or

coastal southern California, where 90 percent of the species

have begun nesting activities by mid to late March, the starting

date could be April 1 or 10. In Mexico, it could be even earlier,

and in much of Latin America it could be much earlier. It is

considered important by some investigators to avoid netting

before migrant individuals of breeding species have finished

moving through. Early netting might result in later net avoidance

during the breeding season, thus biasing a few of the

demographic estimates. However, some adjustment for this

factor can be made during analysis and many stations do this

with good results.

A good measure of the establishment of territories is

increased singing. Also, captured males will show a pronounced

cloacal protuberance. Individuals carrying nesting material is

another excellent indication that the breeding season is under

way. The best measure of the start of the breeding season is the

beginning of egg laying. Females normally develop a brood

patch when the first egg is laid.

The termination of the demographic monitoring should be

no earlier than when the local population begins to be augmented

by fall migrants, or by an increase of dispersing individuals

known to have not bred in the local area. In most of temperate

North America, this will usually be about the second or third

10-day interval of August.

For uniformity, May 1 of each year should be considered the

first 10-day period. If a season in a region begins earlier, it

should be backdated from May 1. In fact, the season of monitoring

for most areas in temperate North America will begin May 1

and continue for a maximum of twelve 10-day intervals until

August 28. If a station begins before May 1, it should continue

until late August, unless a pilot project’s data indicate that all

breeding individuals and their young have left earlier.

For most of temperate North America, we recommend,

therefore, the following periods: May 1-10, May 11-20, May

21-30, May 31-June 9, June 10-19, June 20-29, June 30-July

9, July 10-19, July 20-29, July 30-August 8, August 9-18, and

August 19-28.

Censuses conducted on demographic study stations, such

as mist netting stations, need be done only in the first five 10-

day intervals when birds are on territory and actively singing.

In temperate North America, this will be usually from May 1

through June 19. In northern latitudes or higher elevations, the

period could be as late as June 1 to July 9. Point censuses, and

also area searches, should be done once on each plot in each

of the five 10-day intervals, and preferably about the midpoint of the interval.

Migration Operation

Operating a monitoring station in the spring or fall is an

option in many areas. Spring and fall migration data from mist

nets and censuses are confounded by many factors, particularly

local weather, and the questions migration data can answer are

different from those netting during the breeding season can

answer. The data can provide interesting and insightful

information about the timing, composition, and extent of

migration (e.g., Ralph 1978, 1981a; Robbins and others 1959).

The fall migration in particular gives a measure, derived from

many source areas, of the overall productivity of a species. As

mentioned previously, if a mist net program is operated in the

spring in the same area as a breeding mist net array, a few

demographic measures may be altered somewhat.

Nonbreeding Season

The value of winter studies is quite high. Winter is a time

when populations are resident and relatively stable, thus

providing excellent data on survivorship and mortality. It is

very likely that habitat associations, for example, are much

more defined in the winter than in the summer (e.g., Huff and

others 1991, Manuwal and Huff 1987). The methods outlined

here have full applicability in the non-breeding seasons, both

in North America and Latin America. In the tropics, mist

netting throughout the year at the same site would clarify

many questions about molt, skull, and plumage patterns.

Maintenance of a Study Plot

Plots should be permanently marked with stakes, markers,

or flagging that will survive over at least one winter. Rebar
6 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

(steel reinforcing bar), rock cairns, or tags driven into landmarks

all work well. Tags are available from biological supply

companies. In general, markers should be laid out along a

compass direction, be placed at regular intervals, and be visible

at any point between the markers. Each marker should correspond

to a numbered grid point on a map. In colder areas, be aware that

in years with heavy snowfall, plot markers can still be buried in

the spring. Net and census points should also be permanently

marked. Be sure to record in your journal net height and angle

of placement (use stakes or give a compass direction).

Customized maps of the study area should be traced from

a large scale map or from aerial photographs. Landmarks, grid

points, and net and census points should also be sketched.

Blank maps can be used for spot mapping censuses, vegetation

mapping, and other figures.

Each monitoring station ideally should be operated indefinitely.

Although objectives will vary, we suggest that at a minimum,

capture arrays of nets and nest searches should be operated for

four consecutive years, and census plots for three years.

Journal Keeping

Journal keeping is an essential tool of all field biologists.

The importance of regular, accurate journal keeping cannot be

overemphasized. It is not uncommon for journals to be

subpoenaed in court. The Grinnell method (Herman 1989) is

the most widely used by vertebrate ecologists and is extremely

detailed. Here we provide guidelines for basic information

that may be useful for monitoring landbird populations.

As a minimum we recommend recording the following on

a daily basis:

• Netting information: (a) the number and location of each net

operated; (b) the exact hours of each net operated; and (c) the

total capture and recapture rate for each species at each

monitoring array.

• Censuses and nest searches: the number, location, and timing

of each census conducted and the hours of nest searches.

• Personnel information: list the activities of each biologist

conducting field work, including areas censused, net locations

operated, and other activities.

• List of all birds seen or heard: basically presence/absence

data; provide any interesting notes on potential breeding or

other behavior of note.

• Weather data: in addition to the basic weather data that

should be taken (see below), a general one- or two-sentence

statement on the day’s weather is also helpful.

• Plant phenology: a list of what is blooming or in seed may

help interpret changes in bird distribution.

• Interesting observations of mammals, herptiles, insects, and

other natural history observations should also be included.

Training and Numbers of Personnel

Training is extremely important because the level of training

and experience will greatly affect the reliability of the data

collected. Training must be something that is continuous

throughout the field season. It is necessary to transmit

expectations early and often in data taking or responsibilities

for certain tasks.

The length of time to train personnel will vary greatly

depending upon the quality and interest of recruits. For many

census procedures, the mechanical aspects can be taught in

two or three 2-hour sessions. However, for a person who has

minimal skill in identification of plant or animal taxa, it can

take a week or longer, depending upon the taxon, and the

person’s previous experience. The suggestions for censusers

in Kepler and Scott (1981) are especially relevant. For a

completely untrained person to learn to remove birds from

mist nets takes at least 2-3 weeks of intensive training. This

training should include at least 3-4 hours of removing birds

from nets each day. Training for nest searching requires a

similar time commitment.

Probably the most important aspect of training is the testing

of the observer. This should be done regularly in the field by the

most experienced personnel available to make sure that data are

accurate, and of high quality. This can also be accomplished by

regularly checking data sheets as they come in from the field.

Any delay prevents feedback to the field crews.

The number of persons required to operate a monitoring

station depends upon several factors. If nets are the method of

choice, we suggest a minimum of two people, one of whom is

well-trained in removing birds from mist nets, and one of

whom is well-trained in identification of birds by sight, song,

and call. The less skilled person can be of great assistance, and

with proper training can contribute much to the monitoring.

Censuses and netting are both morning activities, and under

some circumstances they can be conducted concurrently if the

censuser’s position is known to the netter and he or she can be

called upon for help if capture rate is moderately high. The

health of the birds is of paramount importance, and all efforts

to prevent injury must be taken. Nest searches can be conducted

throughout the day, although it is most productive in the

morning.

When conducting censuses, it is best to rotate observers, if

at all possible, so that no observer censuses any given point

more than the others.

Syllabi for training in the methods contained in this handbook

have been prepared. These are for the use of persons experienced

in the methods, so that they can efficiently pass on the methods

to others. The syllabi are available from the senior author.

Data To Be Taken

Below we outline several types of data to be taken and

provide sample forms for each. We have also prepared data

entry programs using IBM compatible computers for these

forms. Clean forms for reproduction and these programs can

be obtained by contacting the senior author. These programs

can use either standard entry systems such as dBASE or

simple BASIC compilers.

For each point count census point, mist net location, and

nest site, we suggest that the “Location and Vegetation Form”

be filled out. It is described in detail in the Habitat Assessment

section and contains important location information for data

base files. At the minimum, for all monitoring programs, this
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 7

Figure 1—An idealized capture station of about 40 acres (16 ha) set

in an area of more than 250 acres of habitat “A.” Nine census points

are set at uniform spacing of about 150 m to estimate population

levels. Ten net locations are placed in sites where high capture rates

are likely, along a stream, near a spring, and other areas where

vegetation is dense, in order to monitor population and demographic

parameters.

100 m

Census Point

Processing Site

Spring

Mist Net Location

Dirt Road

Paved Road

Habitat “A”

“C”

“B”

“D”

“C”





Census

Point

Spring Mist Net Location

Stream

Processing

Site

Dirt Road

Carretera

location data should be taken.

All the data forms have constant the following information,

to help relate between data bases:

• State or province—The 2-column code for each.

• Region—An 8-column code, designated by the

investigator. Often, the name of the USGS quad, a prominent

landmark, or a nearby town will provide the best code name.

• Station/Location—This is a 6-column unique identifier

designated by the investigator to separate, within each region,

the location of the various data points. We recommend that the

station be a 4-letter code. The net location, point count census

point, or nest number will be a 2-number code.

Constant-Effort Mist Nets and

Banding

Scope

The capture of birds in nets can give the biologist an insight

into the health and demographics of the population of the birds

being studied. For instance, the proportion of young birds

captured in mist nets has been shown to be a good measure of

the productivity of birds during the previous few weeks

(Baillie and others 1986). The sex ratio of a population can be

used to assess the species’ differential survivorship the previous

year and the ability of the population to increase. The mist net

capture rate gives a measure of the number surviving during

the previous winter. The marking of individuals gives the

biologist insight into degree of dispersal between different

habitats and survivorship between years (e.g., Peach and

others 1991). Finally, weight, when compared to measures of

body size such as wing length, can give a measure of individual

fitness.

Mist nets have been used for a long period to capture birds.

Recently they have been used to monitor populations. Although

some have used them to assay population size (e.g., Karr

1981), for most species, censuses are the best method for this,

as netting provides relatively fewer data points per unit time.

Netting, however, is the method of choice to provide

information about the various attributes of the population, for

instance, age and sex ratios and physiological condition.

Over the years numerous aids have been developed for

field workers, with an emphasis on capture techniques and

data taking (e.g., Baldwin 1931, Bub 1991, Lincoln 1947,

Lincoln and Baldwin 1929, Lockley and Russell 1953, McClure

1984). O.L. Austin introduced mist nets to North American

biologists in 1947 (Keyes and Grue 1982), and he, Low

(1957), and Bleitz (1957) were all pioneers in their use.

The procedure detailed below is essentially identical to the

“Constant Effort Sites” (CES) Scheme of the British Trust for

Ornithology (Baillie and others 1986). The standards of

operation are also identical to those of the Monitoring Avian

Productivity and Survivorship Program (MAPS) (DeSante

1992a). We suggest the use of a series of mist net arrays, as in

the British program, to be operated on 10 to 12 intervals during

the breeding season, coupled with point count censuses. These

data will provide an index of adult population size and

changes at each station. The proportion of young birds in the

catch will provide a measure of post-fledgling productivity.

And finally, between-year recaptures can provide a sensitive

measure of adult survivorship and recruitment. With these

data, managers will have information on the possible causes

of landbird declines or their remedies.

The monitoring of populations with mist nets is no more

complicated than other techniques, but placement and operation

should be done rather uniformly; thus we present below more

details about this method than about others.

Net Placement

Operating a capture array of mist nets is a complex

undertaking, but very rewarding. Much useful information

can be gained from reading Bleitz (1970), Keyes and Grue

(1982), or McClure (1984). We outline below some guidelines

for operation of nets and their placement (fig. 1).
8 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

A field crew of two people can usually set up and monitor

an array of 8-12 mist nets quite easily. We suggest 10 as an

appropriate number. If the biologists are especially skilled, or

the bird density quite low, a few more nets may be operated.

However, if too many nets are established in an array early in

the season when capture rates are relatively low, the likely

influx of post-breeding birds later in the season, in July and

August, may severely tax the crew’s resources.

Distance between nets is a very important consideration

because of the effect of net dispersion on the precision of data

from capture-recapture analyses. In order to increase the

probability of capturing a bird banded the previous year, one

should place the nets as far apart as possible, thus intersecting

the most territories. However, it is absolutely critical that nets

also be close enough to each other that a person can visit all net

locations in a maximum of 10-15 minutes walking, preferably

less, if no birds are caught. On flat, level terrain, this array

would be about 0.5-0.6 miles (800-1000 m) in length. If 10

nets are placed in a circle or rectangle, this would allow about

an average of 75-100 m between nets, and would cover

approximately 5-10 ha. In steep or rough terrain, nets should

be closer, and the area covered less. In all cases, nets should

be spread out as uniformly as possible.

Nets should be placed at the same location and orientation

for all 10-day intervals in each year and preferably between

years. In the event the vegetation changes between years at a

given location, the nets will measure this change, rather than

changes in population of the birds. For this reason, care should

be exercised in placing nets in locations where the vegetation

will remain relatively stable through the life of the study. For

example, successional changes, for instance, from a clear-cut

to pole-sized trees over 10 years, would be unacceptable for a

site. However, when the changes over a decade would be much

less marked, the site would be permitted. If, due to unforeseen

circumstances, the vegetation is changed markedly at one or

two net locations of an array, the nets can be moved to locations

with similar vegetation to allow better between-year

comparisons. This should be a last resort, and only done after

consultation with knowledgeable participants in the program.

Although few problems arise from placement of census

points in areas of relatively high human impact, capture arrays

must be located with more care. In some areas nets can be left

in place (but closed) between capture days if the chance of

encounter by visitors is extremely low. In most areas, it is

advisable to rig the nets to allow easy removal at the close of

a day’s effort.

Baiting, artificial water, or taped vocalizations should not

be used at any time to attract birds to the nets.

 Net Locations

The best locations for the nets are usually on an edge of a

habitat. Examples of edges include the boundary between a

forest and a field, the boundary between two forests types (e.g.,

an upland pine and a pine/alder association in a valley), brushy

portions of wooded areas, at the edge of a pond, and along a

riparian corridor. Especially productive are areas where a

habitat type has a narrow section, for instance a hedgerow, that

narrows at a gate or where a natural gap funnels the vegetation

along a watercourse. Birds, especially shrub species, will

naturally be funneled into a net at that spot. Observations of

bird movements will often suggest appropriate net locations.

The highest rate of capture is usually found in wetter areas

within a given habitat type. If at all possible, natural running

or pooled water should be available throughout the summer in

the capture array, as it will draw birds from the immediate

area. An array aside a major watercourse with a well-developed

and wide riparian corridor will tend to monitor this habitat, but

will also monitor the surrounding habitats. In many regions of

the country, the riparian zone is the only place where sufficient

numbers of birds can be captured.

The major goal of a mist net array is to capture birds, not to

monitor the birds of a specific habitat. Census methods or nest

search are more appropriate for this. An array set in a uniform

habitat, such as an old-growth stand of coniferous trees, will

usually catch relatively few birds, even if located along a

watercourse in that habitat. There are possible exceptions to

this, such as eastern deciduous forests (T. Sherry, pers. comm.).

A reasonable goal for capture rate is approximately two

birds per net per day. This would result in the capture of

approximately 200 or more birds during the season. Typically,

the capture rate in the breeding season will be high during the

first 10-day period, decline thereafter, and usually increase

again during post-breeding dispersal, in July and August in

temperate North America.

Erecting and Operating Nets

In order to operate nets properly, the trammels (the horizontal

shelf strings that support the net) should be taut horizontally.

Except with 6-m nets, this usually involves the use of tie cords

bracing the pole upright. These can be arranged at 120o angles

to the net, with one end secured to the pole and the other to

nearby rocks, bushes, or stakes. When operated, the netting

material should not be stretched apart to its full extent, but

should allow some slack between the trammel lines; otherwise

birds will bounce off the tight net.

If the habitat is higher than the typical net height of 2.5 or

3 meters above the ground, a stacked net can be considered.

Although some birds may be missed, it is better to use single

nets, rather than to stack them one above another, unless a

particular location has a great abundance of birds. Even canopydwelling species invariably spend at least some time at lower

levels, whether to nest, take water, or forage. The additional

time spent putting up a stacked net can usually be better

employed by establishing another net in the array. McClure

(1984) describes several plans to stack nets; the simplest is to

use a strong metal pole, perhaps 8-10 feet long, such as metal

electrical conduit pipe. Connect two lengths together with a

sleeve (a 10-cm section of conduit slipped over a 20-cm long

pipe and glued in place), and tether the pole with a rope. The

net can be easily lowered and raised using this method.

For single nets, we suggest the following simple method of

putting them in place, adapted from Dennis P. Vroman (pers.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 9

comm.). Clear all vegetation from a net lane 2 m wide to prevent

vegetation from becoming entangled in the net. Drive one piece

of 1-m by 3/8-inch piece of steel reinforcing bar (rebar) into the

ground with a small hand sledge hammer at one end of the net

lane on a slight backward angle to the net. Insert a 5-foot section

of sawn 10-foot, 0.5-inch or larger, galvanized steel conduit

over the rebar. Repeat at the other end of the net lane.

A single net can be kept on a round metal spool (used to hold

bulk electrical wire), with a 6.5-inch diameter rim and 3.5- to

4-inch long axle or shaft. Place the loops of the net over the top

of the upright conduit; then unroll the net towards the second

pole, being careful to keep the loops in order. A second 5-foot

section can be placed on each pole in a conduit connector or a

sleeve atop the first conduit to allow the net to be fully opened.

When a net is to be closed, it should be spun to keep it from

unraveling. To do this most effectively (preferably with two

people from both ends simultaneously), leave the topmost

trammel separated from the others on the pole, and spin the net

on the lower trammels into a tight roll. Then quickly bring the

top trammel down atop the roll to keep it from unraveling.

This will allow the net to be opened much more quickly than

if the net had been spun around all the trammel lines.

To roll up the net, keep all the support cords together and

centered on the axle as the net is rolled up to allow easy

unrolling. Use a rubber band to hold the loops in place at the

end of the rolled net. Poles and rebar can be hidden under

vegetation near the net location to save set-up time.

Nets are also commonly put in cloth bags. To take down the

net, it is rolled up on small folds and put into the bag, as the

biologist moves from one end towards the other.

Net Specifications and Maintenance

A variety of net types can be purchased, but we strongly

suggest that the same type be used throughout the life of the

study. The net color should be black in forest or brush habitats.

The net mesh should be either 30 mm or 36 mm in stretched

diameter. The larger net catches more thrush-sized birds, but

smaller birds can become more severely tangled. Nets 12 m in

length are preferred, although in certain sites a half-net of 6 m

long can prove useful. (If a 6-m net is used, its use for one hour

equals a half net-hour.) In addition, some suppliers offer

“extra-full” nets that provide more capture area. They also

offer “tethered” nets that are resistant to bunching by the wind

because they are fastened to the trammels. If a nontethered net

is obtained, it can easily be tethered by placing drops of a

liquid cement along the top trammel.

A net should be replaced when it fades badly or becomes

degraded by the sun so that it breaks very readily. A net can

be tested by putting two fingers into the net and gently parting

them. Nets sustain damage from branches, misuse, large

birds, and from the rare occasions when a badly tangled bird

must be cut out of the netting. The life can be prolonged by

repairs with a strong black nylon twine or thread. Holes

should be repaired promptly, or the net replaced, as they

affect the efficiency of capture, and make it difficult to figure

out how to extract a bird.

Operation of Nets

Net Hours

To minimize variability and make comparisons from

different locations, standardization of the number of nets and

the number of hours nets are operated has long been advocated.

It is extremely important that nets be operated on the same

schedule between years, so as to allow direct comparisons. A

standard “net” is considered to be 12 m long and 2.5 m high.

For calculating effort, one standard mist net operated for one

hour is a “net-hour.” Two nets stacked atop one another would

be considered two nets, although one net location. If operated

for one hour, they would total two net-hours.

Although there are methods of compensating for varying

number of nets operated in different time periods (Ralph

1976), these are best implemented during migratory periods

when there is a high turnover of individuals between days.

During the breeding season, when populations are more

stable, it is best to operate nets on as regular a schedule as

possible. This includes the number of nets, the number of

hours, the time of day, the number of days, and the number of

days between operations.

We recommend that biologists use the “Record of NetHours” form (fig. 2). The data are recorded on a daily basis, as

follows:

• State or province—The 2-column code for each.

• Region—An 8-column code, designated by the

investigator. Often, the name of the USGS quad, a prominent

landmark, or a nearby town will provide the best code name.

• Station—A 4-letter code for the station that contains the

mist net array.

• Year.

• Operator(s).

• Net location—Place a 2-column number identifying each

net location. Most arrays will have no more than 10 locations,

and thus would be numbered 1-10.

• Number of nets—This number is usually one, but if a

stacked net is used, or if a net is within 10 m of another, they

are considered the same location, and the number of nets is

entered here.

• Month and day—One line for each day of operation, but

if a net location is operated for more or less time than the other

nets, it should get a line to itself.

• Open and close times—Using the 24-hour clock, record

the time of starting to open and the time of starting to close the

nets.

• Hours open—Calculate the number of hours open to the

nearest tenth of an hour (e.g., 4 hours, 20 minutes is 4.3 hours).

• Number of net-hours—Multiply the number of nets by the

hours open, and enter here.

• Total net-hours—For each day, total the number of nethours.

Time of Day and Number of Checks

Nets should be opened within 15 minutes of local sunrise

and operated for a minimum of 4, and preferably 6, hours per
10 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 2—The “Record of Net-Hours,” for recording and summarizing net-hours.

day. Nets should be checked every 45 minutes (more often in

inclement or very hot weather) and absolutely not more than

once each hour. That is, the net round should begin no longer

than 45 minutes after the start of the previous round. Nets

should be opened in the same order each day, and closed in the

same order that they were opened. It is very desirable that the

number of hours for each net location should be the same for

all 10-day intervals and for all years. Each station should be

operated once per 10-day monitoring interval throughout the

breeding season. We recommend that arrays be run no more

than once per 10-day interval. Running arrays more than twice

per interval greatly lowers capture probability per net hour. If

sufficient time is available, it is far more productive to set up

another array, rather than increasing effort at a single station.

When To Close Nets

The nets should not be operated in rain, wind, and extreme

heat. If already open when these conditions occur, they should

be closed, because precipitation is heavy enough for the birds’

feathers to become wet enough to lose their insulation. Strong

winds can cause severe tangling. In general, a steady wind of

more than 10 mph or occasional gusts to more than 15 mph

should be watched carefully for their effect on netted birds,

and the nets should be closed if necessary. Finally, in situations

with excessive heat and direct sunlight with little wind, netted

birds can quickly overheat and die. On such hot days, birds

should not remain in an exposed net for more than 15 minutes.

A certain amount of mortality may occur whenever wildlife

is handled or trapped. However, mortality rates in most

netting projects usually approach zero, and generally average

less than 1 percent when mortality does occur. If mortality

consistently occurs in nets, or exceeds an average of 1 percent,

it is likely that birds are not being processed quickly enough,

probably during their removal from the nets. Under these

circumstances, scrutinize closely the criteria for closing nets

and the expertise of the people running the station.

Removing Birds from Nets

Below we suggest some methods for extracting birds from

mist nets. The methods are used by most netters, were derived
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 11

from ideas of Shreve (1965), and were later modified and

augmented by Ralph (1967, 1988). Practice and careful review

of these techniques are essential. Most importantly, be careful.

The life and health of the birds are of primary concern.

As you work on a badly tangled bird, it is important to

remember that the bird can usually be backed out easily,

unharmed, in the direction from which it entered the net. You

must first take the time necessary to figure out exactly how the

bird went into the net. Observe carefully from which side the

bird entered the net, and between which trammels it went, in

order to find the opening of the pocket the bird made. Do not

just grab the bird, tempting as it may be. Start on the side of the

net which the bird entered; part the trammels and netting

loosely, and look into the pocket caused by the weight of the

bird. Because the tail is the last to enter, look at its position to

get a clue about how the bird entered the net. Back the bird out

the way it went in, step by step. A light touch is the most

important prerequisite for all methods. After determining

where the bird entered, several standard procedures are used

for removing birds, but different species and different problems

will require some improvisation.

We describe the various methods used to remove birds

from nets below. No one method will suffice for all birds,

because each bird flies into a net differently. Combinations of

methods will often be necessary. In all methods it is often

desirable to know where the strands of net are amongst the

bird’s feathers. This knowledge can help you decide where to

move your fingers next. By far the best method is to pull gently

at the exposed netting and see where feathers move on the

bird. This will tell you where the net strands are binding,

leading to quicker removal.

Body Grasp Method

This method has recently been used by some stations, and

it has been found to surpass other methods in ease of learning,

reduced injury to the birds, and speed of removal. About 9 of

10 birds can be removed with this method.

1. Find out from which side of the net the bird entered. Find

the opening of the pocket caused by the weight of the bird.

2. You have three choices at this point. (1) If the bird’s body

is accessible, without any netting in the way, and the net free

of the back and head, just put the bird into the “bander’s grip,”

with your palm against its back, your index and middle fingers

on either side of the neck, the left wing held with your thumb,

and the other fingers curled around the body and the right

wing. Then proceed to step #7 below. (2) If the net is tangled

around the head and wing, just slip your fingers over the body

and under the wings. This usually involves your thumb around

the breast and your fingers over the bird’s back, and down over

its sides and under the wings and carefully around the curve of

the body. (3) If the body is too tangled to be available for a

body grasp, then one of the other methods below must be used.

3. With the body now firmly secured, back the body out of

the net to expose at least the bend of one of the wings. Then,

remove the net from the wings. Flick net threads from the bend

of the wings, working from the underside. Generally your

thumb should be placed under the thread(s) on the underside

of the wing and your forefinger placed on the outer bend of the

wing as a fulcrum to flick the thread over. Often at this stage

it is helpful to pull gently on the exposed portions of the still

tangled threads in order to free them or to see where they are

caught.

4. When one wing is free, slip your fingers over the nowexposed wing, securing it against the bird’s body. Then, pull

remaining loops from around the neck, working from the back

of the head forward, in the manner of removing a T-shirt.

5. Remove the net from the other wing, as above.

6. The bird should now have gradually been put into the

“bander’s grip.”

7. Pull the bird up and away from the net, and it will usually

free its own feet in an effort to fly. If the toes are caught,

untangle them by pulling strands gently. You will notice that

if the heel joint is straightened out, the bird’s toes have a

tendency to relax, so that the netting can be more easily

removed. If the bird is clutching the net firmly, extract the feet

by (1) first freeing the opposable toe (the “thumb”) by sliding

the threads over it and lifting it away from the other toes; (2)

with the fingers, straightening the other three toes out; and (3)

sliding the netting over the toes with repeated strokes.

This method, when administered with a nimble hand and a

light touch, is very easy on the bird because the only firm

contact is on the sides of the neck. It is also a time saver,

because feet untangle themselves. The method works best

with a recently caught bird that has had little time to entangle

itself, but is applicable to most birds.

Feet First Method

The original, and perhaps still the most widely used method,

is somewhat slower but is usually the way that beginners are

taught. Its main disadvantage is that it requires holding the

legs, sometimes causing injury or breakage. It involves the

following steps:

1. As before, find the side of the net the bird entered.

2. If you (the bander) are right-handed, grasp both tibiae

(the tibia is the feathered part of the leg above the bare tarsus)

from the rear of the bird using your left hand so that your

fingers point towards the bird’s head. The bird should be

upside down in the net when you have your grip.

3. Put your index finger between the tibiae, and press your

thumb against the bird’s right tibia and your middle finger

against the left tibia. This leaves your right hand free to

remove net strands from the entangled legs and feet.

4. Most importantly, make certain that all threads are

pulled down and off tibiae and thighs below the heel joint, the

prominent joint between the tibia and tarsus. These threads are

sometimes high up on the thigh at the flank.

5. Untangle the toes by the method described in the body

grasp method above.

6. Pull the bird up and away from the net, still holding the

bird upside down by the feathered tibiae, above the bare

tarsus. Flick net threads from the bend of the wings, working

from the underside. Generally the thumb should be placed
12 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

under the thread(s) on the underside of the wing and the

forefinger placed on the outer bend of the wing as a fulcrum

to flick the thread over. Often at this stage it is helpful to pull

gently on the exposed portions of the still tangled threads in

order to free them or to see where they are caught.

7. When both wings are free, pull remaining loops from

around the neck, working from the back of the head forward. Be

sure to secure the bill by placing the thumb against the tip while

pulling the net over the head in order to protect the delicate neck.

Rollover Method

A third method requires a little practice but is applicable to

almost every situation:

1. As always, determine the side of the net entered.

2. Grasp the left (or right) leg above the tarsus and release

the foot.

3. Release the left (or right) wing; release the head, then the

other wing. Grasp the bird normally with the “bander’s grip.”

Finally, free the right foot.

This method requires an experienced “feel”—the bird is

rolled over and released in order of foot, wing, head, wing, and

foot. This method is especially recommended when one of the

legs is particularly badly tangled. Work so as to free that leg last.

Processing

Once the birds are removed from the nets, put each individual

in a separate, small cloth bag, and transport to the processing

site. It is probably best to have a central processing site, rather

than to process birds at each net as they are captured, because:

(1) a biologist rapidly circulating around the nets can better

monitor the captures, in case of an influx of birds that might

necessitate shutting down some nets temporarily; and (2) it

lessens the disturbance in the vicinity of the nets. Further, if

processing becomes delayed, it is always preferred to have the

birds out of the nets and stored in bags. Bags should be made

from opaque cloth, and sewn so that the seams (and possible

loose threads that can catch toes) are outside. Hang bags from

hooks or branches to prevent them from being stepped on, and

out of direct sunlight. They should be washed often.

Birds should be released at the processing site except for

females (indicated by a brood patch) and dependent juveniles

(indicated by a frizzy appearance and a growing tail). They

should be released at the point of capture.

Recaptures provide the most important data in a constanteffort mist netting program. We suggest, if some birds have

to be released without processing, that recaptures have a

much higher priority for processing than unbanded birds. If

birds have to be released without complete processing, we

suggest that the following be regarded as the priorities, in

order: (1) band number (if a recapture); (2) species; (3) age

(usually involves skulling, or diagnostic plumage characters);

(4) new band number (if previously unbanded); (5) sex; and

(6) other measurements or data. Please notice that the species

and age are the two variables which are considered absolutely

critical. If these are not accurately and completely recorded,

the time and money spent in the monitoring has minimal

value to the objectives.

Special Problems

Tongue Caught in Net

The mouth structure of birds, especially thrushes, thrashers,

and woodpeckers, allows net threads to catch behind the

tongue. While the bird’s head is held between your index and

middle finger, your third and fourth fingers and thumb can

hold the net near the side of the mouth and relieve pressure on

the tongue by pulling the net backward along the side of the

head. A pencil, crochet hook, or a sharp twig can be manipulated

with your free hand to lift the thread from behind the cleft of

the tongue. Until one becomes deft at releasing the tongue in

this manner, a small pair of a scissors is invaluable. Usually,

clipping a single strand of mesh will do the trick.

Badly Caught Birds

As a last recourse, to remove a strand from a tongue, or to

rapidly extract a bird in distress it is sometimes necessary to

cut a few threads with a scissors, a stitch ripper (sewing tool

that cuts threads along seams), or a sharp knife. The most rapid

method is to find an area with few (or only one if possible)

layers of netting. Clip as few strands as possible, just enough

to bring the bird through the net. Then free the bird in the

normal manner. It should be very rare to need to cut more than

three strands. Before releasing such a bird, look carefully to

ensure that no net remains on the bird.

Data Collection

One of the first steps a biologist must take to make the

capture work more meaningful is to properly record the data

presented each time that a bird is caught. Much of the following

was extracted from Pyle and others (1987) and Ralph (1967,

1988). The identification guide by Pyle and others (1987)

should be included in all netting kits.

At each net location, we suggest that the Location and

Vegetation Form (fig. 15, discussed in detail below) be filled

out. The Location information on the first three lines of the

form is vital to data base management.

For each individual captured or recaptured, we recommend

that the following data be recorded. In addition to date, time,

and location, it is imperative that the species be accurately

identified. It is also vital that the age and the sex of the birds

be determined. We highly recommend that determining the

amount of skull pneumatization be a top priority, as essentially

all analyses depend upon accurate ageing. Age and sex

determinations are generally complicated by the highly variable

nature of size, plumage, and molt patterns in each species. We

recognize that a certain percentage of individuals cannot be

reliably aged, sexed, or identified with any one or even all of

the published criteria. Remember that with age and sex it is

better to be cautious than inaccurate. If the bander is unsure of

an age/sex class, we recommend that the record be conservative,

by recording the age or sex as unknown, and separately noting

which class seems likely. By using the skull pneumatization

and the literature carefully, determinations can be made with

above 99 percent confidence. Information on how the bander

aged and sexed the bird can be used to screen improperly
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 13

Figure 3—The contrast between juvenal and

nonjuvenal body feathers. The differences

are most apparent with undertail coverts and

feathers of the nape and back. Taken from

Pyle and others (1987).

juvenal non-juvenal

processed birds. We also highly recommend that the breeding

condition of adults, the extent of juvenal plumage and molt,

and the wing chord also be recorded.

For the various attributes below we suggest a letter or

numeric code. We strongly suggest that whatever codes are

used at a station should be used consistently between years, or

certainly within a year. Alpha (letter) codes have the advantage

of being mnemonic in nature, increasing accuracy. Numeric

codes have the advantage of retaining the order of progression

from small, none, or few, to large or many.

Plumage Attributes

The first plumage (subsequent to the natal down) acquired

by the nestling and retained by the juvenile fledgling is called

the juvenal plumage (note the difference in spellings). The

body feathers of this plumage are replaced during the first

prebasic molt, which almost always occurs within three months

of fledging and usually takes place on the breeding grounds.

Juveniles are readily aged by many criteria and are generally

sexually indistinguishable by plumage. The juvenal plumage

is usually more streaked or spotted than that of the adult, and

juveniles will often have wing bars where the adult has none.

Juvenal feathers also have a more loosely structured contour

(fig. 3), most evident in the feathers of the nape, back, and

undertail coverts. In addition, many nestling characteristics

are evident in young juveniles which can also be helpful in

separating them from adults. The gape of nestlings and early

juveniles is swollen and more brightly colored than that in

adults, and the inside of the mouth is also brighter in tone, or

paler in hue, or both, in juveniles than in adults. Several

characters useful for separating first-year birds from adults

can be applied to juveniles. In particular, summer adults in

alternate (breeding) plumage should show very worn flight

feathers while those of juveniles should be relatively much

fresher. And, of course, the pneumatization process is just

beginning in juveniles, whereas it should be complete (or

nearly so) in adults. Finally, eye color is useful for separating

juveniles of many species, being generally grayer and paler in

juveniles and redder and darker in adults. In summary, biologists

should have no trouble with the separation of juveniles from

adults during the summer months, when all criteria are used.

In most passerine species, however, birds in juvenal plumage

cannot be reliably sexed by in-hand criteria alone. Only in a

few species, in which differences occur in the color pattern of

the flight feathers, can juveniles be sexed by plumage.

As juvenile birds go through their first molt, normally in

the fall, they assume a plumage that is often similar to that of

adults, especially to that of the adult female in sexually

dimorphic (having a male and female plumage) species.

Inspecting the skull is now recognized as being the most

reliable technique for ageing these birds.

Age Classes

The various age codes suggested below follow, for the

most part, the system used by the U.S. Fish and Wildlife Bird

Banding Laboratory and the Canadian Wildlife Service Bird

Banding Office, as listed in the North American Bird Banding

Manual (CWS and USFWS 1991). The system is based

primarily on the calendar year. Following are the age

designation, the alpha code used by the Services, a suggested

one-letter abbreviation (or optional numeric code) for purposes

of this handbook (where different from the Services’ code),

and a definition of the age class.

Unknown (U or 0). Age cannot be determined with absolute

confidence.

Local (L or 4). A young bird incapable of sustained flight.

Hatching Year (HY) (H or 2). A bird in its juvenal or first

basic plumage during its first calendar year (i.e., from its

fledging until December 31 of the year that it fledged).

Second Year (SY) (S or 5). A bird in its second calendar

year (i.e., January 1 of the year following fledging through

December 31 of the same year).

After Hatching Year (AHY) (A or 1). A bird in at least its

second calendar year. This code is more significant after the

breeding season, when it implies an adult. Before the breeding

season, it essentially means “unknown” (either SY or ASY).

After Second Year (ASY or 6) (O [older]). An adult in at

least its third calendar year (i.e., a bird in at least the year

following its first breeding season and second prebasic molt). A

bird known to be in its third year, or older, should be indicated

by “O,” and a note should be made in the Notes columns.

Skull Pneumatization

Determining the amount of skull pneumatization, also

known as ossification, is the best method of ageing most

species of birds during the summer and fall months and, for

some species, is proving useful through the early winter and

even into spring. The importance of this method cannot be

understated. If you take only one datum besides species, skull

pneumatization is quite probably the next most important.

This technique came into common usage in the late 1960’s.

Biologists are strongly urged to become proficient at skulling

and to skull most passerine species throughout the year. When

a fledgling passerine leaves the nest, the section of the skull

overlaying the brain (frontals and parietal) consists of a single
14 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 5—Two good holds for skulling a bird. It is best to look to the

side of the mid-line of the skull. Taken from Pyle and others (1987).

Figure 4—The two common patterns of skull pneumatization, from a

very young bird (“a”), to a completely pneumatized bird (“e”). Taken

from Pyle and others (1987).

y



yyyyyy

yyyyyy

yyyyyy

yyyyyy

yyyyyyyyyyyy

peripheral

pattern

medial line

a pattern

b

c

d

e

y

yyyy

yyyyyyyyyy

  

layer of bone. From fledging until the bird is four to 12 months

old (depending mostly on the species), a second layer of bone

develops underneath the first. The two layers are then separated

slightly by spaces or air pockets and joined by small columns

of bone. This process is called skull pneumatization.

The pattern generally follows one of the two progressions

illustrated in figure 4, but may show other variations. Smaller

species tend to show the peripheral pneumatization pattern,

and larger species the median line pattern. Individuals of

certain species may show either pattern, however, and the

exact shapes of the unpneumatized areas or “windows” will

also show substantial individual variation.

Any passerine found with a partially pneumatized skull

(fig. 4a-c) can be reliably aged as a hatching year bird, with

the exception perhaps of occasional summer or early fall

birds with small windows (fig. 4d). In most North American

passerine species, the skulls of the earliest hatching year

birds become completely pneumatized in October and

November, and the latest birds become complete between

November and January, but for purposes of this monitoring

effort during the breeding season, all hatching year birds will

have incomplete pneumatization.

In some (perhaps many) species, small unpneumatized

windows may normally be retained until spring and even

early summer. This is most commonly seen in the longer

distance migrants such as certain flycatchers, swallows,

thrushes, and vireos. Birds with windows greater than one

millimeter in diameter (fig. 4d) are probably reliably aged as

Second-years through June of their second year. Birds with

smaller windows are not necessarily in their hatching year,

because some small proportion (probably less than 1 percent)

of individuals will never show complete pneumatization.

Birds with small windows in July and August are most likely

to be advanced young of that year.

The Process of Determining the Extent of

Skull Pneumatization

Unpneumatized areas of the passerine skull usually appear

pinkish or dull reddish, whereas pneumatized areas appear

grayish, whitish, or pinkish-white, with small white dots

indicating the columns of bone. The color or contrast between

these two color patterns, or both, can usually be seen through

the skin of the head, especially after the head has been wetted

to allow parting of the feathers, and to make the skin more

transparent.

To skull a passerine, start by holding the bird in the position

shown in figure 5. This hold facilitates skulling because the

skin can more readily be moved around the skull, allowing a

large area of the skull to be viewed through a small area of

skin. In order to see the skull, the feathers need to be parted

such that a small opening of bare skin is created. This can be

accomplished without wetting the feathers, but is much more

easily done if a small amount of water is applied to the head

(do not apply detergent or alcohol solutions). During cold

weather, the few drops of water used to make the skin more

transparent should have no effect on the bird’s ability to
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 15

maintain its temperature. If there is concern about this, simply

put the bird out of the wind in a dry bag for a few minutes

before releasing it.

It is usually easiest to part the feathers by running your

thumb or finger forward over the crown, against the direction

in which the feathers lie, and then moving the feathers off to

each side. In the summer and early fall, when most young

birds are just beginning the pneumatization process, it is

good to start at the rear and the side of the skull and work up

towards the crown. Later in the fall, the parting should be

made higher up on the crown (in the areas just above and

behind the eyes), where the last unpneumatized windows

usually occur. With thicker-skinned birds, one can improve

viewing by parting the feathers on the side of the head or neck

(where the skin is more transparent) and moving the skin up

to the crown. When the skulling process is finished, the

feathers can be smoothed back into place.

It is usually best to hold the bird under a fairly strong lamp

or in indirect sunshine to achieve the best lighting conditions

for viewing. Very bright light often creates a glare off the skin.

It is often helpful to move the head around, because different

angles of light can make it easier to see through the skin. We

strongly recommend using a magnifying device such as a

visor that slips over your head.

Move the skin back and forth—the spots will be stationary,

and thus visible. If the tiny white dots are not visible, one is not

properly viewing the skull, or the bird is a very young juvenile

with an entirely pinkish skull. “Seeing” a boundary between

whitish and pink areas is not enough, because one might be

seeing only bone structure unrelated to pneumatization. Start

looking at the skull at a point at its base and slightly to one side.

Continue looking forward until just halfway between the eye

and the top of the crown. If at no point the dots disappear and

are replaced by a clear pink area, the skull is fully pneumatized.

Because the pneumatization usually proceeds centripetally

and anteriorly, be sure to examine the area between the eyes

of all birds with pneumatized skulls, to make sure that they are

not “advanced” immatures.

Also look for entirely pinkish skulls in very young birds

(fig. 4a) in June-July and for contrasts between the pneumatized

and unpneumatized areas in older birds (most frequently after

August). Small windows (fig. 4d) should be carefully looked

for at all times.

Any of several factors may make it difficult or impossible

to see the pneumatization of the skull. These include: the skin

of the head being too thick; large amounts of fat in the skin

during fall migration and winter; and dark, or otherwise

opaque, skin (especially in molting or injured birds). It is

especially difficult to see the pneumatization of the skull in

molting birds, because of the thickening and the excessive

flaking of the skin.

We suggest codes for categories of skull pneumatization.

Because the critical differences are often in the 0-5 percent or

95-100 percent categories, care should be taken. It can make a

great deal of difference in evaluating the age during the

breeding season to know that a skull had only small windows

(e.g., 98 percent pneumatized) and could have been either a

second-year bird or perhaps a young bird, as opposed to one

that was perhaps 70 percent and almost assuredly a young bird.

The codes we suggest are:

N or 0 - No white spots showing, only a single, thin layer of

bone covers the entire brain.

T or 1 - Trace of pneumatization at the very back of the skull,

usually appearing as an opaque, grayish crescent or

a very small triangular area. Between 1 and 5 percent

of the skull is pneumatized.

L or 2 - Less than one-third pneumatized, but some is obvious.

Generally the posterior part of the cranium has a

triangular or circular area of small white dots, usually

distinctly contrasting with the nonpneumatized area.

H or 3 - Half the skull pneumatized, between one-third and

two-thirds complete. Typically, most of the rear half

is complete, as well as part of the front, extending

back to the eyes. The front is usually difficult to see,

because of dense, short feathers.

G or 4 - Greater than two-thirds pneumatized, but at least a

small area not complete, less than 95 percent complete.

A or 5 - Almost complete pneumatization, between 95 percent

and 99 percent complete. These birds show a tiny

dull, pinkish area or “windows.”

F or 6 - Fully complete pneumatization.

U - Unknown, skull examined, but extent of pneumatization

not determinable.

Sex Determination

The best method for determining the sex of sexually

monomorphic passerine birds during the breeding season is by

the presence or absence of the cloacal protuberance in the

male, and the brood patch, which primarily occurs in females.

All North American landbirds develop at least one of these

characteristics, at least partially, and most are reliably sexed

by them during the late spring and summer months. Latin

American birds are less well-known, but these guidelines

should generally apply.

Cloacal Protuberance—In order to store sperm and to

assist with copulation, external cloacal protuberances, or

bulbs, are developed by male passerine birds during the

breeding season. They usually begin to develop early in the

spring and reach their peak size in 3-5 weeks. Depending on

the species and the number of clutches attempted during the

breeding season, cloacal protuberances will recede from mid

to late summer.

Although the cloacal regions in females will sometimes

swell slightly, or show a small protuberance, it rarely approaches

the size of those in the males (the Wrentit appears to be an

exception). If the swelling forms a gradual slope on the

abdomen ending with the cloacal opening pointing towards

the tail, then it is probably a female in breeding condition.

When the female is most swollen in this area, she will usually

also have a brood patch. A typical male protuberance essentially

forms a right angle to the abdomen and is somewhat larger at

the top than at the bottom.
16 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 7—Brood patches in different stages of development. Taken from Pyle and others (1987).

Figure 6—On the left, a cloacal protuberance at its peak in a male passerine. On the right a nonbreeding

male (class = 0), a male beginning to be in breeding condition (class = S), and a male in full breeding condition

(class = M). Class “L” would show a more prominent protuberance. Taken from Pyle and others (1987).

  





full breeding

partial breeding

non-breeding

O

S

M





yyy

yyyyyy

a bc

(full breeding) (partial breeding) non-breeding

To view the protuberance, blow the feathers apart in the

region of the vent. The shape of the protuberance can be

somewhat variable, and nonbreeding males may not always

develop one. After a little experience with the shape of the

cloacal region during the nesting season, biologists should

have no problem separating breeding males from females.

We have categorized cloacal protuberances into four size

categories (fig. 6): none (N or 0), small (S or 1), medium (M

or 2), and large (L or 3). As one becomes familiar with the

various extents of protuberances, one can make a judgment on

the relative size.

Brood Patch—Incubation or brood patches are developed

by incubating birds as a means of transferring as much body

heat as possible to eggs or young in the nest. In most landbirds,

females perform all or most of the incubating, and develop

more substantial brood patches. The presence of a distinct

brood patch can thus be used to reliably sex breeding females

of almost all passerine species.

The development of the brood patch begins with the loss of

the feathers of the abdomen, about 3-5 days before the first

eggs are laid (Blake 1963). Shortly thereafter, the blood

vessels of the region begin to increase in size, and the skin

becomes thicker and filled with an opaque, whitish fluid.

Figure 7a illustrates a full brood patch as viewed by blowing
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 17

the feathers of the breast aside. A few days after the fledglings

leave the nest, the swelling and blood vascularization will

begin to subside. If a second clutch of eggs is laid, the process

(except for defeathering) will be repeated. A new set of

feathers on the abdomen are usually not grown until the

prebasic molt, after completion of breeding. Between the end

of nesting and the onset of molt, the skin of the abdomen will

often appear grayish and wrinkled. Many young, and especially

juvenile, passerine birds have little or no down or feathers on

the belly; therefore the belly of some young look much like

that of an adult who is just beginning to develop a brood patch,

but the area will be quite smooth and usually a pink or dark red.

In most North American passerine birds, the male does not

develop a brood patch in the breeding season. Slightly fewer

feathers may be present on the abdomen than are found in the

winter, but the breast retains a feathered appearance. In a few

groups, in North America, notably the mimids, vireos,

Myiarchis flycatchers, and a few other species (see Pyle and

others 1987), the male will assist with incubation and develop

an incomplete brood patch. This will include partial or complete

feather loss and slight to moderate vascularization and swelling,

which rarely or never approaches the extent of development

typically found in females of the same species. Only in the

Wrentit and the woodpeckers does the male develop a full

brood patch.

We suggest recording brood patch in the order of its

development as follows:

N or 0 - No brood patch present—Breast more or less feathered.

Nonfeathered areas of the breast and abdomen smooth

without evident vascularization. In some species

such as hummingbirds, and in most young birds, the

breast is normally not feathered.

S or 1 - Smooth skin—A loss of breast and some abdomen

feathers, but most of the area is still rather smooth

and dark red.

V or 2 - Vascularized—Abdominal skin thickened with

increased fluid and vascularization. This is the peak

of incubation.

W or 3 - Wrinkled—Abdomen skin thinning, wrinkly, and

scaly.

M or 4 - Molting—New pin feathers are coming in on the

abdomen. Nesting is usually completely over by this

point.

Measurements

The standard reference for measuring birds is Baldwin and

others (1931), which outlines virtually every possible

measurement. Although old, it is commonly listed as available

in catalogs of used natural history books.

Size, as indicated by specific measurements such as wing,

tail, or tarsus length, is often a useful characteristic for

identifying, ageing, and especially, for sexing passerine birds

in the hand. In almost all passerine species, the size of males

of a given population will average larger than that of the

females by about 5-10 percent. The extent to which the sexes

overlap in size depends on both the species and the particular

measurement being considered. Measurements also vary with

age, but to a lesser extent than with sex. For example, juvenal

primaries tend to be slightly (2-5 percent) shorter than adult

primaries. Within each sex class, immature birds with juvenal

primaries will have shorter wing lengths than adults. When

coupled with weight and fat, size can also give a strong

indication of the health of a bird.

When identifying, ageing, or sexing passerine birds it is

important to use measuring techniques that are strictly

standardized with those of published samples. In the following

sections we recommend standardized methods for obtaining

the measurements. All linear measurements should be recorded

in millimeters (mm).

Wing Length—Although various methods of measuring

wings are employed, we recommend that you measure the

wing chord, because this is the length most frequently used

and most widely published for North American birds, and is

the most consistent between measurements. The wing chord

is measured from the bend of the wing to the tip of the longest

primary, across the natural arc of the primaries (fig. 8). While

taking the wing measurement, avoid the tendency to flatten

the natural curve of the wing, thus getting a measurement that

is 2-5 percent longer than proper.

To measure the wing chord it is best to have a thin ruler with

a perpendicular stop at zero. Insert the ruler under the wing,

and place the bend of the wing (carpal joint or “wrist”) snugly

against the stop. To avoid differences due to carpal compression,

we recommend that the bend of the wing be pushed against the

stop with no more pressure than the wing itself applies when

the ruler is moved up to the wing. Once the wing is in place,

make sure that the line between the carpal joint and the tip of

the longest primary is parallel with the edge of the ruler, gently

lower its tip to the ruler so that it touches it, and read the wing

chord length (fig. 8).

When measuring the wing it is important to make sure that

the longest primary is not broken, bent, or molting. Bent

primary tips should be straightened. Older and more worn

primaries will result in a shorter wing measurement and

should be noted.

Weight—Because bird weight varies substantially with

geographic population, condition of the individual, and season

or period within the life cycle of each particular species, this

measurement is not as useful for ageing, sexing, or identifying

birds as is the wing chord. Weight, however, is an important

indicator of the health of the bird, especially when coupled

with wing length and fat content. It should always be recorded,

when possible, to the nearest tenth of a gram.

Molt

Types of Molt—Relatively little is known about the timing,

sequence, and extent of molt in many species, especially in

Latin America. A proper understanding of molt can be extremely

helpful in the accurate ageing and sexing of passerine birds in

the hand. With a few known exceptions, molting is confined

to two periods within the annual life cycle of North American

passerine birds, just before and just after the breeding season.
18 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 8—Above, a good hold for measuring the wing chord, and below, the measurement of the wing chord and

flattened wing. The wing chord is preferred in North America. Taken from Pyle and others (1987). 8 0 4 0 4 0 6 0 5 0 4 0

















wing chord (wg) flattened wing [wg (flat)]





Thus, most adult passerine birds display two plumages, the

basic (winter) plumage and the alternate (summer or breeding)

plumage. The molt that occurs just before the breeding season

is called the prealternate molt; that occurring just after the

breeding season is the prebasic molt. All North American

passerine birds have a prebasic molt, and just over half

(predominantly migratory species) have a prealternate molt.

The prebasic molt usually occurs from July to September on

the breeding grounds and occurs in both recently fledged birds

and adults that have completed nesting activities for the year.

With one or two exceptions, the prebasic molt in adult passerine

birds is “complete” (fig. 9; includes all body and flight feathers),

whereas hatching year birds of most species typically replace

the body feathers and some coverts, but not the primary

coverts, and flight (wing and tail) feathers (except the central

two tail feathers) during a “partial” first prebasic molt.

As you blow apart the feathers on the various areas of the

body, you can easily determine which feathers are molting by

the presence of a cylindrical sheath around the base of the

molting feather. When the feather is fully grown, this sheath

is preened off and the feather ceases its traffic with the body

and is thus fully grown.

Birds have three types of flight feathers: the rectrices, or tail

feathers, and the outer (primary) and inner (secondary) wing

feathers. The rectrices are numbered in pairs, beginning with

the central ones (the “decks”) as #1, and proceeding outward

in both directions usually to #5 or #6, depending upon the

taxon. In some species the decks are sometimes molted by the

young at the same time as their body feathers. The remaining

rectrices molt in an ascendant sequence from #2 through #6.

The secondaries are long flight feathers attached to the skin

at the ulna, the bone of forearm. These are numbered by all

authors beginning at the bend of the wing and proceeding

inward toward the body. This is the usual order of molt, except

that the three innermost secondaries (tertials) molt like body

feathers and may be molted by juveniles. They are also often

molted concurrently with the longer secondaries.

The primaries are the long flight feathers attached to

bones of the “hand.” These are numbered in most of the

North American literature from the wrist-joint (bend of the
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 19

Figure 9—An example of a wing during complete molt of an adult. Notice the worn primaries 7-9 and secondary 7.

Taken from Pyle and others (1987).

les covs

mid covs

alula

gr covs

pp covs

p10 (if present)

old pp

growing pp

new pp new ss

growing ss

old ss

s6

s7

s8

s9

s3

s2

s4

s5

s1 p1 p2 p3 p4

p5 p7

p8

p9

p6

(new, growing, old)

terts

wing) outward, which is the sequence that nearly all birds

molt these feathers.

Pyle and others (1987) have provided a complete

enumeration of the numbers of flight feathers of passerine

birds in North America.

Recording Molt—A basic system of recording molt in the

flight feathers is to record presence or absence of molt in the

primary feathers or the secondaries (except the innermost

three). We suggest that the molt in the tail feathers not be

recorded, except as a note. You should always check both

wings, because birds often lose feathers accidentally

(“adventitious molt”). Flight feather molt is “S” if symmetrical

and normal, “A” if adventitious, and “N” or “0” if none.

More detailed recording of molt can be conducted using

the British Trust for Ornithology’s method (Ginn and Melville

1983).

Body molt can be recorded by a subjective determination

of none (N or 0), trace (T or 1) (a few, perhaps adventitious

molting feathers), light (L or 1) (involving more than one

feather tract), medium (M or 2), or heavy (H or 3) molt.

Extent of Juvenal Plumage

We suggest that the extent of juvenal plumage be recorded,

because it is a good indicator of the age of a young bird and the

timing of breeding. Juvenal plumage can be coded in the

following: N or 0 = no juvenal body plumage; L or 1 = less than

half of juvenal plumage remains; H or 2 = more than half of

the juvenal plumage remains, some first basic plumage is

visible; F or 3 = full juvenal plumage, bird has not started first

prebasic molt. For a more objective measure, the bander could

estimate the percent of juvenal plumage.

Primary Feather Wear

Feather wear could be a useful indicator of age, because it

seems likely that the juvenal generation of flight feathers may

wear faster, and thus show more wear at any given time, than

later, adult generations of feathers. Faster wear results from

the rapid growth of juvenal feathers resulting in weaker

feathers, and the protracted molt of adults. In some species

during especially the early breeding season, adult flight feathers,

molted after the previous breeding season, are much older

than juvenal feathers and can help age birds.

Examine the outer four or five primaries to determine wear,

and classify them according to the following scale: N or 0 = No

wear, the feather edges are perfect, and the entire edge is light,

including the tips; S or 1 = Slight wear, the feather edges are

slightly worn with no fraying or nicks, and the edge is often

light-colored, except at the tips; L or 2 = Light wear, the

feathers are definitely worn, but with little fraying and few

nicks; M or 3 = Moderate wear, considerable wear with

definite fraying, and nicks and chips are obvious along the

edges; H or 4 = Heavy wear, feathers very heavily worn and
20 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

frayed, and the tips often worn completely off; and X or 5 =

Excessive wear, feathers are extremely ragged and torn, the

shafts are usually exposed well beyond the vane, and all the

tips are usually completely worn or broken off (one wonders

how well the bird can fly).

Fat

The amount of fat on a bird may indicate periods of stress,

low availability of food, low fledging weight, and other

conditions that give insight into the viability of an individual.

Especially as birds prepare for migration, subcutaneous fat is

accumulated and is visible beneath the skin as white, yellow,

or light orange areas easily seen in contrast to the red muscular

areas. The fat can be most easily seen on the abdomen and the

furculum. The furcular or interclavicular region is the

depression formed between the attachments of the pectoralis

muscles to the furculum (the “wish-bone”) and coracoids,

forming a “V” running toward the spinal cord and pectoral

girdle, through which the neck protrudes. You can assign a fat

class on the basis of how much fat you can find:

Fat Class Furculum Abdomen

N or 0 No fat, the region is concave No fat

T or 1 Trace, deeply concave, scattered None, or a trace

patches, less than 5 percent

filled.

L or 2 Thin Layer, less than a third Trace or thin layer

filled.

H or 3 One-Half filled in small patches Small patches, not

covering some areas.

F or 4 More than 2/3 Filled, level with Covering pad, slightly

clavicles mounded

B or 5 Slightly Bulging Well mounded

G or 6 Bulging Greatly Greatly distended mound

V or 7 Very large fat pads of furculum and abdomen meet

Data Entry

We include a standard data form (fig. 10) that we encourage

you to use. Fill in all the information, and print neatly in soft,

black pencil. For codes not shown, and for exact definitions,

see CWS and USFWS (1991). Right justify data in appropriate

fields. Do not use ditto marks. If data are repeated on the next

line, use a slanting line in the field from upper left to lower

right, or a vertical line in the center of each column. If data are

not collected, leave the column blank, or enter 9’s for numerical

data. If a band is lost or destroyed, indicate this in the code

column and also in the species column. On any one sheet place

only the records for one band size or the recaptured birds.

When starting a new series of bands, or a new calendar year,

always start a new banding sheet. The sheet is broken into the

following categories:

• Heading material: State code, region code, band size (“R”

for recaptures, entered on a separate sheet), page number (for

each band size), and year of banding or capture.

• Recorder and bander—Place the initials of the recorder

and bander here, and their full names at the bottom of the page

(these are not entered into the data base).

• Code—This column tells if it is a: new banding (N);

recapture (R) (a bird previously banded); unbanded bird (U)

(place 9’s in the band number columns); destroyed band (D);

lost band (L); or a changed band (C) (a band that replaced an

old or worn band—make a note of the old band number).

• Band number—The full, right-aligned number of the first

band on the first line. Thereafter, the final three digits of new

bands only. Do not use dashes in this field to separate prefix;

rather, right align all numbers. On recapture pages, the full

band number should be entered each time.

• Species—An abbreviation of the species name (e.g., Blcap Chick, for Black-capped Chickadee). The abbreviation is

not entered into the data base, but is a check against the errorprone species codes below, such as Barn Swallow (BARS)

and Bank Swallow (BANS).

• Species Code—The four-letter code of species name

(e.g., BCCH). The list of these for North America is in CWS

and USFWS (1991). A Latin American version has not yet

been prepared, but biologists can use the first two letters of the

genus and the first two letters of the species names. This will

suffice for many species.

• Age—The single letter or numeric codes as indicated

above.

• How aged—Use the following codes: A, adult plumage;

B, brood patch; C, cloacal protuberance; E, eye color; F,

feather wear; H, hatching year (first winter) plumage; I, inside

of mouth or any part of bill; J, juvenal plumage; M, molt; P,

plumage in general; S, skull; T, tail length; W, wing length; or

O, other (explain this code in the Notes section). Write the

codes in their order of importance to your age determination.

• Sex—Use M for male, F for female, and U for unknown.

• How sexed—Use the codes as in “how aged.”

• Skull—Record the code above that indicates the percent

of skull pneumatized.

• Cloacal protuberance—Use the code described previously

• Brood patch—Use the code described previously.

• Fat—Use the codes described previously.

• Body molt—Use the codes described previously.

• Flight feather molt—Use the codes described previously.

• Flight feather wear—Use the codes described previously.

• Juvenal plumage—Record the extent of this plumage,

using the codes described previously.

• Wing length—Record to the nearest millimeter.

• Weight—Record to the nearest tenth of a gram.

• Status—Among the most common are: 300, normal and

released; 301, color-banded; and 615, injured and released.

The full list of status codes is in CWS and USFWS (1991).

• Date—Month, day, and year, all in numbers.

• Capture time—Using the 24-hour clock, record to the

nearest 10 minutes, e.g., 6:24 a.m. is 062, 4:48 p.m. is 165, etc.

• Station/location—Record an abbreviation using four letters

for the station’s name and two numbers for the net location; a

total of six columns used.

• Notes—Record any useful additional data, such as:

sequence of color bands, if present; suspected ages or sexes of

birds coded “U”; information on unusual wing lengths; or why

an “other” code was used for how aged. If additional data are

taken, such as an unusual age category, they should be placed

in the “Notes” columns, in order to keep primary data consistent.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 21 Figure 10—An example of a form for recording data from captures.
22 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Sources of Equipment1

Advertisements for supplies and good articles on capture

techniques can be found in the publication “North American

Bird Bander.” Persons doing mist netting or banding should

join their regional Association and receive this, the joint

publication of the Western Bird Banding Association (BBA),

1158 Beechwood St., Camarillo, CA 93010 (Colorado and

west); Eastern BBA, R.D. #2, Box 436A, Hellertown, PA

18055 (Appalachians and east); or the Inland BBA, 81

Woodshire Drive, Ottawa, IA 52501.

Mist Nets

Nets can be purchased in the United States at the following:

Association of Field Ornithologists, c/o Manomet Bird

Observatory, Box 936, Manomet, MA 02345 [telephone

(508) 224-6521]. A wide assortment of nets.

Avinet, P.O. Box 1103, Dryden, NY 13053 [telephone and

FAX: (607) 844-3277]. They have a wide selection of

nets, banding tools, scales, poles, color bands, and other

material.

Eastern Bird-Banding Association, Gale W. Smith, R.D.

#2, Box 131, Kempton, PA 19529. An assortment of nets.

Color Bands

The only source of split-ring plastic color bands for landbirds

that we have found is A.C. Hughes, Ltd., 1 High Street,

Hampton Hill, Middlesex TW12 1NA, England. Avinet (see

above) carries a limited supply of Hughes’ bands.

The best bands for most species are the “Plastic Split

Rings” in solid colors. We have found their five most visible

and separable colors are Red, Yellow, Light Blue, Dark Blue,

and Orange. If more colors are needed, some investigators

have found White reasonably separable from the standard

aluminum band, and the Black and the Dark Green separable

from the Dark Blue. Hughes’ sizes (and their Fish and Wildlife

Service approximate equivalents) are: XF (0), XCS (1), XCL

(1B), XB (1A), and X3 (2).

Optical Device for Skulling

An excellent one is OptiVisor, a binocular magnifier

available in 2.5, 2.75 and 3.5 powers. Available from the

manufacturer Donegan Optical Company Incorporated, P.O.

Box 14308, Lenexa, Kansas 66285-4308, or call them at (913)

492-2500 for a distributor near you.

Wing Rulers

Rigid tempered steel rules with a stop at the end are very

good for measuring wings. Sizes are 15 cm, 30 cm, and 60 cm.

Available from Chris N. Rose, 98 Lopez Rd., Cedar Grove, NJ

07009.

Banding Pliers

The best have holes in jaws to fit standard U.S. band sizes,

with a split pin on top for even band opening. Three pliers are

available: one will open all of band sizes 0, 1, 1B, and 1A;

another for sizes 2 and 3; and one for sizes 3B, 3A, and 4.

These are available from Roger N. MacDonald, 850 Main St.,

Lynnfield, MA 01940, (617) 334-3448.

Scales for Weighing

Electronic scales are widely available for under $300, and

Pesola scales and a spring balance field scale are available

through Avinet (see above). A good general purpose one has

a capacity of 300 g and a readability of 0.1 g. The Ohaus CSeries costs under $200 and Acculab has one under $150.

With a capacity for most birds, Acculab has a pocket balance

with 80 g capacity for under $100. These are available from

many scientific supply houses, such as Markson, P.O. Box

3944, Houston, Texas 77253 (800-528-5114).

Bags for Holding Birds

Washable bags can be made, or cotton mailing bags can be

purchased. An ideal size for most small birds is 6 by 9 inches,

or somewhat larger. U.S. Government agencies can purchase

excellent cotton mailing bags from the General Services

Administration.

Bird Banding Laboratory and Office

All capture work must be done under very strict regulations

and permits. Permit applications in the United States can be

obtained from the Bird Banding Laboratory, U.S. Fish and

Wildlife Service, Laurel, Maryland 20708. In Canada, the

address is Canadian Bird Banding Office, Canadian Wildlife

Service, Environment Canada, Ottawa, Ontario K1A 0H3.

Special permits are also needed from most states and provinces,

and the above offices can supply information on them. Many

Latin American countries also require permits.

The Bird Banding Laboratory and Office provide excellent

support for all activities relating to capture, and permittees

receive bands at no cost. However, they have limited resources

for supporting banding work and cannot honor all requests for

permits. Applicants for permits must show evidence of

qualifications and must have a well-justified need to band.

Permittees are expected to provide accurate and timely reports

of birds banded.

Nest Searches

Nest searches provide the most direct measurement of nest

success in specific habitats. They also allow identification of

important habitat features associated with successful nests

and insight into habitat requirements and species coexistence.

Knowledge of the appropriate cues and techniques for finding

nests allows large numbers to be found, thereby providing

vital information about many species. Nest searches have an

1

The use of trade or firm names in this publication is for reader information

and does not imply endorsement by the U.S. Department of Agriculture of

any product or service.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 23

advantage over constant-effort mist netting, in that the measures

of success are direct and habitat-specific. However, they are

more limited as to the area surveyed and do not measure

individual survivorship. Mist nets sample birds from a larger

area, and the data derived may therefore have wider

applicability, but are not habitat specific.

In this section we describe aids and standardized techniques

for locating and monitoring success of nests, adapted from

Martin and Geupel (in press).

Nest Sites

Nest finding is labor intensive (DeSante and Geupel 1987,

Ricklefs and Bloom 1977), but most observers can improve

their ability to locate nests in a matter of days with training

and practice.

The behavioral observations and clues described below

work effectively for a variety of species. However, our

experience includes a small subset of species and habitats and,

in particular, is largely restricted to forest and shrub habitats.

Other methods may be more effective in other habitats. For

example, cable-dragging (Higgins and others 1969) and ropedragging (Labisky 1957) may be more effective for many

grassland species. In particular, all species, and even some

individuals, differ in nest placement and behaviors near the

nest. The patience and alertness of observers, and their

familiarity with the habitat and behavior of individual species,

are the most important influences on effectively locating nests.

Nest finding can be a frustrating task; patience is an

important asset. It is a good idea to set a goal of finding at least

one nest daily. More than one nest will be found on many days,

but if at least one nest can be consistently found every day, the

numbers of nests over the season will rapidly accumulate.

Methods

The particulars of plot sizes and numbers will vary according

to the purpose of the study or activity, the habitat involved, and

the density of birds. As a general guideline, we recommend

that two study plots be established for each person that searches

for nests. The searchers should work alternating days on these

two plots for the entire nesting season. This provides consistent

monitoring and allows the person to become familiar with the

plot. In general, eight plots, each 40-50 ha, would usually be

necessary to be established in forest habitat to find sufficient

numbers of nests (ca. 20 nests per species) for the range of

species typically found in any given forest, but smaller plots

(ca. 10 ha) can be established in areas with higher densities.

In general, one should try to develop as quickly as possible

a search image for the nests of various species. T. Sherry (pers.

comm.) notes that he routinely finds 25-50 percent of his nests

by constantly scanning appropriate potential nest locations in

the vicinity of an active female.

During Nest Construction

Ideally, nests should be located during construction to

provide the best estimates of nest success. This is also usually

the easiest time to find nests because of the high level of

activity and, in some areas, forests are not leafed out, making

the task of following the female much simpler (T. Sherry,

pers. comm.). We advise biologists to spend the maximum

amount of time early in the season when the finding rate is

maximum. Nest building begins by May in most areas of

North America, although permanent residents and some groundnesting species will begin earlier. Only the female constructs

the nest and incubates the eggs for most small terrestrial birds

(Kendeigh 1952, Silver and others 1985). Exceptions include

woodpeckers, vireos, and wrens. Thus, the most effective way

of finding most nests is by locating and following females,

although males may provide some cues. Some nests in the

shrub layer can be found by random search. Ground nests in

forests are usually the most difficult to find. It is best to watch

the female as she is gathering nesting material without using

binoculars, because when she flies, she can be followed more

easily with the naked eye.

Females tend to be extremely furtive during nest building.

A mated female can be recognized by copulations or by her

movements around the territory unharassed by the male.

Females should always be checked with binoculars, especially

during and after long, direct flights, to determine whether

nesting material is being carried. Many birds will carry very

fine material, not obvious upon casual inspection, such as

spider webbing and hair for lining nests.

Sitting near sources of nesting material (e.g., failed nests,

thistles) or open areas with a good view of the territory can

help detection of nest-building females. Observers should use

different paths across plots to increase the probability of

randomly encountering females near undiscovered nests.

Follow a bird with nesting material from a distance to avoid

disturbance. Do not interrupt a long flight. If the bird disappears

in a patch of vegetation, begin to scan for potential nest sites.

Be patient and wait for another visit by the bird. If the area

where the female disappears is near the nest, the female will

spend time in the area. At the same time, be aware that the

female may move out of the back side of the patch to another

patch that contains the nest.

Some individuals tolerate nearby observers and behave

normally, but most species are very wary of observers. If the

observer is too close to the nest, the bird often will sit on a

perch somewhere near the nest site until the observer leaves.

Eventually the bird will drop the nesting material if the

observer does not move away. Thus, such behavior is an

indication that the observer is too near the nest and should

move quickly away. Obtain a new position at some distance

(ca. 15 m) hidden by vegetation. Observe the female arrive

with nest material and leave without it from the same location

several times. Be aware that a female can skulk into one patch

of vegetation and leave unobserved to move to a different

patch, then return the same way, to give the appearance of

nesting in the first patch. Some species such as MacGillivray’s

Warbler, Hooded Warblers, and Sage Sparrows will walk on

the ground for several meters to approach the nest secretly.

Birds can often be detected by watching for movement of the

vegetation where they are otherwise hidden. Where the

vegetation stops moving is usually the nest site.
24 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Mapping the male’s position as he sings around the territory

can often reveal a center of activity from which the male can

often see the nest (T. Sherry, pers. comm.). The observer then

can scan appropriate nest sites nearby, or at least increase the

chance of catching a glimpse of a wary female.

Once the suspected nest site has been identified, back away

quickly. Verify the status and location a few hours later, being

careful that the female is absent. Do not approach the nest

while the female is watching; disturbance at this early stage

can cause abandonment. After quick verification, the area

should be left and not visited for four days.

During Egg-Laying

This is the most difficult stage for finding nests because the

female may visit the nest only when she lays an egg, and most

species lay one egg per day. The female will sometimes sit on

the nest during egg-laying when weather is particularly harsh.

Nest visitation becomes more frequent with more eggs in the

nest (Kendeigh 1952).

Behavioral cues are useful at this stage. When either parent

gets near the nest, they will look at it. If an egg-laying female

detects a predator in the area, such as an observer following

her, she will sometimes check the nest. Another good cue is a

female staying in an area without actively feeding. She will

often look at the nest site repeatedly, aiding location of the

nest.

Finally, copulatory behavior can be used during both nestbuilding and egg-laying. Copulation often occurs in the same

tree above a nest, on the same branch, or in the next tree.

Examine carefully the area immediately adjacent to copulatory

activity.

During Incubation

The beginning of incubation can be estimated as when

females suddenly “vanish,” and males increase singing. Some

behavioral cues can help locate nests. Females start foraging

faste during the incubation and nestling stages, probably

because their time is more limited. Females that are making

rapid hops, quick short flights, and rapid wing flicks will

probably return to the nest soon. On average, most passerine

females are off the nest for 6-10 minutes and on for 20-30

minutes (e.g., Zerba and Morton 1983).

Observers can find females by alertly moving through the

study plot, but sitting down in a spot for 20-30 minutes is also

useful. A female leaving a nearby nest can thereby be detected.

Females can also be detected by call notes, although species

differ in the types of sounds. Females of many taxa (e.g.,

gnatcatchers, warblers, Emberizine finches) chip or call just

before leaving, or just after leaving, the nest. This behavior

seems to be a communication note to the mate. Females of

other species use other vocal signals, e.g., thrushes give a

chuck or mew sound; tanagers often give a characteristic

sound near the nest or during copulation; and some taxa (e.g.,

Emberizine finches and icterines) have a nest departure call

(McDonald and Greenberg 1991), often answered by the

male. If you detect, follow, but then lose a vocalizing female,

immediately return to the original location where she was

detected, and you may often find her again before she returns

to the nest.

Males can also be of some help. When the female is off the

nest, some males quietly guard the nest or follow her (for

example, the Gray Catbird) (Slack 1976). A quiet male may

indicate presence of a foraging female or a nest nearby. In

many species, especially cavity-nesters, males will feed

incubating females (e.g., Lyon and Montgomerie 1987; Martin

and Geupel, unpubl. data; Silver and others 1985). Males of

some species (e.g., Chestnut-sided Warbler) use singing perches

that are in direct view of the nest. Males sitting on a perch,

looking towards the same spot, may indicate a nest.

Males can sing anywhere in the territory while a female is

incubating, but he can become silent when the female is about

to leave, or has left, the nest (T. Sherry, pers. comm.). When

this occurs, he will often make a long flight over to where the

female is starting to forage (and sometimes will incite her to

leave the nest). Sherry suggests being alert to these flights

because they provide valuable clues to where the nest vicinity

is, and can also help the observer detect females, which are

often difficult to find considering how long they stay motionless

during incubation.

A female foraging off the nest is fairly tolerant of people,

but observers should be inconspicuous. As she returns to the

nest, she is more cautious. This can be used to an observer’s

advantage. First, a relatively long flight after foraging is

probably a return to the nest, and is often along the same route.

Quickly running in her direction for about 25 m may often

result in a resighting, because the disturbance will keep her

from returning to the nest, giving more time to relocate her. If

she is near the nest, but cautious about approaching, she will

bounce between a few branches, and may also forage rapidly.

Eventually, she will start to move down toward the nest several

times and then suddenly fly back up, apparently indecisive. If

the observer is too close to the nest, the bird will continue to

bounce, and will sometimes fly off, only to return within a few

minutes. The observer should then back off and watch. If it is

cold, do not keep her off the nest for too long. If the female has

been followed for more than 30 minutes without results, then

she probably is not on a nest, unless both sexes incubate.

If a female disappears into a tree or shrub, the nest is probably

in or next to it. Memorize the area where the female disappeared

and choose potential nesting sites before approaching. Moving

quietly, begin tapping potential nest shrubs with a stick. Listen

for the flush of the female off the nest. If unsuccessful, the site

can be revisited for careful searches.

In many species, nest site preference seems to be an

evolutionarily conservative trait (Martin 1992). Some birds

greatly prefer their nest to be in or under certain plant species,

or in particular patch types (Martin and Roper 1988, Martin

unpubl. data). Describe and visit nest sites from previous

years to aid new observers in finding nests.

During the Nestling Stage

Finding nests during the nestling period is the easiest,
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 25

because both males and females commonly bring food and

remove fecal sacs. Males are normally the easiest to follow, as

they tend to be less cautious. Nests can usually be found from

a distance using binoculars because of the constant activity of

the parents.

In some species a singing male can indicate the nest

location. He may sing, for example, less and less as he starts

to gather food to carry to the nest, become silent when he is

about to approach the nest, and then resume loud song

immediately after leaving the nest (T. Sherry, pers. comm.).

Additionally, Sherry notes that birds will often become reticent

to go to a nest with a human nearby, so that if a bird becomes

relatively inactive (hopping around, not taking long flights) in

a particular area, or dropping prey, then the nest is probably

nearby. In this case, the observer should either search intensively

in the vicinity, if likely nest spots are nearby, or back away to

give the bird a chance to become calm and go to the nest.

Knowledge of the nesting cycle allows an observer to

anticipate when to start looking for a new nest. Most species

will renest after a nesting failure, although this varies among

and within species (Geupel and DeSante 1990a, Martin and Li

1992). Reconstruction usually begins within 10 days, and the

earlier in the nesting cycle that failure occurred, the farther

apart the nests are likely to be (citations in Martin 1992).

Multi-brooded species may renest in as little as 8 days after

fledging. Sometimes the female will begin nesting while the

male is still tending the fledglings of the previous brood

(Burley 1980).

Nest Monitoring

Each nest found needs to be checked every 3 to 4 days to

determine its status. Careful attention to checking nests is

critical for data quality, because the number of days that nests

have eggs or young is used to calculate daily mortality rates,

the most effective measure of nest success (Mayfield 1961,

1975). Nests should be checked from a distance the day before

expected fledging, and every other day thereafter. A chart

showing nests as they are found and the expected date of

fledging is extremely helpful. If nestlings appear ready to

fledge before the next scheduled visit, then the next visit

should be sooner. Calculations of nest success should terminate

with the last day that young were observed in the nest. Nests

should also be checked more frequently about the time of

hatching, if the length of the incubation period is desired.

With canopy nests, mirrors attached to telescoping aluminum

poles can check contents of nests. These are available from

stores stocking swimming pool supplies, and are commonly

up to 4-5 m. A window-washing pole to 12 m is also available

(Tucker Manufacturing Company, 613 Second Ave. S.E.,

Cedar Rapids, Iowa 52406; 319 363-3591). T. Sherry (pers.

comm.) suggests a convex mirror to allow views from a

variety of angles from the ground. Mounting a small flashlight

next to the mirror can illuminate the nest contents in cloudy or

rainy weather. Often binoculars must be used to view the nest

in the mirror.

Careful and detailed observations should be recorded if a

nest predation event is observed. If the nest appears inactive

from a distance, it should be approached to verify. If the eggs

or young appear to be gone, then check the nest structure and

immediate area, perhaps up to 6-10 m (T. Sherry, pers. comm.)

for evidence. Any evidence (e.g., shell fragments, hole in nest,

nest torn up) should be fastidiously noted. When the young

fledge, they commonly perch on the edge, flattening it, and

leave fecal droppings in (or on the edge of) the nest. These

would indicate possible successful fledging. Observers should

try to verify success by seeing fledglings or by hearing adult

alarm calls or begging calls of the young. Fledglings normally

do not move very far in the first couple of days, although some,

such as Rufous-sided Towhee, may move 100 m in a few

hours. Some species or individuals may carry food up to 24

hours or longer after predation of their nest, including to

unrelated fledglings from neighboring territories.

Nestlings may be banded when the primaries first break

sheath. Banding may provide valuable information on juvenile

survival and dispersal. Always have an assistant with you to

record data, and be careful the nestlings do not jump out as you

try to remove them (use two hands). Avoid banding in the

morning or during cold or wet periods.

Filling Out the Forms

Two types of data sheets are used to record data about the

nest site and nest activity. One set (“Nest Check Form”— fig.

11) is used in the field to record information when nests are

checked. To prevent loss, and serve as a backup and summary

record for each nest, the “Nest Record Form” (fig. 12) should

be maintained at some permanent location. The Record Form

should be updated daily, to prevent data loss.

All observations should be recorded on the Check Form

and transferred to the Record Form, including visits with no

activity. This is particularly critical for canopy or cavity-nests

where nest contents cannot be viewed.

Nest Check Form

Data are collected in the field and are recorded on the

Check Form. One to several nests can be recorded on a single

form. When a new nest is found, its location is carefully noted

at the bottom of the form, and the form may be needed in the

field over the next few visits to relocate the nest. The data

taken should include:

• State or province—The 2-column code for each.

• Region—An 8-column code, designated by the

investigator. Often, the name of the USGS quad, a prominent

landmark, or a nearby town will provide the best code name.

• Station—A 4-letter code for the station that contains the

nest search plot.

• Year.

• Observer’s initials.

• Nest number—A unique, identifying 2-column number

for the nest site. We would expect that at each station, for each

species, no more than 100 nests would be found.
26 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 11—An example of a Nest Check Form for recording in the field the status of nests and information on where the nest is located.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 27

Figure 12—An example of a Nest Record Form that is kept at a permanent location for recording data from the Nest Check Form, as well as

the nest site and characteristics data.
28 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

• Species name—The 4-letter code, based on CWS and

USFWS (1991).

• Date—Month, Day, Year.

• Time—Use the 24-hour clock.

• The activity of an adult if either building (“build.”) or

incubating (“on”), by putting an “X” in the blank.

• The observer should record the contents of the nest

whenever it is approached close enough for careful observation.

If the contents are actually observed, this should be noted by

an “X” in the observed box (“obs.”). If the contents are

counted accurately, the number of eggs, young, or both, are

noted. Age of the nestlings should be estimated when possible

because it can help determine the nest fate by providing

information on length of time that nests were active. Age

estimates should be recorded in Notes.

The form also includes space for a description of one or

more nest sites that the observer finds on this day. The

description should be sufficiently detailed to allow anyone to

locate the nest. Take compass readings from a fixed point

(e.g., a stake or grid point) to establish a reference location.

Nest Record Form

This form is filled out each day upon return from the field,

and should contain the following data:

• Header data

State or Province

Region

Species code

Year

Nest number

The number of attempts at nesting that this record

represents for that pair for that season.

• Nest Checks. These are the data transcribed from the

Check Form, and are the same as for that form.

• Dates and Period

The following dates should be tabulated, as they become

available: date of finding of nest (and contents when

found), date of first egg laid, date of clutch

completion (and number of eggs laid in final clutch),

date of hatching of last egg (and number of nestlings

produced), date of fledging (and the number of

fledglings), or nest failure, and date when last active.

Outcome, a written description of the fate of the nest.

Cause of failure (codes: UN = unknown because not

revisited; FY = fledged, with at least one young seen

leaving or in vicinity of nest; FP = fledged young, as

determined by parents behaving as if dependent

fledgling(s) nearby, FU = Suspected fledging of at

least one young, but uncertain (e.g., no adult behavior

observed); FC = fledged at least one host young with

cowbird parasitism; PO = predation observed; PE =

probable predation, nest empty and intact; PD =

predation, damage to nest structure; AB = nest

abandoned prior to eggs; DE = deserted with egg(s)

or young; CO = failure due to cowbirds; WE = failure

due to weather; HA = failure due to human activities;

and OT = other).

Period = the number of days nest was observed for the

following: days during the egg laying, incubation, and

nestling period.

Success = for each period, based on the following codes:

S = Successful, D = Depredated, N = status unknown/

nest not occupied, U = status Unknown/nest occupied

fate unknown, M = Mortality other than predation, A

= Abandoned, F = Female died, Z = abandoned, no

(zero) eggs laid.

Predation Risk from Monitoring

Locating and monitoring nests have potential to increase

predation (Major 1989, Picozzi 1975, Westmoreland and Best

1985). With proper precautions, such biases can be eliminated

or minimized (Gottfried and Thompson 1978, Willis 1973).

Finding the nest normally creates the most distress to adults

and disturbance to the nest site because subsequent visits are

brief. Some evidence suggests that predation rates are higher

on the first or early visits than subsequent visits (Bart 1977,

but see Bart and Robson 1982).

Therefore, we suggest the following when locating nests:

• Minimize distress calls by adults; never allow them to

continue for more than five minutes;

• Do not approach a nest when any potential nest predators,

particularly visually-oriented predators (e.g., corvids), are

present;

• Minimize disturbance to the area around the nest; and

• Do not get close to nests during nest building, as birds will

abandon if disturbed before egg-laying, particularly during

the early part of a season.

To lower the probability of predation or brood parasitism

from checks, we recommend that you

• Check from as great a distance as possible, using

binoculars to look into the nest or climb up to look from above;

• Approach nests on different paths on subsequent visits,

using paths that are quick, quiet, and that minimize vegetation

disturbance;

• Never leave a dead-end trail to the nest, but continue

walking in a different direction;

• If avian predators are common, check other bushes

without nests, and always assume a predator is watching;

• Be quick and accurate during nest checks and nestling

banding;

• Minimize the number of observers;

• Use a pen or stick to check nests to prevent human scent

from being left on or near a nest.

Vegetation Measurement

We suggest two methods of vegetation measurement: (1)

the nest and the plant containing it; and (2) the nest site and

random points in the plot. The entire plot should be measured

with a series of points, as outlined in the section “Methods of

habitat assessment” below.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 29

The Nest and Nest Plant

Measurement of the vegetation of the nest site is an important

research tool and has some application to monitoring. If you

wish to determine this aspect of habitat, we suggest that you

measure the vegetation as soon as a nesting attempt terminates.

Be careful at the beginning of the season, as an empty nest may

not yet have eggs. Some species or individuals will delay

laying as long as eight days after completing nests. Do not

delay measuring the vegetation, because foliage density around

the nest changes rapidly.

We suggest the following measurements (fig. 12), of the

plant containing the nest. All measurements should be in

centimeters.

• Plant species common name.

• Plant species genus.

• Plant height.

• Nest height.

• Plant “dbh” (diameter at breast height), stem diameter of

the nest substrate, usually measured at 0.25 m above the

ground, because many nests are in substrates less than “breast

height.”

• Nest distance from edge—Distance from edge of plant,

inward to the nest.

• Canopy cover—The canopy cover at chest height should

be measured using a densiometer. This is a measure of the tree

canopy, and should be measured as close to the nest as

possible, but not under the canopy of the nest plant if it is a

shrub.

• Nest distance from center/stem—Distance of the nest

laterally from the main stem.

• Number of support branches—The number of branches

actually supporting the nest.

• Diameter support branches—Average diameter of stems

supporting the nest.

• Nest concealment—Measured by estimating percent of

the nest concealed by foliage cover in a 25-cm circle centered

on the nest from a distance of 1 m from above (overhead

cover), from below, and from the side (side cover) in each of

the four cardinal directions.

• Compass direction—Direction from the nest to the main

stem of the substrate.

• Total percent cover nest substrate—The percent cover of

the plant containing the nest, using the outer margin of the

plant as the boundary. This is most useful in shrubs.

The Nest Site and Random Points

Vegetation in the patch surrounding the nest can provide

information on differences in microhabitat choice among

species.

We recommend using vegetation sampling methods based

on a series of points, as outlined in the section “Methods of

habitat assessment,” below, or those described in Martin and

Roper (1988) with some modifications (obtainable from

Martin). The point method involves measuring habitat features

in the nest patch in circular releves of 11.2-m radius centered

on the nest, smaller than the 25- to 50-m releves for general

habitat assessment, detailed below. In addition, non-use sites

should be sampled with the same protocol at 35 m from the

nest in a direction parallel to the contour of the plot (to stay

within the same microhabitat type when possible). The sampling

plot should be centered on the plant stem nearest to the 35-m

point that is of the same species and size as that used for the

nest. Random plots can also be established in a grid to obtain

a stratified random sample of the vegetation. Comparisons of

random versus nest plots can indicate choice of microhabitat

types. Comparisons of nest versus non-use plots then provide

information on choice of habitat patches within a microhabitat

type. These sampling protocols keep the methods relatively

compatible with other sampling schemes (e.g., James and

Shugart 1970), but also allow tests of hypotheses about the

interactions between choice of nest site and predation risk or

habitats chosen for nesting.

Censusing

The assessment of population size should be an integral

part of any monitoring program. Various methods have been

employed and thoroughly tested (see Ralph and Scott 1981).

Abundance of birds has long been used to measure habitat

suitability but is often retrospective, giving trends without

any possibility of determining causation, and can even be

misleading (van Horne 1983).

It is desirable to use a method that allows the biologist to

census as many points as possible in the time available, thus

gaining as many independent data points as possible. That is,

it is much better statistically to census five points in a 10-day

interval, than to count at one point five times. The farther apart

each of the five points, the more likely the data can be

extrapolated to a larger region.

Below we outline four major methods. Two of these, the

point counts and the spot mapping methods, are the most

common ones used (for definitions see Ralph 1981b). The

point count is probably the best for most surveys and has been

adopted as the standard method for monitoring (Ralph and

others, in press). The methods for both are taken in part from

the excellent book by Koskimies and Vaisanen (1991). In

addition, a strip transect count and an area search method are

also presented. The latter is especially popular with volunteers.

General Considerations

Time of Day

The best time for censusing at most temperate latitudes

during the breeding season is usually between 5 and 9 a.m.

Under most circumstances, no counts should be done after 10

a.m. Exceptions could be in the non-breeding period. It is best

to start within 15 minutes of local sunrise. Examining pilot

data is the best way to determine when detection rates are the

most stable. In general, the period between official sunrise and

the ensuing 3-4 hours is usually relatively stable. For most

species, during the period between dawn (first light) and
30 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

sunrise, the number and rate of birds singing is somewhat

higher than the rest of the morning. For maximum comparability

in detection probabilities for species among points, it will be

best to start at sunrise rather than at first light.

Census Period

Breeding season point counts should be run during the time

of year when the detection rates of the species being studied

are most stable. Within the breeding season, the months of

May, June, and the first week in July are best for counting most

passerines in North America. However, stable counting periods,

when the rate of singing of the various species has stabilized,

are as early as April in the Southeast and Southwest and may

extend later in the boreal zones. In Latin America the breeding

season will be longer, and censuses can profitably be conducted

throughout the year.

Weather

Birds should not be surveyed when rain or wind interfere

with the intensity or audibility of bird sounds, when fog or rain

interfere with visibility, or when cold weather shuts down bird

song activity.

Point Counts

We suggest two levels of point counts. Extensive point

counts are intended for a series of points, placed at a minimum

of 250 m apart, largely on roads or trails over an entire region.

Intensive point counts are placed within a mist net or nest

search plot.

The account below is based on Hilden and others (1991),

and the standards are taken from Ralph and others (in press),

as adopted by the Point Count Workshop of the Monitoring

Group of the Neotropical Migratory Bird Conservation

Program, held in Beltsville, Maryland, November 1991.

Background and Aims

In many countries point counts are the main method in

monitoring the population changes of breeding landbirds.

With the point count method it is possible to study the yearly

changes of bird populations at fixed points, differences in

species composition between habitats, and abundance patterns

of species. The point count method is probably the most

efficient and data-rich method of counting birds. It is the

preferred method in forested habitats or difficult terrain. Point

counts involve an observer standing in one spot and recording

all the birds seen or heard at either a fixed distance, or

unlimited distance. This method can be conducted one or

many times at a given point. The North American Breeding

Bird Survey of the U.S.D.I. Fish and Wildlife Service is such

a method.

The point count method applied to landbirds does not

provide reliable data on waterfowl; however, rails and waders

are counted well. Some landbirds also pose problems as they

are particularly quiet, loud, nocturnal, or flocking. If these

species are of particular interest, the method may be modified

to accommodate them.

Equipment and Time Needed

One should not start point counts without good identification

skills, including a knowledge of the songs and calls of birds.

Details on training for distance estimates are given in Kepler

and Scott (1981). In the tropics, learning all the songs and calls

of all species at all times of the year is difficult in practice. In

many areas it takes an experienced observer 4-8 weeks to

identify 80-90 percent of the species. In temperate zones, this

can often be done in less than 2 weeks.

For the census one needs a map, a pencil, notebook, a watch

that shows seconds, and binoculars. The route and the points

are marked on a survey map and, if necessary, in the field with

plastic tape or streamers to ensure that the same points are

found in the following years. The observer may move from

one point to another by foot or with a vehicle.

The time needed for censusing one point count route is

usually no more than four morning hours, depending on the

distance between the points and the method of travel.

Choosing a Counting Route for Extensive Point

Counts

An extensive point count route should encompass all the

habitats of a region, if possible. In addition, it should include

any mist net or nest searching plots in the region. In choosing

a route and laying out the points for census, use a systematic

rather than random sampling design, either on roads or off

roads. Systematic gridding of points is preferable to the

random placement of points in most cases. Systematic

placement can include placing points at designated distances

along roads. Do not stratify by habitat, unless separate estimates

for a habitat are required. If the goal is to estimate population

trends for an entire management unit, then point counts should

be spaced evenly throughout that unit, or along the road

system in an area, without regard to current habitat

configurations.

Observers should attempt to carry out censuses primarily

on tertiary roads, then secondary roads, and should avoid

wide, primary roads. Off-road censuses should be carried out

on trails, if possible, in major habitats not covered by road

systems. Using roads, travel time can be reduced to as little as

1-2 minutes between sampling points. Under optimal road

conditions, up to 25 5-minute point counts can be conducted

in one morning. In an off-road situation, the number of point

counts one observer can conduct during a morning varies

between 6 and 12. Roadside habitats usually do not sample all

of the available habitats. In this situation, a collection of both

on- and off-road surveys can be created that best fits local

conditions. Although a road modifies the surrounding habitats,

we feel that tertiary road systems (i.e., narrow dirt roads)

allow for birds to be counted in approximately the same

proportions as off-road surveys.

The minimum distance between point counts in wooded

habitats is 250 m. Birds previously recorded at another sampling

point should not be recorded again. In virtually all habitats,

more than 99 percent of individuals are detected within 125 m

of the observer. In open environments, this minimum distance
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 31

should be increased because of the greater detectability of

birds. Along roads, where travel by vehicle is possible, distances

of 500 m or more should be used.

Choosing Points for Intensive Point Counts

The intensive point counts are conducted within a study

plot for mist nets or nest searches. We suggest between 9 and

16 points in a grid of 3 by 3, 3 by 4, or 4 by 4 points. For most

analyses, the birds counted from these points will be combined

into a single mean. Therefore, the distance between points is

less critical than for extensive point counts where each point

is intended to be statistically independent. The points on an

intensive census grid should be adjusted to fit within the

netting array or nest search plot so as to fully census the area.

It is very important not to include areas much beyond the array

or plot boundaries. These are covered by the extensive point

count censuses. For example, a census grid of nine points, 100

m apart, would cover 4 ha. Allowing for an effective radius of

censusing of perhaps 50 m outside this grid, the area covered

expands to about 9 ha. A grid of 12 points 150 m apart would

have an effective area of about 22 ha. Thus a census grid

should have points that are between 75 and 150 m apart,

depending upon the area to be covered and the number of

points to be included. Under most circumstances 9-12 points

should be more than adequate.

Field Work

The censuser should approach the point with as little

disturbance to the birds as possible. Counts should begin

immediately when the observer reaches the census point.

Time spent at each count point should be 5 minutes if travel

time between counting points is less than 15 minutes (for

greater efficiency) and 10 minutes if travel time is greater than

15 minutes. If a survey is primarily for inventory and few

points will be surveyed, then 10 minutes is appropriate. Data

should be separated into those individuals seen or heard

during the first 3 minutes (for comparison with Breeding Bird

Surveys) and those additional individuals heard in the remaining

2 and 8 minutes.

The details of each point are recorded: the reference number,

name of the point, date, and the time. The species are written

down in the order they are observed. For each species, the

number of individuals is recorded separately for those within

a circle of 50 m around the censuser and for all those outside

the circle, out to an unlimited distance. In noisy environments,

dense foliage, or in tropical forests, observers have found that

25 m was preferred. The distance is that at which the individual

was first observed. For birds near the 50-m border, the

category may be confirmed by measuring paces to the border

when the counting is over. If a bird flees when the censuser

arrives at the point, the bird should be included according to

its take-off place. Birds that were detected flying over the

point, rather than detected from within the vegetation, should

be recorded separately.

Estimating distances requires experience, so a new censuser

should measure the length of steps in different terrain, and

then check the distance to several singing birds in order to

make the estimating of distances routine. Estimating may be

eased by either natural or artificial landmarks.

If there are several males of the same species around a

point, one may sketch in the margin the directions and distances

of each singing male with an arrow to ensure that they are not

confused. Juvenile birds or birds that fledged during the

current breeding season should be recorded separately.

A bird flushed within 50 m of a point’s center as an

observer approaches or leaves a point should be counted as

being at the point if no other individual is seen during the count

period. It is advisable that this be recorded separately.

If a flock is encountered during a census period, it may be

followed after the end of the period to determine its composition

and size. An observer should follow such a flock for no more

than 10 minutes. This is especially useful during the winter. A

bird giving an unknown song or call may be tracked down

after the count period for confirmation of its identity.

No attracting devices or records should be used, except in

counts for specialized groups of birds.

Filling in the Forms

The data taken at point counts are of two types, the location

information and the census data. The location data are contained

in the first three lines of the “Location and Vegetation Form”

(fig. 15, described below) and contain information about each

census point. We also suggest that the vegetation data be taken

(see Habitat Assessment, below). We suggest two types of

census data forms. One involves mapping and the other direct

recording.

Mapping Point Counts—This method of taking data involves

the recorder placing on a map (fig. 13) the location of each bird

detected (D. Welsh, pers. comm.). We suggest that species

codes be used on the map, with a single letter for the most

common species, and the full 4-letter code for other species.

The birds’ activities can be recorded by the various mapping

symbols given in figure 13. The circle on the map can be the

50-m radius, enabling the observer to keep track of individuals

easily. The orientation of the observer (“DIR”) should be

entered on each form by placing the compass direction in the

box at the top. Separate time periods are easily kept by using

different colored pencils, e.g., birds seen in the first 3 minutes

in black, and those seen within 3-5 minutes in red.

The data are then transcribed onto the Point Count Data

Form (fig. 14), described below.

Direct Recording Point Counts—This method involves a

single-step process of the observer recording the observations

directly on the Point Count Data Form (fig. 14). Many observers

do not think that it is necessary to map the location of birds in

order to keep track of individuals. Using this method, an

observer tallies in pencil each individual detected by placing

a “tick” mark (a single line), or another code, in the appropriate

column. Codes, for example, can be used to separate out

singing vs. visual-only birds (S and V) and age categories.

When field work is over, the actual number in each distance

and time category can be written in ink for data entry.
32 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 13—A recording form for mapping the location of birds during point counts with some mapping symbols. Taken from Welsh

(pers. comm.)
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 33

Figure 14—An example of a data form for recording point count

data. Birds are recorded separately within or outside a 50-m circle

around the observer, and in the first three minutes or later in the

census. The data are recorded as “tick” marks in each box; then later

the actual numbers of birds, as derived from the data, are summarized

and recorded.
34 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

Figure 15—The Location and Vegetation Form. The upper portion should be filled out at all study locations. The bottom portion quantifies

vegetation.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 35

It uses a method that, while quantitative, mimics the method

that a birder would use while searching for birds in a given

area. Essentially this is a series of three 20-minute point

counts in which the observer can move around in a somewhat

restricted area. In this way unfamiliar calls can be tracked

down and quiet birds can be found.

Preparation

The observer should be reasonably familiar with most (if

not all) bird species likely to be encountered at the plot. This

method allows the observer to track down unfamiliar birds,

but walking the plot before a survey with a person familiar

with the birds allows the observer to be more efficient.

Choosing a Plot

The plot should allow relatively easy detection and

identification of birds (by sight or calls) and allow the observer

to move about freely. The plot should be sufficiently large to

provide three separate search areas (or plots), each about 3 ha

in forest or dense woodland, but larger areas of 10 ha or more

can be used in more open habitats. In very dense forest, smaller

areas of 1-2 ha can be used. The search areas can have adjoining

boundaries or can be in completely separate regions of the plot.

More than three search areas can be established within a plot,

but the same search areas must be used on each visit.

Time of Day

Because of the intensive nature of this method, it can be

carried out longer into the morning than other methods. However,

it should continue no later than five hours after dawn.

Field Work

Walk throughout the plot for exactly 20 minutes in each

search area, stopping or moving to investigate sightings or calls

when appropriate. Record numbers of birds of each species seen,

heard, or both seen and heard in the search area during this time.

Record birds outside the search area separately, but concentrate

on finding as many birds as possible within the plot. The

observer may find it easier to tape record observations and then

transfer results onto paper soon after the survey. An accompanying

person can serve as a recorder. A single survey is completed after

at least three areas have been searched at a plot.

Filling in the Form

A standard form is suggested, listing the species found and

a running tally of the number of birds, both on and off the area.

These tallies can be totaled on the right of each area for each

species.

Spot Mapping

Background and Aims

The mapping method is based on the territorial behavior of

birds. By marking the locations of observed birds on a detailed

map during several visits within a breeding season, it is

possible to count the number of territories in an area and

estimate the density of birds. Spot mapping is not usually used

The specific data suggested are as follows:

• State or province—The 2-column code for each.

• Region—An 8-column code, designated by the

investigator. Often, the name of the USGS quad, a prominent

landmark, or a nearby town will provide the best code name.

• Station—For intensive point counts, we suggest a 4-letter

code, the same as that used for the mist net array or nest search

plot. For extensive point counts, we suggest a code relating to

the general area or road. In general, we expect that a given

station will have no more than 50 points.

• Month, day, and year.

• Observer

• Visit number—Indicate how many visits this year will

have been made to these points at the end of this day’s census.

• Point Number—The 2-column census point number.

• Time—Using the 24-hour clock.

• Species—The 4-letter species code.

• Tally of individuals—This is a series of five fields. The

major subdivisions are those birds detected at less than, and

more than, 50 m, and those birds flying over, but not landing

within detection of, the observer. Within the two distance

categories, observers can separate out those detected in the

first three minutes, and the next two minutes. Observers

wishing to separate out behavioral, age, or sex categories can

note them with an appropriate letter code. Otherwise, “tick”

marks (e.g., 3 = ///) can be used.

Repeating the Count

In general, a station should be sampled only once each

season. Counts can be repeated if the goal is good estimates of

the community at certain, specific points, such as a small area

of rare wetland habitat.

The timing of the census of each route should be kept

constant from year to year; it should not differ by more than

seven days from the date of the first count. If the phenology of

the spring differs, then the date can be changed. The start of the

count should not differ by more than 30 minutes from that of

the first year. If possible, the same observer should census the

route every year.

Strip Transects

Strip transects are very similar to point counts, but the

observer records all birds seen or heard while traversing each

section of a trail. Each section is then the unit of measurement,

and can be 100 m or 250 m long. This method is best used in

very open terrain where the observer can devote his or her full

attention to the birds, and not worry about footing.

In this method the observer should attempt to cover a given

amount of trail in a fixed amount of time, e.g., 100 m in ten

minutes.

Area Search

Background and Aims

The area search method has been adopted for a nation-wide

survey, the Australian Bird Count (Ambrose 1989), and was

chosen over several others because of its appeal to volunteers.
36 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

as a general method for broad-scale monitoring of breeding

landbirds, because it requires more time and field work than

single-visit point counts and line transects. However, the

method should be applied when fairly precise pair numbers

and densities as well as the distribution of territories in small

study areas or patchy habitats are to be studied. The standard

mapping method is less suitable for species that live in

colonies or loose groups, or species with large or no territories.

In general, one or two observers make repeated visits (a

minimum of 8) to specific plots during the breeding season.

Some habitat analysis is also required. Standard methodology

as described by Robbins (I.B.C.C. 1970) is also used by The

Cornell Laboratory of Ornithology’s (CLO) resident bird

counts. The latter program, known as the “Breeding Bird

Census” (BBC), is a continent-wide program that welcomes

contributors and publishes results of North American plots

annually in the Journal of Field Ornithology. The CLO also

encourages “Winter Bird Population Studies” (WBPS) on the

same plot. For more information, write to: CLO, Resident

Bird Counts, 159 Sapsucker Woods Road, Ithaca, NY 14850;

Telephone (607) 254-2441. The basics of the method are

contained in Koskimies and Vaisanen (1991). We present

here enough information for a biologist to evaluate the

technique. The methods of especially data recording,

evaluation, and analysis are complex and detailed.

Equipment and Time Needed

One needs 30-40 copies of a very detailed map (preferably

1:2000, or, in open areas, 1:3000 may be acceptable), a

compass, and flagging for marking the area.

The time needed depends on the size and terrain of the

census area as well as on bird density, with higher densities

requiring the mapping of more individuals. Usually about 10-

30 hectares in a wooded area or 50-100 ha in an open area may

be counted in one morning. Thus, in forest it takes 10 mornings

to census 30 ha by the ordinary 10-visit version of the mapping

method (about 50-60 hours of field work). In addition, it can

take as many as 40 hours (4 hours per census morning) to

prepare the species maps, and about 5-10 hours to analyze

them. In total, one could spend as many as 100 hours censusing

30 ha of forest during one breeding season. Marking the 50-

by 50-m plot in the field takes about 25 hours before the first

census season.

Drawing a Map and Marking the Area

The census area should be as round or square as possible in

order to minimize border length, because territories along

edges are difficult to analyze. After the area has been chosen,

a detailed map (known as a visit map) is drawn of it before the

first census. The recommended scale for the map is 1:2000. A

survey map (1:20,000) and field experience should be used in

drawing. Boundaries of the area and landmarks such as edges

between habitats, streams, roads, paths, buildings, big rocks,

and trees are marked on the map. There should be enough

landmarks on the map so that the observer can locate the

positions of birds accurately on the map. One copy of the map

is needed for each visit, and enough copies should be reserved

for making the species maps. If there are only a few natural

landmarks, a grid of 50-m squares can be established with the

corners of the squares marked with plastic flagging with

coordinates written on them.

Census Period and Number of Visits

Because of differences in phenology of arrival and nesting,

the visits should cover a period long enough to ensure that

each species is easily observable on at least three visits. There

should be 10 visits in a standard mapping of forest birds. If the

bird density is very high and the nesting period of the community

is long, 12 visits are recommended. The visits ought to be

evenly distributed over the census period. Fewer visits can

suffice in open habitats, where bird densities are usually lower

than in forests, or where the season is short (e.g., tundra or

alpine grasslands).

Time of Day

The main census time is 5 a.m. to 10 a.m. when the birds sing

most actively. After a very cold night counting can be delayed.

During very warm weather it should be prolonged because of

the lower activity of birds. Two visits should be made in the

evening: the first in the beginning of the census period (especially

for counting thrushes), the second about two or three weeks

later (especially for counting nocturnal singers). If there are

several nocturnal or dusk-active species breeding in the area,

these two censuses should be added to the ordinary program of

10 morning visits, for a total of 12. In northern temperate zones,

owls, woodpeckers, and crossbills breed early and should be

censused by extra visits in March and April.

Field Work

A clean map is reserved for each visit. Each visit should

cover the area as evenly as possible, and no place should be

farther from the route than 25 m (dense vegetation or high

density of birds), 50 m (sparse vegetation, few birds) or 100 m

(open habitats). The route you follow through the plot should

be on a grid twice the size of the distances above, for example,

50 m in dense habitat. Successive visits should be started at

different points, especially if you think that a part of the area

is getting attention at the expense of the rest. Simultaneous

observations of two individuals of the same species singing or

seen must always be recorded carefully so that birds can still

be separated from their neighbors after they have moved,

which frequently happens during a census visit.

Even while you are busy censusing, you should not walk

very slowly, because then, for example, a bird uttering alarm

calls may attract other birds to congregate nearby. Therefore,

walk with moderate speed and record the birds all the time.

Stop frequently to “hunt” for simultaneous observations of

different individuals of the same species, to listen, and to mark

the birds on the map. If you are not sure whether there is only

one bird or two, you can return to the area censused already to

make sure which is the case. In open areas it is often useful to

search for the birds with binoculars.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 37

The ordinary speed of censusing is 10-12 min/ha, or 5-6 ha/

hour when the bird density is about 300-500 pairs/km2. If the

density is very high, the censusing speed slows down to 3-4

ha/hour (15-20 min/ha). When the density is very low or only

some of the species are being censused early in the spring, one

may walk a little more rapidly; however, at least eight minutes

should be allowed for each hectare.

There are many advantages to slow and thorough censusing:

(1) one can gather simultaneous observations effectively by

following the movements of individual birds in different parts

of their territories; (2) one can pay special attention to species

difficult to detect; and (3) one can search for nests and check

those found earlier. All observations are marked on a map using

standard codes which are given in the detailed instructions in

Koskimies and Vaisanen (1991). All observations are transferred

from the field maps to exactly the same locations on the speciesspecific maps. There should be a separate map for each species.

Other Considerations

Color Banding Individuals

Observer variability can be a great problem in many of the

censusing schemes described above (Verner and Milne 1989).

The color banding of individuals allows field identification

and survival estimates of individuals without recapture and

can greatly enhance spot mapping efficiency, the ability to

find nests, and basic life history information. Furthermore, it

allows more detailed observation of behavior including

breeding biology, survival, and foraging ecology. Colorbanding and other auxiliary markers must be authorized by the

Bird Banding Laboratory.

Methods of Habitat Assessment

Many applications of habitat analysis are in the literature

(e.g., Verner and others 1986). It is not our intention to outline

what analyses can be done, but to emphasize that, at the least,

vegetation information should be taken at each of the stations.

Objectives of vegetation assessment can be many, but among

the most common are to relate, in one way or another, the

changes in bird composition and abundance to differences in

vegetation. These vegetation changes can be either changes

over time, or differences between habitats. Two adequate, but

relatively time-consuming, methods of habitat assessment are

those of James and Shugart (1970), used primarily in forested

habitats, and Noon (1981). An excellent and rapid method

which could be substituted for the method of estimating stand

characteristics below is that of MacArthur and MacArthur

(1961) which involves estimating foliage density. The technique

uses horizonal measurements to estimate density by relating

the percentage of a board that is obscured by foliage. This

method has been tested and found reliable by Conner and

O’Halloran (1986) and Conner (1990).

If managers wish to characterize the interactions of birds

and habitat in a region, then some kind of habitat classification

with sampling in proportion to the relative abundance of

habitat in that region is the optimal design. This sampling,

stratified by habitat, should be done with the guidance of a

biometrician.

We present two alternatives here. One is that used to type

vegetation into broad habitat classifications, as the Constant

Efforts Site vegetation assessment technique does, or a more

specific one, involving estimation of stand characteristics. We

strongly suggest the latter method, as being more useful for

monitoring.

Broad Habitat Classification

Objectives—This method provides brief, overall

classification of vegetation and a map that allows other

investigators to evaluate the habitat of your station. These data

should be the minimum collected on vegetation at any

monitoring station. If more detailed vegetation data are

collected, then this level need not be taken.

Considerations—The information collected should provide

enough data to determine the vegetation types. The method

will not provide quantitative information for correlative

analyses and ordinations.

Procedures—It is best to make a map of the main areas of

habitat within the station on a yearly basis, sometime in June.

Prepare it on the scale of approximately 1:2000 (approximately

1 foot to a half mile [1 m to 2 km]). Include the major

vegetation types, extending it at least 100 meters beyond the

outermost net or capture location. Indicate on the map: trails,

roads, ditches, streams, marshy areas, net or census points,

open water, and broad habitat boundaries. Also on the map

should be a reference point identifiable on a U.S. Geological

Survey topographic map or equivalent.

Use colored lines to separate habitat types, and record the

following on a form:

• Habitat type: broad category such as forest, brush, marsh,

field, etc.

• Shrubby vegetation: list the shrub species comprising

more than 10 percent cover in order of their percent cover in

each type.

• Trees: list the tree species comprising more than 10

percent cover in order of their percent cover in each type.

• Height of vegetation: record the approximate average

height of the canopy of forest or brush to the nearest meter.

• Ground layer: describe the vegetation of the ground layer

in terms of the common name of the main species groups

present, e.g., ungrazed grass, bare ground with nettle, rushes,

etc.

• For wet areas: indicate the water depth in June, or for

temporary ponds and streams, give the period that water was

present.

Estimation of Stand Characteristics

Objectives—This is a system for assessing habitat

characteristics in an efficient and timely fashion at vertebrate
38 USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993.

monitoring stations. It is taken from a method developed by

Bruce Bingham and C.J. Ralph.

Considerations—The information collected will provide

enough data to determine the vegetation formation, association,

and major structural characteristics. The types of data are

those which have some logical relationship with bird

requirements for feeding or nesting. The method provides

enough quantitative information for correlative analyses and

ordinations. It is flexible so that it can be applied to any

vegetation formation, including deserts, grasslands, and forests.

Procedures—Establish a releve, a variable radius plot

centered, for example, on a census point. The size of the plot

will vary, depending on the homogeneity of the vegetation

composition, and the density of the vegetation. Generally, this

would be a radius of less than 50 m, and often about 25 m.

Walk around the point for no more than 5-10 minutes, or until

you stop adding new species, whichever is less. Once the

search is stopped, the distance from the stopping point, or the

outermost boundary of vegetation that the observer can see

from the point center, is the radius of the plot and is treated as

a boundary for estimating relative abundance.

If the point has more than one vegetation type, then establish

two releves. An example would be along a road, with a clear

cut on one side, and a mature forest on the other. No more than

two releves should be established at a point.

Determine the number of major layers of vegetation within

your releve by their dominant growth form: tree layer (T),

shrub layer (S), herb (H), and the ground cover (moss and

lichen) layer (G).

In a forest with all layers, the tree layer is the uppermost

stratum, dominated by mature trees. It may be a single layer,

or consist of two or more sublayers recognizable by changes

in density and canopy status (see below). The shrub layer is

dominated by shrubs or small trees. The herb layer is dominated

by low-growing plants, typically nonwoody, although seedlings

and other reproduction of trees and shrubs may be present. The

ground layer is dominated by such plants as mosses, lichens,

and liverworts. Bare ground and litter are ignored for this

classification scheme.

We recommend the use of the following height classes for

each stratum, if they are appropriate, because they can make

the process less subjective. For example, the tree layer could

include any plants taller than 5 m (In shorter forests, this might

be lowered to 3 or 4 m, as appropriate). The shrub layer could

then be established at between 50 cm to 5 m. The herb layer

includes any plants less than 50 cm tall. The moss/lichen layer

refers to a ground-appressed, low carpet, less than 10 cm high.

For purposes of bird-habitat association, only species of

trees and shrubs need be identified and recorded in the data

below. For other plants, a common name such as FERN,

HERB, MOSS, or LICH will suffice for most purposes. Plant

ecologists have used some species in the herb or ground cover

layers as indicative of a particular plant association. In this

case, the species should be recorded.

Determine the average height of each major layer present

and dominant plant species. It is desirable to have additional

information on structure, such as the maximum and minimum

d.b.h. of canopy trees and total percent cover value of each

layer.

Determine relative importance of species in each layer

present. Importance can be expressed as either abundance or

cover. Percent cover is probably the most common, and we

suggest using it.

Below is a detailed description of the data we suggest be

taken and recorded as on figure 15. The data are separated into

Location Data and Vegetation Data.

Location Data:

State or province—The 2-column code for each.

Region—An 8-column code, designated by the investigator.

Often, the name of the USGS quad, a prominent landmark,

or a nearby town will provide the best code name.

Site data.

• Latitude and longitude—For each point, latitude and

longitude should be recorded as the southeast corner of

 the 1-minute block containing the point, as determined from

accurate topographic maps. For example, 40°

53’20"N,

124°08’45"W would be reduced to 4053-12408.

• Elevation to nearest meter, by using an altimeter.

• Aspect of the slope (the compass direction the observer

faces when looking down hill) to the nearest degree, with a

compass.

• Percent slope, with a clinometer.

• Presence (+) or absence (-) of water within the releve.

• Plot radius, distance from the center to the edge of the

releve.

Vegetation Data:

Vegetation Structure and Composition

• Total cover—Estimate the cover of each of the four

layers, according to the established scale such as BraunBlanquet (Mueller-Dombois and Ellenberg 1974) or

Daubenmire (1968). We recommend the Braun-Blanquet Cover

Abundance Scale, which is: 5 = >75 percent cover; 4

= 50-75 percent cover; 3 = 25-50 percent cover; 2 = 5-25

percent cover; 1 = numerous, but less than 5 percent cover, or

scattered, with cover up to 5 percent; + = few, with small

cover; and r = rare, solitary, with small cover.

• Height—Record to the nearest decimeter (0.1 m) the

average height of the lower and upper bounds of each

of the four layers.

• Species—Record the species by a 4-letter code (using

the first two letters of the genus and the first two of the species)

with the greatest cover (foliage or crown cover) within each

layer’s boundary.

• D.b.h.—For each layer where trees are present, record

the diameter at breast height to the nearest centimeter of the

largest tree in the layer and also for the smallest trees.

• Species—Record the species of trees used for minimum

and maximum d.b.h. measurements

• Number of sublayers—Sublayers are useful to give the

plant ecologist a quick overview of the structure of a layer, and
USDA Forest Service Gen. Tech. Rep. PSW-GTR-144-www. 1993. 39

are primarily relevant to the tree layer, although sometimes

seen in the shrub layer. Record the number of sublayers visible

in each primary layer. Record “1” if the layer is uniform and

“2” or more if more than a single layer is divided into

sublayers. In a primary layer, sublayers are sometimes obvious

because of one or more species with shorter heights than the

dominant species of the upper portion of the layer. In addition,

sublayers are sometimes formed by cohorts of one or more

size classes, possibly related to some event. For example, the

tallest trees in a stand may form an open (low-density) layer

of emergent individuals. Beneath that may be a denser layer of

trees forming the main body of the tree layer. Below this

denser layer may be another open or closed layer of trees that

are intermediate to the main body of the canopy. This layer

may consist of shade tolerant species or reproduction. Biologists

should be cautioned that extreme precision is not required for

this estimation, and unless sublayers are very obvious, they

should not be recorded.

Species composition data

• Sublayer—For layers where sublayers have been

recognized, record the sublayers with a letter designating the

primary layer, followed by a number (e.g., T1, T2, T3, S1,

etc.), indicating the sublayers by decreasing heights.

• Cover or cover abundance value, as above, using the

Braun-Blanquet method—Because of the difficulty of

determining crown covers independently for species of trees

in a canopy, sometimes basal area cover of stems (trunks) has

been used for tree layer species and crown cover for species in

other layers. We suggest the cover abundance value for

consistency.

• Species—Record the species’ name for each plant

species making up at least 10 percent of the cover.

Additional/optional information can be integrated into

the method, if desired:

Snags: list layers with snags present; separate into those

with a d.b.h. of less than 10 cm and those larger.

Logs: list those less than 10 cm diameter at large end by

abundance or cover class, and those greater than 10 cm.

Comments—This type of vegetation assessment is limited

by the size of the plot and the amount of estimation required.

For example, a plot of even 50 m in radius obviously does not

include all vegetation inhabited by birds heard or seen from a

census point. This would require a plot of 200 m or more

radius. However, most birds detected at a point are within 100

m, and many are within 50 m. Further, time limitations would

require much more time spent monitoring vegetation than

spent counting birds.

When observers are required to estimate, a substantial

amount of error is introduced. What effect the degree of

observer error likely with estimation would have on conclusions

should be established. The principal source of error in this

method of vegetation assessment is the determination of

percent cover and heights. Intensive training can moderate

this source of error, enabling each vegetation assessment to be

placed into at least broad categories or plant associations.

Weather Monitoring

We suggest the following weather measurements three

times per day, at the beginning, midpoint, and end of the

census or capture period in a day. The maximum high and low

temperature from each 24-hr period should also be recorded.

Data from nearby weather stations may also be used. However,

some measurements from the immediate area are more valuable.

• Weather—Use RAIN, DRIZZLE, SLEET, SNOW, or

FOG. (If it is raining and foggy, put down RAIN.) If the above

conditions do not apply, use: OVC (overcast), more than 90

percent cloud cover over entire sky; BRK (broken), 50-90

percent cloud cover; SCT (scattered), 10-50 percent cloud

cover; or CLR (clear), less than 10 percent cloud cover.

• Wind Direction—Using an anemometer, stand facing

into the wind and record the direction to the nearest 1/16th of

the compass, i.e., N, NNE, NE, etc. If winds are variable,

record predominate direction.

• Wind Speed—Record both the average and maximum

speeds.

• Visibility—Estimate visibility to the nearest 250 m if less

than 2 km, otherwise to the nearest kilometer.

• Barometric pressure.

• Temperature—Dry bulb temperature. Record to the nearest

1 degree centigrade.

• Relative humidity.

• Rain—Record from a rain gauge to nearest 0.1 mm.

We suggest a continuous strip chart recorder to measure

temperature and a somewhat permanent station to measure

rainfall.

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