United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Paper

RMRS-RP-21

February 2000

Optimum Timeframes for

Detecting Songbird

Vocalizations in the Black Hills

Todd R. Mills

Mark A. Rumble

Lester D. Flake
Mills, Todd R.; Rumble, Mark A.; Flake, Lester D. 2000. Optimum timeframes for detecting songbird

vocalizations in the Black Hills. Res. Pap. RMRS-RP-21. Ogden, UT: U.S. Department of Agriculture,

Forest Service, Rocky Mountain Research Station. 6 p.

Birds are indicators of vegetation structure and ecological conditions. The singing activity of birds

declines during late-morning periods, which can affect estimates of abundance and conclusions regarding

vegetative conditions indexed by birds. Therefore, it is important to quantify periods of bird activity so

biologists can plan studies. We determined hourly detections from singing males of 22 nongame bird

species in ponderosa pine, quaking aspen, and grassland vegetation types in the Black Hills of South

Dakota. Detections of 12 of 22 species differed among 1-hr intervals after sunrise. Detections of yellowrumped warblers, Townsend’s solitaires, red-breasted nuthatches, western tanagers, and American

robins decreased on count-episodes more than 4 hrs after sunrise. Detections of dusky flycatchers

declined on count-episodes more than 3 hrs after sunrise and detections of black-capped chickadees

were greatest during the first hour after sunrise and declined afterward. Detections of many other species

from songs or calls decreased on count-episodes more than 5 hrs after sunrise. We recommend that bird

counts in the Black Hills be completed within 4 hrs after sunrise so estimates of bird abundance are not

affected by reduced singing among males.

Keywords: forest birds, detection, hourly variation, methods

The Authors __________________________________________

Todd R. Mills is with the Department of Wildlife and Fisheries Sciences, South Dakota State

University, Brookings, SD 57007.

Mark A. Rumble is with the USDA Forest Service, Center for Great Plains Ecosystem Research,

Rocky Mountain Research Station, 501 East St. Joe, Rapid City, SD 57701.

Lester D. Flake is with the Department of Wildlife and Fisheries Sciences, South Dakota State

University, Brookings, SD 57007.

Abstract _____________________________________________

Cover art by Joyce VanDeWater
Federal Recycling Program Printed on Recycled Paper

Contents__________________________________

Page

Study Area ................................................................................................ 1

Methods .................................................................................................... 1

Sampling Design .................................................................................... 1

Bird Counts ............................................................................................ 1

Data Analysis ......................................................................................... 2

Results ...................................................................................................... 2

Birds Associated With Ponderosa Pine ................................................. 2

Birds Associated With Quaking Aspen .................................................. 2

Birds Associated With Grasslands ......................................................... 2

Other Forest Birds ................................................................................. 4

Discussion ................................................................................................. 4

Management Implications ......................................................................... 5

Literature Cited.......................................................................................... 5

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USDA Forest Service Res. Pap. RMRS-RP-21. 2000 1

Bird populations serve as indicators of environmental conditions because they have specific requirements (Cody 1985, Martin and Finch 1995) and

therefore can be used for monitoring effects of land

management activities on other wildlife. However,

several factors cause estimates of bird abundance to

vary and may lead to inaccurate estimates of populations (Ralph et al. 1993). In most studies, more than

90% of birds are identified by their song or call (Carey

et al. 1990). Therefore, if the frequency that birds sing

during the day varies, estimates of bird abundance

may be biased if data are collected during periods of

low singing frequency (Palmeirim and Rabaca 1994,

Robbins 1981). Current recommendations are that

birds should be counted within 4-5 hrs after sunrise

(Ralph et al. 1993, Manuwal and Carey 1991). However, these recommendations were made based on

data from different regions and vegetation types than

occur in the Black Hills. We conducted this study to

determine variation in singing behavior of birds in the

Black Hills so managers could effectively devise studies that accurately estimate bird populations.

Study Area _____________________

This study was conducted on the Nemo, Harney,

Pactola and Spearfish Ranger Districts in the north/

central region of the Black Hills National Forest,

South Dakota, from 1992 to 1994. The Black Hills

encompasses approximately 15,540 km2 in west-central

South Dakota and east-central Wyoming. Elevation

ranges from 1450 m to 1770 m. Major forest vegetation

types include ponderosa pine (Pinus ponderosa)/snowberry (Symphoricarpos albus), ponderosa pine/

kinnikinnick (Arctostaphylos uva-ursi), ponderosa

pine/juniper (Juniperus communis) and ponderosa

pine/bur oak (Quercus macrocarpa) (Hoffman and

Alexander 1987). Quaking aspen (Populus tremuloides)

occurs as climax forest vegetation with paper birch

(Betula papyrifera) in both uplands and in drainages.

Wet (subirrigated) and dry (upland) grasslands or

meadows are interspersed throughout the area. Most

of the ponderosa pine forest has been logged in the past.

Optimum Timeframes for

Detecting Songbird Vocalizations

in the Black Hills

Todd R. Mills

Mark A. Rumble

Lester D. Flake

Methods _______________________

Sampling Design

In 1992, we selected 87, 4- to 32-ha land units

(hereafter referred to as stands) managed by the

Black Hills National Forest based on vegetation type

and structural stage criteria (Buttery and Gillam

1983, Mills et al. 1996) that represented range of

vegetation types and structural stages of forest

conditions in the Black Hills. Prospective stands

were identified using 1:24,000 aerial photographs

and stand inventory data (Black Hills National Forest RIS Data, Custer, SD). Forty-eight stands were

ponderosa pine (7 stands in each of 6 structural

stages and 6 stands in another), 28 stands were

quaking aspen (7 stands in each of 4 structural stages)

and 11 stands were grasslands. Quaking aspen

stands included both uplands and drainages. Vegetation characteristics used for selecting stands were

verified in the field prior to counting birds. In 1994, we

selected 91 new stands using the same vegetation type

and structural stage criteria. Forty-nine of these stands

were in ponderosa pine, 28 in quaking aspen and 14 in

grasslands.

We marked 3 sites in each stand selected in 1992

and 2 sites in each stand selected in 1994. In ponderosa pine, upland quaking aspen, and dry meadow

vegetation types, sites were 150-200 m apart and

150 m from the edge of the vegetation type or structural stage (Manuwal and Carey 1991, Robbins et al.

1989). In drainage quaking aspen and wet meadows,

sites were located >150 m apart, in the center of the

drainage; these vegetation types are too narrow to

locate sites 150 m from an edge.

Bird Counts

At each site, we recorded detections of birds during

the breeding season by using a variable circular plot

(Reynolds et al. 1980) on 2 consecutive days (one

sample session) during 2 sample sessions each year.

Each day we counted birds at a site is hereafter
2 USDA Forest Service Res. Pap. RMRS-RP-21. 2000

referred to as a count-episode. Bird counts extended

from 15 May to 7 July so early and late nesting

species were included (Ralph et al. 1993, Robbins

et al. 1989). Count-episodes began at sunrise and

were completed by 11:45 AM. All observers were

experienced and trained for visual and audio identification of birds in the Black Hills and practiced estimating distances before and during bird counts (Verner

and Milne 1989). We used 3 observers and each observer visited different sites each sample session. We

also reversed the order of daily count-episodes to sites

(Gotfryd and Hansell 1985, Verner and Milne 1989).

During each count episode, we recorded all birds seen

or heard during an 8-minute interval that began upon

arrival at the count point (Verner 1988, Ralph et al.

1993). Birds observed while approaching the census

point were recorded as observations from the count

point. We recorded the time the count began, whether

birds observed were within the vegetation type or

structural stage being sampled, and the activity of

bird. We discontinued bird counts during periods of

moderate to heavy rain, when wind velocities exceeded 10 km/hr, or when temperatures were less than

7°C or greater than 24°C (Manuwal and Carey 1991).

Data Analysis

For data analysis, we truncated the data beyond

50 m of the variable plot center; thus, analyses were

conducted on detections of birds from a 50-m fixed

radius plot and included only birds detected from

songs or calls that were in the vegetation type or

structural stage identified for sampling. We determined the average number of detections, by species, in

1-hr time periods after sunrise for each stand. The

weekly average time of official sunrise was obtained

from the local newspaper. We used KolmogorovSmirnov tests to determine if the frequency of countepisodes in the seven 1-hr time periods differed

among vegetation types and structural stages. Fewer

(p <0.01) count-episodes were completed 7 hrs after

sunrise in quaking aspen and grasslands than the

previous time periods. We determined associations of

birds to vegetation types from Mills et al. (1996) and

restricted our analyses to data from sites in vegetation types associated with individual species. For

example, detections of birds associated with quaking

aspen and grasslands were compared for six 1-hr

time periods using data from sites in these vegetation types, respectively. However, detections of

birds associated with ponderosa pine were compared over seven 1-hr time periods from sites in

ponderosa pine.

Homogeneous variances in our data were uncommon, so we selected statistical tests that did not

require this assumption. We tested the hypotheses

that average detections of birds from songs or calls at

sites did not differ among 1-hr intervals after sunrise

using Welch’s test with Dunnett’s T3 multiple comparison procedure. We assigned statistical significance at α = 0.05 for all tests. We assumed that the

breeding season for birds was sufficiently short that a

factor for time during the breeding season was not

needed in the analysis.

Woodpeckers have large home ranges, vocalize infrequently, and drum for territorial displays. Most

detections of woodpeckers in this study were visual

and were not included in our analyses.

Results ________________________

Birds Associated With Ponderosa Pine

Differences in detections of yellow-rumped warblers (Dendroica coronata) and Townsend’s solitaires

(Myadestes townsendii) were not significant through

4 hrs after sunrise (figures 1A and 1B). Detections of

individuals for both of these species declined on countepisodes more than 4 hrs after sunrise. Detections of

red-breasted nuthatches (Sitta canadensis) and western tanagers (Piranga ludoviciana) were stable for

the first 4 hrs after sunrise, then declined significantly on count-episodes more than 4 hrs after sunrise (figures 1C and 1D). White-breasted nuthatches

(P. carolinensis) were associated with the ponderosa

pine vegetation type, but no differences in detections

by song were noted.

Birds Associated With Quaking Aspen

Dusky flycatchers (Empidonax oberholseri) sang

infrequently during the first hour after sunrise

(figure 2A). Afterward, detections of dusky flycatchers by song increased until 4 hrs after sunrise when

significant declines were noted. Significant declines in

detections of warbling vireos (Vireo gilvus) occurred

on count-episodes more than 5 hrs after sunrise.

Ovenbirds (Seiurus aurocapillus) and common yellowthroats (Geothlypis trichas) were also associated

with quaking aspen in the Black Hills, but no differences in detections from singing males were noted.

Birds Associated With Grasslands

Detections of vesper sparrows (Pooecetes gramineus)

and western meadowlarks (Sturnella neglecta) from

songs or calls did not differ through 5 hrs after

sunrise (figures 3A and 3B). Detections of these

species were highly variable and declined on countepisodes more than 5 hrs after sunrise. Detections of

grasshopper sparrows (Ammodramus savannarum)

were relatively constant throughout the morning.
USDA Forest Service Res. Pap. RMRS-RP-21. 2000 3

Figure 1—Average number of detections per hour of birds associated with

ponderosa pine among seven 1-hr time periods during bird counts in the

Black Hills. Standard errors are indicated by error bars. Time periods with

different letters are significantly different (α = 0.05) using Dunnett’s T3

multiple comparison procedure.

Figure 2—Average number of detections per hour of birds associated with

quaking aspen/paper birch among seven 1-hr time periods during bird counts

in the Black Hills. Standard errors are indicated by error bars. Time periods

with different letters are significantly different (α = 0.05) using Dunnett’s T3

multiple comparison procedure.
4 USDA Forest Service Res. Pap. RMRS-RP-21. 2000

Other Forest Birds

Dark-eyed juncos (Junco hymelis) were vocal

throughout the morning until 6 hrs after sunrise

(figure 4A). However, detections of black-capped

chickadees (Poecile atricapillus) declined steadily

throughout the morning with significant declines

occurring on count-episodes more than 1 hr and again

more than 5 hrs after sunrise (figure 4B). American

robins (Turdus migratorius) were vocal until 4 hrs

after sunrise (figure 4C). Detections of northern

flickers (Colaptes auratus), downy woodpeckers

(Picoides pubescens), hairy woodpeckers (P. villosus),

Swainson’s thrushes (Catharus ustulatus), mountain

bluebirds (Sialia currucoides), chipping sparrows

(Spizella passerina), and brown-headed cowbirds

(Molothrus ater) did not differ among hourly sampling

periods.

Discussion _____________________

The conspicuousness of many bird species varies

throughout the day owing to changes in activity

levels (Palmeirim and Rabaca 1993) and the periods

of highest detectability are not the same for all birds

(Robbins 1981). Ralph et al. (1993) and Manuwal and

Carey (1991) recommend that bird counts should be

completed within 5 hrs after sunrise. But Verner and

Ritter (1986) found no hourly differences in species

abundance measured 4 to 5 hrs after sunrise. Our

results support the recommendations of Ralph et al.

(1993) and Manual and Carey (1991). Biologists planning studies should note that detections of several

birds species declined by more than 50% on countepisodes completed 4 to 5 hrs after sunrise. Bird

counts completed more than 4 or 5 hrs after sunrise

Figure 3—Average number of detections per hour of birds associated with

grasslands among seven 1-hr time periods during bird counts in the Black Hills.

Standard errors are indicated by error bars. Time periods with different letters

are significantly different (α = 0.05) using Dunnett’s T3 multiple comparison

procedure.

would underestimate Townsend’s solitaires, yellowrumped warblers, red-breasted nuthatches, western

tanagers, dusky flycatchers, black-capped chickadees,

and American robins. Vesper sparrows and western

meadowlarks would be underestimated on count-episodes completed during early and late in the morning.

However, it is possible that our presence during low

light conditions of early morning caused these grassland birds to be less vocal. Meadowlarks may be

sensitive to observer intrusions as evidenced by lower

density estimates closer to count points than 30 to

50 m away (M. Rumble, unpubl. data, Rocky Mountain Research Station, Rapid City, SD).

Fatigue by observers could influence the apparent

detection of birds during the late morning. However,

we do not believe fatigue was a factor that caused

fewer detections of some song birds later in the morning. There were many species that no differences in

detectability from songs or calls were evident on countepisodes later in the morning. If observer fatigue had

been a factor, most all species should have exhibited

declines in detections during late morning countepisodes. Most of the bird species that exhibited declines in detection later in the morning were common

and we believe that our ability to detect them from

songs or calls was similar throughout the morning.

It is well known that in songbird surveys, even the

best surveyors seldom record all of the audible birds

(Bart 1985, Robbins and Stallcup 1981). Diurnal

variation in detectability of birds is widely acknowledged (Robbins 1981, Skirvin 1981). Detections of

birds usually depend on certain attributes of singing

and calling (Manuwal and Carey 1991) and bird calls

or songs often vary regionally (Carey et al. 1990).

Training observers to recognize songs and calls of local

birds is important (Verner and Milne 1989).
USDA Forest Service Res. Pap. RMRS-RP-21. 2000 5

Management Implications ________

We recommend that study designs for bird counts

include the following. Count-episodes should be completed within 4 hrs after sunrise. Some species associated with each of the vegetation type categories were

less detectable from songs on count-episodes completed more than 5 hrs after sunrise; thus, their

abundance would be underestimated. Multiple countepisodes at a site should include at least 2 consecutive days and the order of sampling should be

distributed throughout the daily counting period to

minimize effects of time on detectability of birds.

Recommendations of other researchers should be followed (e.g., Verner 1985,Verner and Milne 1989).

Figure 4—Average number of detections per hour of birds associated with both quaking

aspen/paper birch and ponderosa pine among seven 1-hr time periods during bird counts in

the Black Hills. Standard errors are indicated by error bars. Time periods with different letters

are significantly different (α = 0.05) using Dunnett’s T3 multiple comparison procedure.

Literature Cited _________________

Bart, Jonathan. 1985. Causes of recording errors in singing bird

surveys. Wilson Bulletin 97:161-172.

Buttery, Robert F.; Gillam, Bertha C. 1983. Forest ecosystems.

In: Hoover, Robert L.; Wills, Dale L. eds. Managing forested

lands for wildlife. Colorado Division of Wildlife in cooperation

with U.S. Department of Agriculture, Forest Service, Rocky

Mountain Region, Denver, CO: 43-71.

Carey, Andrew B.; Castellano, Valen E.; Chappell, Christopher

[et al.]. 1990. Training guide for bird identification in Pacific

Northwest Douglas-fir forests. Gen. Tech. Rep. PNW-GTR260. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Research Station. 28 p.

Cody, Martin L. (ed.). 1985. Habitat selection in birds. Academic

Press, Orlando.

Gotfryd, A.; Hansell, R.I.C. 1985. The impact of observer bias on

multivariate analysis of vegetation structure. Oikos 45: 223-234.
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Hoffman, George R.; Alexander, Robert R. 1987. Forest vegetation of the Black Hills National Forest of South Dakota and

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Collins, CO: U.S. Department of Agriculture, Rocky Mountain

Forest and Range Experiment Station. 48 p.

Manuwal, David A.; Carey, Andrew B. 1991. Methods for measuring populations of small diurnal forest birds. Gen. Tech. Rep.

PNW-278. Portland, OR: U.S. Department of Agriculture, Pacific

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Martin, Thomas E.; Finch, Deborah M. 1995. Introduction:

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migratory birds. In: Martin, Thomas E.; Finch, Deborah M.,

eds. Ecology and management of neotropical migratory birds:

a synthesis and review of critical issues. Oxford University

Press, New York: xii-xvi.

Mills, Todd R.; Rumble, Mark A.; Flake, Lester D. 1996. Evaluation of a habitat capability model for nongame birds in the

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CO: U.S. Department of Agriculture. Rocky Mountain Forest

and Range Experiment Station. 30 p.

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and compensate for time-of-day effects on bird counts. Journal

of Field Ornithology 65:17-26.

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methods for monitoring landbirds. Gen. Tech. Rep. PSW-144.

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Research Station. 41 p.

Reynolds, R.T.; Scott, J.M.; Nussbaum, R.A. 1980. A variable

circular-plot method for estimating bird numbers. Condor

82:309-313.

Robbins, Chandler S. 1981. Effect of time of day on bird activity.

In: Ralph, C.J.; Scott, J.M., eds. Estimating numbers of terrestrial birds. Studies in Avian Biology 6:275-286.

Robbins, Chandler S.; Stallcup, R.W. 1981. Problems in separating species with similar habits and vocalizations. In: Ralph,

C.J.; Scott, J.M., eds. Estimating numbers of terrestrial birds.

Studies in Avian Biology 6:360-365.

Robbins, Chandler S.; Dawson, Deanna K.; Dowell, Barbara A.

1989. Habitat area requirements of breeding forest birds of the

middle Atlantic states. Wildlife Monograph 103. 34 p.

Skirvin, A.A. 1981. Effect of time of day and time of season on the

number of observations and density estimates of breeding birds.

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Berkeley, CA: U.S. Department of Agriculture, Pacific Southwest Forest and Range Experiment Station. 4 p.

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Zoologica Fennici 26:191-199.

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