BIOLOGICAL REPORT 82(10.131)

FEBRUARY 1987

Library

National Wetlands Research Center

U. S. Fish and Wildlife Service

700 Cajundorne Boulevard

Lafayette, La. 70506

HABITAT SUITABILITY INDEX MODELS:

FORSTER'S TERN (BREEDING)-GULF

AN D ATLANTIC COASTS

f":! - I

lnd Wildlife Service

lepartment of the Interior SK

361

.U54

no . 8 2-

10 .131
MODEL EVALUATION FORH

Habitat models are designed for a wide variety of planning applications

where habitat infonnation is an important consideration in the decision

process. It is impossible, however, to develop a model that perfonns equally

well in all situations. Each model is published individually to facilitate

updating and reprinting as new information becomes available. Assistance from

users and researchers is an important part of the model improvement process.

Please complete this fonn following appl ication or review of the model. Feel

free to include additional information that may be of use to either a model

developer or model user. We also would appreciate information on model

testing, modification, and application, as well as copies of modified models

or test results. Please return this fonn to the following address.

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Thank you for your assistance.

Species _ Geographic

Location-------------------------------- Habita t or Cover Type (s) _

Type of Application: Impact Analysis Management Action Analysis __

Baseline Other---------------------------------- Variables Measured or Evaluated-----------------------

Was the species information useful and accurate? Yes No

If not, what corrections or improvements are needed?-------------
Were the variables and curves clearly defined and useful? Yes No

If not, how were or could they be improved?

Were the techniques suggested for collection of field data:

Appropriate? Yes No

Clearly defined? Yes No

Easily applied? Yes No

If not, what other data collection techniques are needed?

Were the model equations logical? Yes No

Appropriate? Yes No

How were or could they be improved?

Other suggestions for modification or improvement (attach curves,

equations, graphs, or other appropriate information) ----------

Additional references or information that should be included in the model:

Model Evaluator or Reviewer Date ------------ ------- Agency _

Address _

Telephone Number Comm: __ FTS _
Biological Report 82(10.131)

February 1987

HABITAT SUITABILITY INDEX MODELS: FORSTER'S

TERN (BREEDING)--GULF AND ATLANTIC COASTS

by

Richard P. Martin

ana

Phillip J. Zwank

Louisiana Cooperative Fish and Wildlife Research Unit

Louisiana State University

Baton Rouge, LA 70803

Project Officer

Rebecca Howard

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Performed for

National Wetlands Research Center

Research and Development

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report may be cited as:

Martin, R.P., and P.J. Zwank. 1987. Habitat suitability index models:

Forster's tern (breeding)--Gulf and Atlantic coasts. U.S. Fish Wildl. Servo

8iol. Rep. 82(10.131). 21 pp.
PREFACE

The habitat suitability index (HSI) model in this report on the Forster's

tern is intended for use in the U.S. Fish and Wildlife Service's (1980)

habitat eval uation procedures for impact assessment and habitat management.

The model was developed from a review and synthesis of existing information

and is scaled to produce an index of habitat suitability between 0 (unsuitable

habitat) and 1 (optimally suitable habitat). Assumptions used to develop the

model and guidelines for model applications, including methods for measuring

model variables, are described.

This model is a hypothesis of species habitat relationships, not a

statement of proven cause and effect. The model has not been field-tested.

For this reason, the U.S. Fish and Wildlife Service encourages model users to

convey comments and suggestions that may help increase the utility and

effectiveness of this habitat-based approach to fish and wildlife management.

Please send any comments or suggestions you may have to the following address.

Informat i on Trans fer Sped a.lis.t

Nationa:l WES'tlafl<ds: Research Cerrter

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

PREFACE " ..

ACKNOWLEDGMENTS " " " " ..

iii

vi

Variables.........•.....

MODEL ..

.. ..

Considerations..............•.....••.•..................

1

1

2

3

3

4

5

5

6

6

7

11

13

15

16

............................... Outputs .

Appl icabi 1i ty .

INTRODUCTION .. " ..

Distribution .....•......•••...••..

Special

HABITAT SUITABILITY INDEX (HSI)

Model

Model Description .........•.....•..........••...............•...

Suitability Index (SI) Graphs for Model

HSI Determination .. Field Use of Models •........•.

Interpreting Model

Life History Overview••.••....••

SPECIFIC HABITAT REQUIREMENTS ••••.••.••••••••..•••••••••••.••••.••

Food and Foraging Habitat.

Nesting Cover•..•..•..•.•.

Wa ter ..

RE FERENCES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••

APPENDIX •••••••••••••••••

Definitions of SI Model ......................................... Classes..•........•.........•.•..•......

17

21

21

v
ACKNOWLEDGMENTS

The narrative and habitat suitability index model for the Forster's tern

was reviewed by the following individuals who have some knowledge of the

nestj,l':I:Q requirements of this species: J. Burger, Rutgers-The State

VniveTs1ty"New Br,u:1!Iswi'ctk.. ,New ,J:erse"y; J • Parne l l , University of North

Carol ina., :WiHrrillgton;andG. 'Peterson, Ce'l"lter for 'Wetland Resources, louisiana

State University" Bat-o.n .R.ot/B}€. Evaluation of model structure and functional re]ationshl~s ~re ~rovided by personnel in the U.S. Fish and Wildlife Service

Ecol@gicalServices Offices at Lafayette, Louisiana, Corpus Christi, Texas,

ar.lrl AnnapoHs,Maryland, and by R. Hamilton and D. Pashley, School of

Forestry, Wildlife and Fisheries, Louisiana State University, Baton Rouge.

Special thanks go to D. Hewitt for typing the draft document. Funding

for the model development and publication was provided by the U.S. Fish and

Wildlife Service. Patrick Lynch illustrated the cover.

vi
FORSTER'S TERN (Sterna forsteri)

INTRODUCTION

Distribution

The nesting range of Forster's terns hosts three allopatric breeding

populations. The first and most important breeding area, in terms of the

number of nes t i ng pairs, includes the western guIf coas t from the

Louisiana-Mississippi border to northern Tamaulipas, Mexico (American

Ornithologists' Union [AOUJ 1983). In addition, small numbers of Forster's

terns have nested in Mobile County, Alabama (Imhof 1976). Although this

species has not been recorded nesting in Mississippi (J. Jackson, Mississippi

State University, Starkville; pers. comm.), it is observed in the coastal

regions of that State every summer, and several thousand nest in adjacent

Louisiana (Portnoy 1977; Clapp et ale 1983). The two largest colonies of

Forster's terns documented in the literature were both in Louisiana: one of

2,750 pairs in Lake Borgne on the Louisiana-Mississippi border and one of

2,263 pairs in Calcasieu Lake (Portnoy 1977).

The second nesting region extends from Long Island, New York, south along

the Atlantic coast to Cape Lookout, North Carolina (Erwin 1979; Portnoy et al.

1981; Parnell and McCrimmon 1984). About 6,000 birds breed in this region

annually (Erwin 1979; Portnoy et al. 1981). Louisiana is the most important

nesting area for Forster's terns along the Atlantic or Gulf of Mexico coasts

in terms of total number of breeding birds and mean colony size (Table 1).

The third and largest breeding area extends from southeastern British

Columbia east to south-central Manitoba, south to central California, northern

Utah and Colorado, northwestern Kansas, and nor~hern Iowa, Illinois and

Indiana (AOU 1983). The number of nesting birds i1n this area has not been

documented, but the largest concentrations of nesting birds are apparently in

the San Francisco Bay region and Manitoba (Gerrard and Whitfield 1971; Gill

and Mewaldt 1979).

Some northward and westward dispersal of Forster's terns is apparent

prior to the southward migration. The southward migration is primarily

through interior North America (AOU 1983). Forster's terns winter along

coastal areas from central Cal ifornia south to Costa Rica on the Pacific

coast, along the Atlantic-gulf coast from Virginia to Costa Rica, and in the

Bahamas and Greater Anti 11 es (AOU 1983). The greatest concentrati ons of

wintering terns occur along the Texas and Louisiana coasts, and coastal

southern California (Rubega et ale 1984; Drennan et ale 1985).

1
Table 1. Recent distribution of Forster "s tern colonies along the Atlantic

and gulf coasts of the United States.

State Colonies Pairs References

New Jersey 6 463 Erwin 1979

Ma ryl and 7 520 Erwin 1979

Virginia 21 1,095 Erwin 1979

North Carolina 13 931 Parnell and McCrimmon 1984

Alabama a a Imhof 1976

Louisiana 34 9,608 Portnoy 1977

Texas 64 1,773 Texas Colonial

Waterbird Society 1982

Total 145 14,390

a A few pairs may nest annually but adequate data are not available.

Oberholser (1974) and Blacklock et al. (1978, in Clapp et al. 1983)

; nd i cated that the number of Forster I s terns nest; ng on the Texas coast has

dec1i ned since the mi d-1940 I S because of the deve1opment of the 1arger

offshore islands and possibly because of bioaccumulation of DDT and PCB.

Recent investigators, however, have postulated that the number of nesting

terns has been stable (Texas Colonial Waterbird Society 1982). This

maintenance of population size may be due to increased availability of

suitable nesting areas on dredged spoil t sl ands.» Because of the limited

availability of quantitative historical data, the t~end for nesting Forster1s

terns in Louisiana cannot be determined.

Life History Overview

Forster's terns along the Atlantic and gulf coasts tend to nest in

single-species colonies of 10 to several hundred birds in saline or brackish

marshes on near-shore islands (Erwin 1979; Portnoy et al. 1981). Only a small

proportion of the gulf coast population nests in mainland marshes: 3.1% of

the colonies and 0.5% of the breeding birds in Louisiana, and 8.5% of the

colonies and 2.8% of the breeding'birds in Texas. In inland areas, floating

mats of aquatic vegetation ("pop-ups") appear to provide important nesting

2
substrate (P. Yakupsak, Lacassine National Wildlife Refuge, Louisiana; pers.

comm.) .

The majority of adult terns arrive at the nesting areas during April

(Erwin 1979). Along the Atlantic and gulf coasts they usually lay eggs in

April and May, and most nesting is completed by the end of July (Kopman 1907,

1908; Bent 1910; Oberholser 1938; Portnoy 1977; Erwin 1979; Parnell and

Soots 1979). Chaney et ale (1978) calculated the mean clutch size of this

species to be about 2 eggs per nest for 92 nests examined along the Texas

coast in 1977. Few published data are available on the incubation period of Forster's terns along the gulf coast, but Chaney et al. (1978) calculated the

period to be 23 days for 6 nests examined. The mean length of time from the

initiation of hatching to emergence is about 2.8 days for each egg (McNicholl

1983). Fledging apparently occurs at approximately 40 days, but varies with

prey abundance (Clapp et al. 1983).

Data on the extent of renesting by Forster's terns are not available for

the gulf coast colonies, but Parnell and Soots (1979) and J. Burger (Rutgers-The State University, New Brunswick, New Jersey; pers. comm) have

recorded renesting after storm damage in North Carolina and New Jersey.

Bergman et ale (1970) postulated that some renesting may occur in the interior

population if the original nest is destroyed.

SPECIFIC HABITAT REQUIREMENTS

Food and Foraging Habitat

Forster's terns tend to forage in protected shallow bays and lagoons near

the nesting colony (Oberholser 1974; Baltz et al. 1979; Chapman 1984). When

foraging over open water away from the co~stal marsh, they concentrate in the

zone between the foreshore and the first bar (Chapman 1984). Forster's terns

selected water less than 1 m deep during two foraging ecology studies in

coastal California (Salt and Willard 1971; Baltz et al. 1979).

Few data on prey size and type are available; however, the diet

apparently consists primarily of small fish, crustaceans, and aquatic insects

(McAtee and Beal 1912; Salt and Willard 1971; Baltz et ale 1979). Menhaden

(Brevoortia tyrannus) and silvery anchovies (Engraulis eurystole) were

important prey items in the late fall and winter in birds collected from

coastal South Carolina, Georgia, and Florida (McAtee and Beal 1912). Baltz et

ale (1979) determined that juvenile shiner perch (Cymatogaster aggregata) and

northern anchovy (Engraulis mordax) were the predominant food items found in

stomachs of birds collected at Elkhorn Slough, Monterey County, California.

Prey size ranges from 1 to 10 cm with a mean of about 1. 5 cm (Ba ltz et a1•

1979; Salt and Willard 1971).

Forster t s terns are apparently not limited by foraging habitat or prey

availability. They nest in or near brackish and salt marshes, which are

important nursery areas for many marine fishes and crustaceans and provide a

dependable source of appropriate-sized prey.

3
Nesting Cover

Formerly, nesting colonies were established on natural marshy islands

that were generally within 2 km of the mainland (Kopman 1907, 1908; Job

1908; Bent 1910). The development of artificial islands by deposition of

dredged spoil has created additi ona1 suitable nesti ng areas (Portnoy 1977;

Chaney et al. 1978; Texas Colonial Waterbird Society 1982). Almost all of the

recent colonies in Louisiana were located on 'natural islands: only 0.8% of

the breeding birds were found on dredged spoil islands in 1976 (Portnoy 1977).

Artificial islands are apparently more important as colony sites in Texas

because of the lack of undisturbed natural islands; 55.6% of the colonies

examined along the Texas coast during 1973-30 were located on artificial

islands (Texas Colonial Waterbird Society 1982). Dredged spoil islands in

saline waters along the upper gulf coast are initially vegetated by smooth

cordgrass (Spartina alterniflora), and sufficient vegetative cover for the

establishment of nesting colonies of Forster's terns should develop within 2

to 5 years (Chaney et al. 1978; Lewis and Lewis 1978).

Forster's terns in coastal colonies characteristically nest on small

marshy islands vegetated with~. alterniflora in saline marsh and~. patens

(saltmeadow cordgrass) in brackish marsh (Bent 1921; Oberholser 1938;

Oberholser 1974; Portnoy 1977; Parnell and Soots 1979; Texas Colonial Waterbird Society 1982; Clapp et al. 1983; G. Peterson, Center for Wetland

Resources, Louisiana State University, Baton Rouge, pers. comm.).

The availability of wind- and wave-accumulated mats of vegetation or wood

(wrack) within Spartina spp , marsh is apparently one of the most important

factors influencing the establishment of tern colonies, although Forster's

terns may occasionally nest in marshes that lack wrack. Wrack is usually

deposited above the mean high-tide line by storm tides and wind during the

winter, and provides quality nesting substrate the following spring. The

presence of wrack was noted by most authors describing the substrate of

Forster's tern nests in the eastern and southern United States (Job 1908; Bent

1921; Oberholser 1938; Chaney et al . 1978; Parnell and Soots 1979; Texas

Colonial Waterbird Society 1982; G. Peterson, pers. comm.). Because wrack

normally accumulates parallel to the island shore, tern colonies tend to be

long and narrow. Wrack consisting of aquatic vegetation such as Zostera

marina is usually only 5 to 10 crn in depth, while wrack composed of coarse

material, such as S. alterniflora culms, regularly,accumulates to 0.5 m (J.

Parnell, UniversitY of North Carolina; pers. comm.}. Apparently, the

important factor determining colony establishment is a quantity of wrack

sufficient to completely cover the underlying marsh vegetation, thus creating

an open ridge within the marsh suitable for nest placement; Forster's terns

will not utilize wrack deposits that have a large amount of marsh vegetation

penetrating the mat (J. Parnell, pers. comm.). Wide wrack deposits are

probably superior to narrow deposits because nests are less readily located by

predators using the elevated ridge as a travel lane. Bleached wrack is

particularly suitable as a nest substrate as it provides good camouflage for

tern eggs and chicks (Burger and Lesser 1978). Because tern nests are

generally shallow platforms (Bent 1921; Chaney et al. 1978), the increased

elevation afforded by nesting on wrack protects the nest and contents from

potential wave and storm tide damage, which is apparently the primary source

4
of nest failure in Texas and North Carolina (Chaney et al , 1978; Parnell and

Soots 1979). Occasionally, terns will establish small nesting colonies on

sand or shell beaches if wrack deposits are present and vegetative cover is

available nearby (Oberholser 1938; Parnell and Soots 1979). Common terns that

used wrack as a nest substrate had greater nesting success than pairs that

nested on the ground (Burger and Lesser 1978).

Vegetative density does not appear to be important except that ground

cover must be sufficient to trap a dense mat of wrack for a nesting substrate

and to provide some protective cover for young chicks (Burger and Lesser 1978;

J. Parnell, pers. comm.).

Water

The physiological water requirement of Forster's terns is probably met

within the saline and brackish habitats surrounding the nesting colonies and

need not be considered as a factor of habitat sui tab t l t ty (Welty 1982).

Special Considerations

Predation. Some characteristics of islands are related to their

potential to sustain terrestrial predators. Rats (Rattus spp. ) , raccoons

(Procyon lotor) and mink (Mustela vison) are predators of nesting terns

(Provost 1947; Austin 1948; Sprunt 1948; G. Peterson, pers. comm.). Thus, the

perceived potential of an island to support a viable population of terrestrial

predators is important when estimating the suitability of the site as tern

nesting habitat. Island elevation, island size, and distance from the

mainland are probably the most important variables determining successful

predator colonization.

Islands with 1ittle rel ief (less than 0.5 m maximum elevation) tend to

have low vegetative diversity and are periodically flooded, so probably would

not support viable populations of terrestrial predators. As island size

increases the maximum elevation tends to increase, thereby increasing

vegetative diversity and the potential to support predators. Islands with

some topographi c vari abi l ity also provi de refuge for predators from hi gh

tides; however, Chaney et al. (1978) postulated that islands less than 20 ha

in size, even if they had some topographical relief and vegetative diversity,

would not support large predators indefinitely.

In Texas, the largest tern colonies were found on small islands up to 1

ha in area (Texas Colonial Waterbird Society 1982). Similar quantitative data

are not available for Louisiana, but Kopman (1907) stated that colonies tended

to be established on "small" islands and G. Peterson (pers. comm.) and S.

Cardiff (Museum of Zoology, Louisiana State University, Baton Rouge; pers.

comm.) stated that the Forster's tern colonies they examined were on islands 1

ha or less in area.

As the distance from the colony island to the mainland increases, the

probability of predator colonization decreases (McArthur and Wilson 1967). In

this model, "mainland" refers to contiguous habitat greater than 20 ha in

area.

5
Avian predators, especially herring gulls (Larus argentatus), can cause

serious egg or chick loss in tern colonies along the northern Atlantic coast.

However, the smaller laughing gull (Larus atricilla) is the common nesting

gull along the southern Atl anti c and gulf coast in the United States; and

predation by this species does not appear to be an important limiting factor

of nesting terns (Portnoy 1977; Burger and Lesser 1978).

Disturbance. Human disturbance and development of traditional nesting

areas are often cited as reasons for tern colony abandonment (Davis 1965;

Gochfeld 1974, 1976; Portnoy 1977). Development of the larger offshore

islands along the Texas coast is considered the reason Forster's terns

abandoned some traditional colony sites (Oberholser 1974). The extremely

large colony of 2,750 pairs of nesting terns in Lake Borgne in Louisiana was

deserted with no apparent fledging of young after the colony was intentionally

disturbed by humans, and some adult birds were apparently shot (Portnoy 1977).

Even if human di sturbance does not cause complete colony abandonment, it can

cause the adul ts to 1eave temporari ly, expos i ng the eggs and young to heat

stress and predation (Portnoy 1977). Only 1 of 32 colonies in Louisiana and

14' of 64 colonies in Texas were noted as being included in State, Federal, or

pri vate refuges and thereby protected from most di sturbances (Portnoy 1977;

Texas Colonial Waterbird Society 1982).

Although we could not find documented reports of Forster'~ tern

susceptibility to oil pollution, the potential for adverse effects is great

because this species nests and roosts at the high-tide line (Chapman 1984).

In addition, the method of feeding (aerial plunging) brings the birds into

regular contact with the water surface. Because the Louisiana and Texas

coasts are important oil production areas and because a 1arge proporti on of

the world's breeding and wintering Forster's terns are found there, special

attention should be given to important nesting and roosting islands should a

major spill occur.

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area and season. The habitat suitability index model in this

report was developed for application within the normal breeding range of the

Forster's tern along the gulf coast of Louisiana and Texas. There are

apparently few differences in nesting habitat requirements along the Atlantic

coast; therefore, the HSI model may also be used to evaluate potential habitat

in coastal New Jersey, Maryland, Virginia, and North Carolina. The HSI model

was developed to evaluate Forster's tern habitat during the breeding season

(March through July).

Cover types. Forster's terns are extremely selective in their choice of

nesting habitat, with a vast majority of the colonies established in saline or

brackish marsh habitats classified as "estuarine intertidal persistent

emergent" by Cowardin et al. (1979).

6
Minimum habitat area. Minimum habitat area is defined as the rm mmurn

amount of contiguous habitat that fulfills all life requisites for a species;

however, the minimum habitat area principle is not readily applicable to

colonial nesting species that travel to a food source. Published data on the

minimum area required by Forster's terns were not found. If quantitative data

on minimum area were collected in the future and the size of the site under

evaluation is less than the minimum, the HSI for this species would be zero.

Verification level. The output of this HSI model is an index between 0

and 1.0 that is believed to reflect habitat potential for Forster's terns.

Earlier drafts of this model were reviewed by J. Burger, Department of

Biology, Rutgers-The State University, New Brunswick, New Jersey; J. Parnell,

Department of Biological Sciences, University of North Carolina, Wilmington;

G. Peterson, Center for Wetland Resources, Louisiana State University, Baton

Rouge. Personnel in the U.S. Fish and Wildlife Service Ecological Services

Offices at Lafayette, Louisiana; Corpus Christi, Texas; and Annapolis, Maryland, also reviewed the model. This model has not been field-tested.

Model Description

Overview. Habitat suitability for Forster's terns is dependent on

several factors. Five factors that influence nesting habitat quality are

incorporated in this model: two variables that describe cover, two variables

describing the physical characteristics of the nesting island, and the amount

of human disturbance in or near the study area. The relationships of habitat

variab1es to the nesting HSI value are illustrated in Figure 1.

Although waves and storm tides can cause serious nest loss, they are

environmental factors and not a component of the habitat per se, so are not

included in this model. An additional factor that may be important when

assessing habitat quality, although quantitative data are lacking, is the

influence of tidal amplitude on colony establishment and success. Wrack

deposition on low marshy islands would be reduced in areas exposed to great

fluctuations in tidal range. In those areas, wrack would tend to accumulate

on islands with some topographic relief. Colonies established in areas of

large tidal amplitude would also have an increased potential for colony damage

by waves and storm tides. In North Carolina, Forster's terns only nest in

Pamlico and Cove Sounds, which are exposed to relatively little tidal

fluctuation, indicating that colony site selection may be influenced by tidal

amplitude (J. Parnell, pers. corrnn.).

The avail abil ity of water and prey is assumed to be non-l imiti ng withi n

the breeding range of Forster's terns; therefore, those variables are not

considered in the model.

The HSI model uses the known habitat requirements of Forster's terns to

assess the potential suitability of a given site to support a nesting colony.

The assumptions for each variable and the associated primary data sources are

presented in Table 2. The following section explains the logic used in the

formulation of the HSI equation.

7
0:>

Habitat variable

VI Percent of island covered

with S. alterniflora or

S. patens

V2 Wrack quality

V3 Is 1and size

V4 Distance of island from

mainland or other island ::> 20 ha in area

Vs Disturbance level

Component

____------------~---Nesting cover

- Isl and characteristi cs ~HSI

---------- Oi sturbance

Figure 1. Relationship of habitat variables to the habitat suitability index (HSI) for

nesting Forster's terns.
Table 2. Variable sources and assumptions for Forster's tern suitability

indices.

Variable

V

3

Source

Oberholser 1938

Chaney et ale 1978

Burger and Lesser 1978

Oberholser 1938

Portnoy 1977

Texas Colonial Waterbird Society 1982

Parnell and Soots 1979

J. Burger, pers. comm.

Chaney et ale 1978

J. Parnell, pers. comm.

G. Peterson, pers. comm.

McArthur and Wilson 1967

Portnoy 1977

Texas Colonial Waterbird

Society 1982

Davis 1965

Gochfeld 1974

Gochfeld 1976

Portnoy 1977

Assumption

Marsh vegetation (Spartina alterniflora or ~. patens) is necessary for wrack

accumulation, nest support,

and chick cover.

Optimal sites contain an

abundance of wrack for nest

substrate.

The relative protection from

wave damage afforded a colony and the potential of an

island to support terrestrial

predators are dependent on

island size.

The potential for predator colonization decreases as the

island becomes increasingly isolated from the mainland.

Human disturbance is

detrimental to tern colony

establishment and nesting

success.

Cover component. Vegetative cover is an important component of Forster's

tern nesting habitat. Coastal nesting colonies are normally located in stands

of S. alterniflora in saline marsh and, occasionally, ~. patens in brackish

marsh. Spartina spp. are important components of the nesting habitat because

they trap and are a major component of wrack and provide escape cover for

chicks. Decumbent patches of Spartina spp. also provide secondary nesting sites. This model assumes that optimal nesting habitat is found on low,

periodically flooded saltmarsh islands vegetated with near-monotypic stands of ~. alterniflora with canopy cover of at least 25% (VI).

9
Islands with some elevational relief have increased vegetative diversity

when compared with low, regularly flooded islands (Lewis and Lewis 1978).

Because vertebrate species diversity is correlated with increased spatial

heterogeneity and plant species diversity (MacArthur and MacArthur 1961), the

potential for an island to sustain a viable population of one or more

predators increases with an increase in elevation. Islands with a maximum

elevation less than 0.5 m above mean high tide are normally only vegetated

with 5partina spp. and do not support woody. vegetation. Because vegetative

composition ;s dependent on island topography, the cover variable, VI' includes an elevational component. .

The presence of wrack (V2) is very important for colony establishment.

Quantifying this variable is difficult, so it should be measured on a relative

scale. Optimum habitat contains extensive wrack deposits that completely

cover the underlying marsh vegetation and provide a substrate that elevates

the nest above mean high tide. The importance of annual replenishment of

wrack is supported by the observation that common terns that used wrack as a

nest substrate generally did not use wrack from previous years due to

compaction and decomposition (Burger and Lesser 1978).

Island characteristics. Two variables--size and distance from the

mainland--are used to quantify the potential of islands to support tern

predators and provide protection from wave damage to the colony. As the size

of the island (V 3) increases, the potential to support a viable predator

populati on increases, primari ly because of increased topographi c di versi ty , Islands greater than 20 ha are relatively unsuitable for the establishment of

nesting colonies because they potentially support large predators. The

minimum island size required to support Rattus spp. is not known, although

these predators probably do not persist on small, periodically flooded, marshy

islands.. However, rice rats (Oryzomys palustrus) are widely distributed in

marsh habitats along the Atlantic and gulf coasts (Wolfe 1982), and although

they can be an important predator of marsh wren (Cistothorus palustris) eggs

(Kale 1965), we could not find any published data documenting rice rat

predation of tern eggs. It is assumed that the 51 score for island size

is low (0.2) for islands up to 0.1 ha in size, is equal to 1.0 between 0.1 and

1.0 ha, and decreases to 0.1 at greater than 20 ha. The suitability of very

small islands (i.e., less than 0.1 ha) is low due to the high probability of a

colony being damaged by waves: colonies established deep within a marsh on

larger islands are protected by the wave-damping effect of the vegetation.

The island isolation variable (V 4) is derived from the island

bi ogeography theori es of MacArthur and Wi 1son (1967); as the di stance from

the mainland increases, the potential for successful predator colonization

decreases. Conversely, increasing insularity exposes the nesting colony to

potential severe wave and tidal damage, which may impose an upper limit on the

distance of high quality island study sites from the mainland. Optimum

nesting islands are assumed to be separated from the mainland by 1 to 3 km of

water sufficiently deep to create an effective predator barrier (greater than

O.S m deep at mean low tide).

Disturbance. Forster's terns are sensitive to human disturbance of the

nesting colony (V S). It is assumed that commercial or recreational boating

10
near a nesting colony does not adversely affect nesting success if the vessels

do not approach closer than 100 m. The impact of occasional, short duration

human disturbance on the ground in or near the colony is also limited.

Apparently, nesting gulls can become habituated to regular, low-intensity disturbance so the effects of a given level of disturbance may vary among

colonies: the level of disturbance is most critical in previously undisturbed

colonies (Burger 1981). Nearby occupied houses, especially if free-roaming

pets are present, are signs of potentially serious disturbance. Frequent use of the study area by recreational vehicles is not compatible with tern

nesting. Optimum nesting habitat is found in areas such as refuges where

human disturbance between 1 March and 30 July can be restricted.

Suitability Index (SI) Graphs for Model Variables

This section provides graphic representation of the relationship between

individual habitat variables and habitat suitability for For-ster ' s terns on

islands with estuarine (E) intertidal persistent emergent cover. The SI

values for each variable are read directly off the Y axis of each graph; 1.0 =

optimum suitability, 0.0 = unsuitable.

The SI graphs are based on the assumption that the suitabil ity of a

particular variable can be represented by a two-dimensional linear response

surface.

Habitat

E

Variable

Percentage of the

study area vegetated

with Spartina alterniflora or S.

patens with canopy

cover at least 25%.

11

Suitability Graph

1.0

)( 0.8

CD

'tJ

C

- 0.6

>-

::

:c 0.4 tV

::

::J

en 0.2

0.0

0 20 40 60 80 100

%
Habitat Variable Suitability Graph

2 3 4 5

Class

1

l-

.

1.0

0.0

1) unsuitable

2) poor

3) fair

4) good

5) optimum.

Qua1ity of wrack

deposits during the nesting season

(1 March to 30 July) (see Appendix for

class definitions)

E

.

I0.2

0.4

1.0

0.6

E V3 Island size

)(

1) less than 0.1 ha CD

2) 0.1-1.0 ha

"

c

- 3) 1.1-10 ha

~~ 10.1-20 ha ::

greater than 20 ha. .a

as

::

:I

UJ

0.0

1 2 3 4

Class

5

E Distance of the

island from nearest

mainland or island

greater than 20 ha

(water must be a

minimum of 0.5 m

deep at mean low

tide).

)(

CD

"

C

-

2 3 <4 5

km

1

o.o....... ........_--..__...."...

o

12
Habitat Variable Suitability Graph

E

HSI Determination

Disturbance during

the nesting season

(see Appendix for

level definitions)

1) extreme

2) moderate

3) minimal

4) none.

1.0

)( 0.8

cP

"

.5 0.6

;>.

:a 0.4 (U

::s

UJ 0.2

0.0

1 2 3

Class

4

The following equations integrate the five individual 51 values to obtain

the final H5I value for Forster's tern nesting habitat.

Component Equation

Cover (C)

Island Characteristics (IC)

Disturbance (D)

H5I =:

1/2

(5I V x SIV ) 1 2

The variables VI and V2 were assumed to be very important; therefore any

compensatory interaction would be weak and the equation for cover incorporates

a geometric mean of those variables. An arithmetic mean is calculated from

the individual SI values for V3 and V4 because those variables interact in a

compensatory fashion: a low 51 score for one of the variables can be

compensated by a high score for the other variable.

An intermediate calculation utilizing the cover (weighted) and island

characteristic components is necessary prior to calculating the overall H5I.

13
This step is important because a large island, while scoring low on the

characteristic component, could be a potentially high-quality nesting area if

it scored high on the cover component. It is conceivable that a large island

vegetated almost entirely with Spartina would be more suitable than a small

marshy island because of the wave-damping effect.

A geometric mean is then calculated using the resulting two components of

the equation because the compensatory relationship is perceived to be weak.

Sample HSI values were derived from three hypothetical study areas (Table

3). The first study area represents a relatively large expanse of coastal

marsh recently separated from the mainland by subsidence. Habitat suitabil ity

of the area is limited primarily by size and proximity to the mainland, which

would indicate a high potential for predators. Study area 2 represents a

large island that exhibits a relatively great amount of topographic relief

and, therefore, marsh vegetation is restricted to the perimeter. This

scenario can develop on natural islands or by deposition of dredged material.

These islands are preferred by sportsmen and recreationists, so disturbance is

l i ke ly , The third study area is a small, low, marshy island. The only

limiting factor would be the ability to trap and retain a large, dense mat of

wrack .

Table 3. Calculation of suitability indices (SI), component indices, and

habitat suitability indices for three hypothetical study areas using habitat

variable (V) measurements and Forster1s tern HSI model equations.

Model Study area 1 Study area 2 Study area 3

component Data SI Data SI Data SI

VI 100% 1.0 20% 0.2 100% 1.0

V2 Good 0.8 Optimum 1.0 Good 0.8

V3 15 ha 0.2 100 ha 0.1 0.5 ha 1.0 I

V4 0.1 km 0.2 3.0 km 1.d 2.5 km 1.0

V5 Minimal 0.8 Moderate 0.5 None 1.0

C 0.89 0.45 0.89

IC 0.2 0.55 1.0

D 0.8 0.5 1.0

HSI 0.73 0.49 0.96

14
Field Use of Models

Since HSI models are often based on limited data, the biological

precision of the model should not be overestimated and intensive ground

evaluation of the study site is usually not warranted. Variables VI through V4 can be adequately evaluated by examining high-resolution aerial photography

(Table 4).

While examination of wrack quality immediately prior to the breeding

season wi 11 provi de data on the suitabil ity of the site for the current

nesting period, a projection of future wrack conditions may be difficult.

Wrack accumulation is dependent on island characteristics such as elevation

and vegetative cover; however, stochastic events such as storm tides are also

important and are not predictable. If wrack is present during the on-site

investigation it can be assumed that future wrack quality will remain constant

unless the proposed project will alter the island1s characteristics. However,

Table 4. Suggested methods for field measurement of variables used in the

Forster1s tern model.

Variable Suggested technique

The proportion of the study site covered by Spartina alterniflora or ~. patens can be

visually estimated from aerial photographs and

validated by field examination of the area.

Wrack quality can be estimated from high resolution,

large-scale aerial photography or by ground

reconnaissance for the period immediately prior to

the breeding season. If wrack is absent,

consultation with persons familiar with the site will

be necessary.

Island size can be measured with a planimeter from

aerial photography.

The distance of the study site from the nearest

mainland can be estimated from aerial photography

or maps.

Potential colony disturbance can be determined from

interviews with local residents, sportsmen, or refuge personnel. On-site inspection and aerial photography will reveal signs of extreme or permanent sources of

disturbance.

15
the absence of wrack does not necessarily indicate that the island will be

unsuitable in future years: assigning a 51 of 0.0 for V2 based on the absence

of wrack may result in an overall low H5I value for an otherwise high-quality

site. If wrack is absent, future wrack quality can be estimated by consulting

persons familiar with the site: an average over the past 3-5 years would be

acceptable.

Interpreting Model Outputs

The breeding H5I for Forster's terns derived from field application of

this model only represents habitat potential, and may not accurately predict

nesting density. The model should be used to compare the potential of two or

more sites to support nesting terns. If different study areas are examined,

the one with the higher H5I should have the potential to support a larger

colony of nesting Forster's terns. In addition, this model can be used to

estimate the effect of a proposed project on the habitat suitability of a

specific site.

16
REFERENCES

American Ornithologists' Union. 1983. Checklist of North American birds.

6th ed. American Ornithologists' Union, Washington, O. C. 877 pp.

Austin, O. L. 1948. Predation by the common rat (Rattus norvegicus) in the

Cape Cod colonies of nesting terns. Bird-Banding 19:60-65.

Baltz, D. M., G. V. Morejohn, and B. S. Antrim. 1979. Size selective

predation and food habits of two California terns. West. Birds 10:17-24.

Bent, A. C. 1910. Report of A.C. Bent on the Breton Island Reservation.

Bird Lore 12:280-282.

Bent, A. C. 1921. Life histories of North American gulls and terns. U.S.

Natl. Mus. Bull. 113. 337 pp.

Bergman, R. D., P. Swain, and M. W. Weller. 1970. A comparative study of

nesting Forster's and black terns. Wilson Bull. 82:435-444.

Blacklock,

waters.

140 pp.

G. W. 1978. Birds

Annotated checklist.

(Unpubl. MS.)

of Padre and Mustang Islands and adjacent

Welder Wildlife Foundation, Sinton, Tex.

Burger, J. 1981. Effects of human disturbance on colonial species, particularly gulls. Colon. Waterbirds 4:28-36.

Burger, J., and F. Lesser. 1978. Selection of colony sites and nest sites by

common terns Sterna hirundo in Ocean County, New Jersey. Ibis 120:433-449.

Chaney, A. H., B. R. Chapman, J. P. Karges, D. A. Nelson, R. R. Schmidt, and

L. C. Thebeau. 1978. Use of dredged material islands by colonial seabirds

and wadingbirds in Texas. Dredged Material Res. Program Tech. Rep. 0-78-8.

U.S. Army Engineer Waterways Exp. Stn., Vicksburg, Miss. 317 pp.

Chapman, B. R. 1984. Seasonal abundance and habitat-use patterns of coastal

bird populations on Padre and Mustang Island barrier beaches. U.S. Fish

Wildl. Servo FWS/OBS-83/31. 73 pp.

Clapp, R. B., D. Morgan-Jacobs, and R. C. Banks. 1983. Marine birds of the

southeastern United States and Gulf of Mexico. Part III: Charadriiformes.

U.S. Fish Wildl. Servo FWS/OBS-83/30. 853 pp.

17
Cowardin, L. M., V. Carter, F. C. Golet, and E. 1. LaRoe. 1979.

Classification of wetlands and deepwater habitats of the United States.

U.S. Fish Wildl. Servo FWS/OBS-79/31. 103 pp.

Davis, T. H. 1965. Jones Beach tern banding and mortality. Linnean

News-Letter 19(5):3-4.

Drennan, S. R., et ale 1985. The eighty-fifth Audubon Christmas bird count.

Am. Birds 39:427-811.

Erwin, R. M. 1979. Coastal waterbird colonies:

Virginia. U.S. Fish Wildl. Servo FWS/OBS-79/10.

Cape El i zabeth, Ma i ne to

212 pp.

Gerrard, J. M., and D. W. A. Whitfield.

Forster's tern in the prairie provinces.

1971. Breeding distribution of

Blue Jay 29:19-22.

Gill, R., Jr., and L. R. Mewaldt. 1979. Dispersal and migratory patterns of

San Francisco Bay produced herons, egrets, and terns. N. Am. Bird Bander

4: 4-13.

Gochfeld, M. 1974. Waterbird colonies of Long Island, New York: 2. Wantaugh

Parkway Colony. Kingbird 24:47-50.

Gochfeld, M. 1976. Waterbird colonies of Long Island, New York: 3. Cedar

Beach ternery. Kingbird 26:62-80.

Imhof, 1. A.

Montgomery.

1976. Alabama bi rds ,2nd ed ,

445 pp.

Uni vers ity of Alabama Press,

Job, H. K. 1908. Reservation news; Battledore Island, Louisiana. Bird Lore

10:187-189, 276.

Kale, H. W. 1965. Ecology and bioenergetics of the long-billed marsh wren

Telmatodytes palustris griseus (Brewster) in Georgia salt marshes. Publ.

Nuttal Ornith. Club 5:1-142.

Keller, C. E., J. A. Spendelow, and R. D. Greer. 1984. Atlas of wading bird

and seabird nesting colonies in coastal Louisiana, Mississippi, and Alabama:

1983. U.S. Fish Wildl. Servo FWS/OBS-84/13. 127 pp.

Kopman, H. H. 1907. Report of exp1orati on of seabi rd col onies. Bi rd Lore

9:233-240.

Kopman, H. H. 1908. Inspection of East Timbalier Reservation, Louisiana.

Bird Lore 10:229-231.

Lewis, R. R., Ill, and C. S. Lewis. 1978. Colonial bird use and plant

succession on dredged material islands in Florida. Vol. 2: Patterns of

plant succession. U.S. Army Engineer Waterways Exp. Stn. Dredged material

Res. Program Tech. Rep. 0-78-14, Vicksburg, Miss. 169 pp.

18
MacArthur, R. H., and J. W. MacArthur. 1961. On bird species diversity. Ecology 42:594-598.

MacArthur, R. H., and E. O. Wilson.

Princeton University Press, N.J.

1967. The theory of island biogeography. 209 pp.

McAtee, W. L., and F. E. L. Beal.

rapacious birds in relation to man.

30 pp.

1912. Some coemon game, aquatic, and

U.S. Dep. Agric. Farmer's Bull. 497.

McNicholl, M. K. 1983. Hatching of Forster's terns. Condor 85:50-52.

Oberholser, H. C. 1938. The bird life of Louisiana.

No. 28., New Orleans. 834 pp.

Dep. Conserv., Bull.

Oberholser, H. C.

Press, Austin.

1974. The bird life of Texas, Vol. 1.

530 pp.

University of Texas

Parnell, J. F., and D. A. McCrimmon, Jr. 1984. 1983 supplement to atlas of

co1onia1 waterbi rds of North Carol i na estuari es. UNC Sea Grant Pub1. ,

UNC-SG-84-07, Raleigh, N.C. 64 pp.

Parnell, J. F., and R. F. Soots, Jr. 1979. Atlas of colonial waterbirds of

North Carolina estuaries. UNC Sea Grant Publ., UNC-SG-78-10, Raleigh, N.C.

274 pp.

Portnoy, J. W. 1977. Nesting colonies of seabirds and wadingbirds: coastal

Louisiana, Mississippi, and Alabama. U.S. Fish Wildl. Servo FWS/OBS-77/07.

126 pp.

Portnoy, J. W., R. M. Erwin, and T. W. Custer. 1981. Atlas of gull and tern

colonies: North Carolina to Key West, Florida (including pelicans,

cormorants, and skimmers). U.S. Fish Wildl. Serv., Washington, D.C.

FWS/OBS-80/05. 121 pp.

Provost, M. W. 1947. Nesting of birds in the marshes of northwest Iowa. Am.

Midl. Nat. 38:485-503.

Rubega, M. A., et ale 1984. The eighty-fourth Audubon Christmas bird count.

Am. Birds 38:396-828.

Salt, G. W., and D. E. Willard. 1971. The hunting behavior and success of

Forster's tern. Ecology 52:989-998.

Sprunt, S. A., Jr. 1948. The tern colonies of the Dry Tortugas Keys. Auk

65:1-19.

Texas Colonial Waterbird Society. 1982. An atlas and census of Texas

waterbird colonies, 1973-1980. Caesar Kleberg Wildlife Research Institute,

Texas A&I University, Kingsville. 357 pp.

19
u.s. Fish and Wildlife Service. 1980. Habitat evaluation procedures (HEP).

ESM 102. Washington, D.C. n.p.

Welty, J. C. 1982. The 1He of birds, 3rd. ed. Saunders College Publishing,

New York. 754 pp.

Wolfe, J. L. 1982. Oryzomys palustris. Mammal. Spec. 176:1-5.

20
APPENDIX

DEFINITIONS OF SI MODEL CLASSES

Qua1ity of wrack depos its

Optimum. Wrack composed of dense mat of Spartina spp. culms or other

vegetation relatively resistant to decay. Mat should be greater than 30-cm

deep and greater than 5-m wide. Little or no underlying marsh vegetation

should penetrate mat.

Good. Wrack mat characteristics similar to "optimum," but depth less

than 30 cm or width less than 5 m, or both.

Fair. Wrack composed of mat of old or readily decomposable vegetation

such asZostera marina. Little or no underlying marsh vegetation should

penetrate mat.

Poor. Wrack characteristics similar to "fai r" but some underlying marsh

vegetation beginning to penetrate mat. This class also includes wrack

depos i ts on bare sand or shell beaches with potenti a1 vegeta t i on cover for

chick nearby.

Unsuitable. Wrack old or decomposed with extensive areas of underlying

marsh vegetation penetrating mat. ~'Jrack deposits on bare sand or shell

beaches with no vegetation nearby for chick cover are also unsuitable.

Disturbance

Extreme. Site regularly used (e.g., daily) for obtrusive recreation

(dune buggies, motorcycles, etc.) or near heavily traveled water routes or

both. Permanent dwellings within 300 m of the site may also be present.

Moderate. Site occasionally used (e.g., weekly) for obtrusive recreation

or with light water traffic near the colony or both.

Minimal. Site visited by sportsmen no more than once a week, and/or

water traffic 1ight and greater than 100 m from site. Recreational use of

site regulated.

None. The site is protected during the nesting season (e.g., a refuge)

or isnot sui tabl s for human use (such as low, regularly flooded, marshy

islands).

21
50212 -101

REPORT DOCUMENTATION II. REPORT NO. [I. PAGE Biological Report 82(10.131)

•• Totl. end Subtltl.

Habitat Suitability Index Models: Forster's Tern (Breeding)--

Gulf and Atlantic Coasts

7. Author<.)

Richard P. Martin and Phillip J. Zwank

9. PerformI". O,...nlzetlon N.m••nd Address

Louisiana Cooperative Fish and Wildlife Research Unit

U.S. Fish and Wildlife Service

Louisiana State University

Baton Rouge, LA 70803

12. Sponsorin. O...nlzetlon N.m••nd Addr•••

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

15. Supplement.ry N~.s

16. Abstr.ct (Limit: 200 words)

1 Recipi.nt·. Acce ••lon No.

l. R.port Oet.

Februa ry 1987

L

... L P.rformin. O,...nlz.tlon R.pt.

10. ProjectlT..k/Wortl Unit No.

11. Contr.ct(C) or Gr.nt(G) No.

(C)

13. Type of R.port & Period Cov.red

I•.

A review and synthesis of existing information were used to develop a model for

evaluating the quality of breeding habitat for Forster's tern (Sterna forsteri). The

model is applicable within the nesting range of Forster's terns along the Gulf of

Mexico and Atlantic coast. It is scaled to produce an index between 0 (unsuitable

habitat) and 1.0 (optimal habitat). Habitat suitability index models are designed for

use with the Habitat Evaluation Procedures previously developed by the U.S. Fish and

Wildlife Service. Guidelines for model application and techniques for measuring model

variables are provided. 4

17. Docu ....nt An.lysi. •. D.scriptors

Mathematical models

Birds

Wildlife

b. Id.ntlfi.rs/Open-End.d T.rm.

Habitat Suitability Index

Habitat

Habitat management

c. COSATI Field/Group

IL Av.llebllity St.t.m.nt

Unlimited release

(Se. ANSI-l39.18)

U.S. GOVERNMENT PRINTING OFFICE: 1987-761-070/40005

Impact assessment

Forster's tern

Sterna forsteri

19. Security CI••• (Thi. R.port)

Unclassified

20. Security CI••• (Thi. Pep)

Unclassified

21. No. of P••••

21

22. Pric.

OPTIONAL FORM 272 (4-77)

(For....rly NTIS-35)

Dep.rtm.nt of Ccmm.rc.
Take Pride in America

Preserve OurNatural Resources

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