FWS/OBS-82/ 10.78

SEPTEMBER 1984

HABITAT SUITABILITY INDEX MODELS:

GREAT EGRET

SK

361

.U54

no. 82-

10.78

and Wildlife Service

Department of the Interior
This model is designed to be used by the Division of Ecological Services in

conjunction with the Habitat Evaluation Procedures.
MODEL EVALUATION FORM

Habitat models are designed for a wide variety of planning applications

where habitat information is an important consideration in the decision

process. It is i:npossible, however, to develop a model that performs 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 application 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 al so would appreciate information on model

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

test results. Please return this fonn to:

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Sl idell, LA 70458

Thank you for your assistance.

Species _ Geographi c

Location------------------------------------ Habitat 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 Appropriate? Yes

Clearly defined? Yes----

Easily applied? Yes----

collection

No

No----

No--

of field data:

Yes No

Yes-- No----

If not, what other data collection techniques are needed?----------

Were the model equations logical?

Appropriate?

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-------------
FWS/OBS-82/10.78

September 1984

HABITAT SUITABILITY INDEX MODELS: GREAT EGRET

by

Brian R. Chapman

Department of Biology

Corpus Christi State University

Corpus Christi, TX 78412

and

Rebecca J. Howard

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Project Officer

Paul L. Fore

Regional Office, Region 2

U.S. Fish and Wildlife Service

Albuquerque, NM 87103

Performed for

National Coastal Ecosystems Team

Division of Biological Services

Research and Development

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report should be cited as:

Chapman, B.R., and R.J. Howard. 1984.

great egret. u.s. Fish Wildl. Servo

Habitat suitability index models:

FWS/OBS-82/10.78. 23 pp.
PREFACE

The habitat suitability index (HSI) model for the great egret presented in

this report is intended for use in the habitat evaluation procedures (HEP)

developed by the U.S. Fish and Wildlife Service (1980) 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 1.0 (opt ima'l ly suitable habitat).

Assumptions used to develop the HSI model and guidelines for model

applications, including methods for measuring model variables, are described.

This model is a hypothesis of species-habitat relations, not a statement

of proven cause and effect. The model has not been field tested, but it has

been applied to three hypothetical data sets that are presented and discussed.

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 on the great egret HSI model to the

following address.

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

PREFACE . . . . ACKNOWLEDGMENTS

INTRODUCTION

Distribution

Life History Overview

SPECIFIC HABITAT REQUIREMENTS

Food and Foraging Habitat

Cover . . . Water . Interspersion . Special Considerations .. HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Model Descriptions .... Suitability Index (SI) Graphs for Model Variables

Component Index Equations and HSI Determination

Field Use of Models

Interpreting Model Output.

ADDITIONAL HABITAT MODELS

REFERENCES

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iii

vi

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1

1

2

2

3

5

5

6

6

6

8

10

14

15

18

18

19
ACKNOWLEDGMENTS

Earlier versions of the habitat suitability index model and narrative for

the great egret were reviewed by Dr. R. Douglas Slack and Jochen H. Wiese. The

model's structure and functional relationships were thoroughly evaluated by

personnel of the U.S. Fish and Wildlife Service's (FWS) National Coastal

Ecosystems Team. Model and narrative reviews were also provided by FWS

Regional personnel.

vi
GREAT EGRET (Casmerodius albus)

INTRODUCTION

The great egret, also called common egret or American egret, is a large

white heron tn the order Ciconiiformes, family Ardeidae. Great egrets stand

94.0-104.1 cm (37-41 inches) tall and have a wing spread to 139.7 cm (55

inches) (Terres 1980). The species is associated with streams, ponds, lakes,

mud flats, swamps, ahd freshwater and salt marshes. The birds feed in shallow

water on fishes, amphibians, reptiles, crustaceans and insects (Terres 1980).

Distribution

The great egret is a common breeding species in all coastal areas south

from southern Oregon on the Pacific coast and from Maine on the Atlantic coast;

in riverine, palustrine and estuarine habitats along the coast of the Gulf of

Mexico; and in the Eastern-Central United States (Palmer 1962; Erwin and

Korschgen 1979; American Ornithologists' Union 1983). The great egret

undergoes an extensi ve postbreedi ng di spersa 1 that extends the range of the

species to most of the United States exclusive of the arid Southwest (Byrd

1978). Young birds hatched in Gulf coast colonies tend to move northward for

a short period (Byrd 1978; Ogden 1978). However, with the onset of colder

weather most great egrets and other herons migrate south and many winter along

the gulf coast in Texas, Louisiana, and Florida (Lowery 1974; Oberholser and

Kincaid 1974; Byrd 1978). Analysis of banding data indicates that many birds

winter in Cuba, the Bahamas, the Greater and Lesser Antilles, Mexico, and

Central America (Coffey 1948). Lowery (1974) suggested that during severe

winters, a higher proportion of the population winters farther south.

Life History Overview

Great egrets nest in mixed-species colonies that number from a few pairs

to thousands of individuals. A colony may include other species of herons,

spoonbills, ibises, cormorants, anhingas, and pelicans. Colony and nest-site

se 1ections begi n as early as December along the gulf coast, but most great

egrets do not initiate nesting activities until mid-February or early March

(Bent 1926; Oberho 1ser and Ki ncai d 1974; Chaney et a 1. 1978; Morri son and

Shanley 1978). Eggs have been recorded from March through early August, and

young have been observed in nests from mid-May through late August (Oberholser

and Kincaid 1974; Chaney et al. 1978). Clutch size varies from one to six eggs

per nest, but three to four eggs is most common (Bent 1926). Incubation period

in a Texas colony ranged from 23 to 27 days (Morrison and Shanley 1978). The

first fl ights of young have been noted about 42 days after hatching (Terres

1980) .

1
SPECIFIC HABITAT REQUIREMENTS

Food and Foraging Habitat

Fish constitute up to 83% of the great egret's diet (Hoffman 1978). Most

fish taken by great egrets are minnow-sized (less than 10 cm or 3.9 inches),

but fish up to 36 cm (14 inches) can be captured and swallowed (Willard 1977;

Schlorff 1978). Other major food items include insects, crustaceans, frogs,

and snakes, while small mammals, small birds, salamanders, turtles, snails, and

plant seeds are occasionally taken (Baynard 1912; Bent 1926; Hunsaker 1959;

Palmer 1962; Genelly 1964; Kushlan 1978b).

Little specific information exists on the food habits of various age

classes of great egrets. An adult great egret weighing 917 g (32.3 oz) (Palmer

1962) may require approximately 110 g (3.9 oz) of food per day (estimated by

using the wading bird weight-daily food requirement model proposed by Kushlan

1978b). Oa i ly food requi rements are undoubtedly hi gher duri ng the nesting

season when adults are feeding young (Kushlan 1978b).

Great egrets usually forage in open, calm, shallow water areas near the

margins of wetlands. They show no preference for fresh-, brackish, or

saltwater habitat. Custer and Osborn (1978a,b) found that feeding habitat

selection in coastal areas of North Carolina varied daily with the tidal cycle.

Ouri ng low tide, great egrets fed in estuari ne seagrass beds. Ouri ng hi gh

tide, freshwater ponds and the margins of Spartina marshes were used. Inland,

great egrets feed near the banks of rivers or lakes, in drainage ditches,

marshlands, rain pools (Bent 1926; Ousi et al. 1971; Kushlan 1976b), and

occasionally in grassy areas (Weise and Crawford 1974). Feeding sites are

generally not turbid and are fairly open with no vegetative canopy and few

emergent shoots (Thompson 1979b).

Great egrets forage singly, in single-species groups, and in mixed-species

associations (Kushlan 1978b). Great egrets generally fly alone to feeding

sites (Custer and Osborn 1978a,b) and may use the same feeding site repeatedly.

The density and abundance of fish at a given location in estuarine habitats may

vary with season, time of day, tidal stage, turbidity, and other factors. If

feeding success is low, great egrets may move to other areas (Cypert 1958;

Schlorff 1978) and join other conspecifics in good feeding habitats (Custer and

Osborn 1978a,b). Most instances of group feeding have been observed during

specific environmental conditions, such as lowered water levels, that tend to

concentrate prey (Kushlan 1976a,b; Schlorff 1978).

Meyerriecks (1960, 1962) and Kushlan (1976a, 1978a,b) provided detailed

information on hunting techniques employed by great egrets. The

II stand-and-waitll and IIslow-wadell methods are used most frequently. Because of

thei r long 1egs, great egrets can forage in somewhat deeper water than most

other herons. In New Jersey, foraging depths ranged from 0 (standing on the

bank whil e fi shi ng) to 28 cm (11 inches), but depths rangi ng from 10 cm to

23 cm (4 to 9 inches) were most commonly used (Willard 1977). In North

Carolina, great egrets fed in water with a mean depth of 25.1 cm (9.8 inches)

in Spartina habitat and of 17.4 cm (6.8 inches) in non-Spartina habitat

(Custer and Osborn 1978b). Mean wat~r depth was 20 cm (7.9 inches) for

2
foraging great egrets in California (Hom 1983). In addition to wading, great

egrets can feed by ali ght i ng on the surface of deep waters to catch prey, a

method rarely employed (Reese 1973; Rodgers 1974, 1975).

Although recent declines of great egret populations in the central coastal

region of Texas occurred simultaneously with declines in coastal marine and

estuarine fish populations (Chapman 1980), no causal relationship has been

proven. At present there are no known management practices that provide

suitable food alternatives for piscivorous species, such as the great egret,

during periods of fish population decline. Known fish nursery and feeding

areas need protection from destruction or habitat alteration to ensure adequate

prey populations for fish-eating birds.

Cover

Nesting. The great egret is a versatile nester, using trees, shrubs, and

ground sites in riparian forest, swamp, and island habitats. Most colony sites

in Texas are on natural or dredged-material islands, but several inland sites

are known (Chaney et al. 1978). Most colony sites in Louisiana are associated

with coastal fresh- or brackish water marshland (Portnoy 1978). In some cases,

great egrets successfully occupy artificial nesting structures (Wiese 1976).

Few colony sites are known that lack a substantial water barrier. Most inland

sites are in swamps where nest trees grow in water at least 0.6 m (2 ft) deep

during the breeding season (Meanley 1955; Wiese 1976). Such colony isolation

may be important to reduce predation (Taylor and Michael 1971) or other

disturbance.

Nest height varies with vegetation height, and nests within a mixedspecies heronry tend to be stratified vertically in an order that correlates

wi th speci es body 1ength (Burger 1978). Thus, great egret nests are usually

higher than the nests of all other species except the great blue heron (Ardea

herodia). Most great egret nests are situated near the top, but just below

the crown, of vegetation (Meanly 1955; Teal 1965; Pratt 1972; Girard 1976;

Wiese 1976; Maxwell and Kale 1977; Portnoy 1978; Thompson 1979; Beaver et al.

1980). Terres (1980) noted that nests were usually about 6.1-12.2 m (20-

40 ft) above the ground in medium sized trees. In coastal shrub-scrub

vegetation, mean nest heights of 2.8 m (9.2 ft) (Maxwell and Kale 1977) and

1.7 m (5.6 ft) (Beaver et al.) have been reported. McCrimmon (1978)

identified several additional characteristics of great egret nest placement

that differ from other species: great egrets nested in larger trees, closer

to the edge of the heronry, and in more open and accessible sites. Trees and

shrub species where great egrets in coastal Texas and Louisiana build nests

are listed in Table 1.

Because great egret nests are 1arge (0.6 m or 2 ft) in di ameter (Gi rard

1976), they are usually supported by several 1imbs that have a combined mean

diameter of 5.9 cm (2.3 inches) (McCrimmon 1978). Thus, suitable nest site

criteria may be related not only to available space, but also to minimum nest

support. If vegetation for suitable nest support is present, great egrets can

nest close to each other. Nearest nest distances of 1 m (3.3 ft) have been

found in densely packed colonies (Beaver et al. 1980).

3
Table 1. Scientific name, common name, and mean vegetation height of all

plants reported as nest species for great egrets in Texas and Louisiana.

Mean

height

Scientific name Common name (m) Reference

Acacia farnesiana Huisache 1 Goering and Cherry

1971

Acer rub rum Red maple 1 Taylor and Michael

1971

Avicennia germinans Black mangrove 1 Chaney et al. 1978

Baccharis halifolia Sea myrtle 1 Burger 1978

Chaney et al. 1978

Celtis lindheimeri Hackberry 5 Chaney et al. 1978

Cephalanthus occidentalis Buttonbush 7 Taylor and Michael

1971

Iva frutescens Marsh-elder 1 Chaney et al. 1978

Portnoy 1977

Nyssa sp. Tupelo 7 Portnoy 1977

Opuntia lindheimeri Prickly-pear 1 Chaney et al. 1978

Prosopis glandulosa Mesquite 2 Chaney et al. 1978

Salix nigra Black willow 5 Wiese 1976

Sambucus canadensis Common elder-berry 1 Chaney et al. 1978

Scirpus spp. Bulrush 1 Oberholser and

Kincaid 1974

Spartina patens Marshhay cordgrass 1 Chaney et al. 1978

Tamarix sp. Salt cedar 2 Burger 1978

Taxodium sp. Cypress 8 Simmons 1959

Zanthoxylum clava-herculis Tickle-tongue 5 Chaney et al. 1978

4
A1though great egrets usually nest in the crowns of trees and shrubs,

ground nests have been reported in Texas (Chaney et al. 1978) and elsewhere

(McCri mmon 1978). Ground nests are rare and usually found adjacent to a

heronry on an island; apparently, ground nesting occurs when there is a lack of

suitable nest sites in trees or shrubs in or near a dense colony.

Colony size of single-species or mixed-species heronries varies from four

nests in a single tree or shrub to several thousand nests scattered throughout

a heterogeneous vegetative association covering 6 ha (15 acres) or more

(Portnoy 1977; Chaney et al. 1978; Nesbitt et al. 1982). Great egret nests

tend to be clumped within a mixed-species heronry because their nest placement

requirements differ from other herons.

There is evidence that herons re-use colony sites (Custer and Osborn

1977). Repeated use of a site may depend upon several factors: (1) prior

(successful) experience at a site (Wiese 1978b); (2) the presence of other

herons, part i cul arly the great blue heron, whi ch begi ns nesting before the

great egret (Chaney et al. 1978); and (3) the remnants of old nests (Wiese

1976). Colony abandonment can result from the destruction of nest vegetation

(Wiese 1979) or from changes in feeding habitat (Custer et al. 1980). Human

disturbance and predation have also been implicated as factors contributing to

colony abandonment (Chaney et al. 1978).

Non-nesting. Great egrets roost nocturnally in communal sites when not

breeding. These sites are usually at the tops of tall trees in dense thickets

or in the tops of short trees on islands or over water (Bent 1926). Roosting

sites may be used for many years, and some may also be used for nesting. The

characteristics of roost sites are similar to the those of nest sites, but no

specific data have been published.

Water

The physiologic water requirement of great egrets is probably met during

feeding activities in aquatic habitats (Dusi et al. 1971). Water depth affects

the quantity, variety, and distribution of food and cover; great egret food and

cover needs are generally met between the shoreline and water 0.5 m (1.6 ft)

deep (Willard 1977).

Interspersion

Suitable habitat for the great egret must include (1) extensive shallow,

open water habitat from 10 to 23 cm (4 to 9 inches) deep (Willard 1977); (2)

food species present in sufficient quantity (Custer and Osborn 1977); and (3)

adequate nesting or roosting habitat close to feeding habitat. Most great

egrets at a colony in North Carolina flew less than 4 km (2.5 mi) from nesting

colonies (and presumably, from roosting sites) to feeding areas (Custer and

Osborn 1978a), but flight distances of up to 36 km (22.4 mi) have been recorded

in the floodplain of the Upper Mississippi River (Thompson 1979b).

Several heronries may be close together. Great egrets from one colony may

fly over or near an adjacent colony, but rarely feed in the same areas as

conspecifics from the adjacent colony (Thompson 1979b).

5
Special Considerations

Human disturbance and habitat alteration are the two factors considered

most responsi b1e for the decl i ne of the great egret throughout its range

(Custer and Osborn 1977; Portnoy 1977; Chaney et al. 1978; Chapman 1980).

Great egrets are sensitive to human disturbance and may abandon nests or entire

colonies as a result of human activity (Goering and Cherry 1971; Mendoza and

Ortiz 1974). Human presence in a colony may cause nest desertion, which leads

to high nestling mortality from exposure, predation, and accidents (Morrison

and Shanley 1978).

Traditional colony sites and nocturnal roosts should be preserved.

Secondary sites of similar ecologic constitution are also important. High

heron density within a colony may destroy nest vegetation by the effects of

guano buildup and, to a lesser extent, trampling (Wiese 1978a,b). When this

type of habitat destruction occurs, great egrets may pioneer adjacent suitable

sites.

HABITAT SUITABILITY INDEX (HSI) MODELS

Model Applicability

Geographic area. The habitat suitability index (HSI) models in this

report were developed for application in coastal wetland habitats in Texas and

Louisiana. Because there are few differences in habitat requirements along the

Atlantic coast, the remainder of the gulf coast, and inland sites in the

Southeastern United States, the HSI models may also be used to evaluate

potential habitat in those areas.

Season. These model s wi 11 produce HSI val ues based upon habitat

requirements of great egrets during the breeding season (February to August).

Because there is no apparent seasonal difference in feeding habitat preference

and because winter nocturnal roosts are similar to nesting sites, the HSI

models may also be used to evaluate winter habitat for the great egret.

Cover types. Great egrets nest on upland islands and in the following

cover types of Cowardin et al. (1979): Estuarine Intertidal Scrub-Shrub

wetland (E2SS), Estuarine Intertidal Forested wetland (E2FO), Palustrine

Scrub-Shrub wetland (PSS) (including deciduous and evergreen subclasses), and

Palustrine Forested wetland (PFO) (including deciduous and evergreen

subclasses). Great egrets may also feed in these wooded wetlands, but

preferred feeding areas may be anyone of a wide variety of wetland cover types

(Table 2).

Mi nimum habitat area. Mi nimum habitat area is defi ned as the mi nimum

amount of contiguous suitable habitat required before an area can be occupied

by a particular species. Specific information on minimum areas required by

great egrets was not found in the 1iterature. If 1oca 1 i nformation is

available to define the minimum habitat area, and less than this amount of

area is available, the HSI for the species will be zero.

6
Verification level. The output of these HSI models is an index between 0

and 1.0 that is believed to reflect habitat potential for great egrets. Two

biologists reviewed and evaluated the great egret HSI model throughout its

development: Dr. R. Douglas Slack, Texas A&M University, College Station, and

Jochen H. Wiese, Environmental Science and Engineering Company, Gainesville,

Florida. Their recommendations were incorporated into the model-building

effort. The authors, however, are responsible for the final version of the

models. The models have not been field-tested.

Table 2. Great egret feeding habitat types. Classification follows Cowardin

et al. (1979).

System Subsystem Class Abbreviation

Estuarine Intertidal Aquatic Bed E2AB

Emergent E2EM

Forested E2FO

Stream Bed E2SB

Scrub-Shrub E2SS

Unconsolidated Shore E2US

Riverine Tidal Aquatic Bed RIAB

Emergent RIEM

Unconsolidated Bottom RIUB

Unconsolidated Shore RIUS

Lower Aquatic Bed R2AB

Perennial Emergent R2EM

Unconsolidated Bottom R211B

Unconsolidated Shore R2US

Intermittent Stream Bed R4SB

Lacustrine Littoral Aquatic Bed L2AB

Emergent L2EM

Unconsolidated Bottom L2UB

Unconsolidated Shore L2US

Palustrine Aquatic Bed PAB

Forested PFO

Emergent PEM

Scrub-Shrub PSS

Unconsolidated Bottom PUB

Unconsolidated Shore PUS

7
Model Descriptions

Overview. Separate HSI models were developed to evaluate great egret

feedi ng and nesting habi tats. No attempt was made to integrate these two

models into a single, overall habitat model for the following reasons. As

noted previously, most great egrets fly less than 4 km (2.5 mi) from nesting or

roosting sites to feeding areas, but they may travel up to 36 km (22.4 mi).

HSI models are intended primarily for use in impact assessment and may be

applied in relatively small study areas. The study area for great egret mayor

may not contain both feeding and nesting cover types, and great egrets may use

habi tat outsi de the study area boundari es. An HSI mode1 i ntegrati ng food and

nesting requirements may assign a low or no value to an area with cover types

that supply only one of these requirements when the remaining requirement is

met outside the area. Similarly, a single HSI model would downgrade the value

of an area that had high-quality nesting habitat and where birds were bypassing

low-quality feeding sites to use higher quality feeding sites outside the

area. Separate models that evaluate potential feeding or potential nesting

habitat quality avoid problems of the type outlined above and retain

simplicity in model application. The relationships of habitat variables to

the feeding and nesting HSI values are illustrated in Figure 1.

Feeding HSI model. Great egret feeding habitat suitability is related to

prey availability. Habitat suitability is optimal when two conditions are

met: (1) the populations of minnow-sized fish are high; and (2) shallow open

water (necessary for successful prey capture), aquat i c vegetation (necessary

for prey survival and reproduction), and deeper water are present in a ratio

that maximizes prey density and minimizes hunting interference. Use of this

model assumes that deep or permanent water environments are not limiting in

coasta1 habitats and that fi sh popul ations are di stri buted uniformly. Because

great egrets hunt a variety of species in many different habitat types, a

general approach to modeling feeding habitat suitability is presented.

Suitability of all wetland cover types for feeding is determined by integrating

two factors: (1) the abundance of prey and (2) the accessibility of prey.

The abundance of prey is determi ned by the abi 1i ty of the habi tat to

support the major prey species, especially minnow-sized fish. It is assumed

that the abundance of major prey species is related to the primary and

secondary productivity of the aquatic habitat; however, few field studies have

documented this relationship. The model assumes that prey abundance is not

limiting in coastal habitats. Therefore, the accessibility of prey is used as

the indicator of feeding habitat suitability.

The accessi bi 1i ty of prey is determi ned by water depth and percentage

cover of aquatic vegetation. A wetland with 100% of its area covered by water

10-23 cm (4-9 inches) deep is assumed to be optimal for feeding by great egrets

(VI)' Although an absence of submerged or emergent vegetation would render

fish species most vulnerable to capture, it is unlikely that many prey species

would use such an area because it totally lacks cover. The model assumes,

therefore, that optimal conditions for both the occurrence and susceptibility

to capture of prey speci es exist when 40%-60% of the wetland substrate is

covered by submerged or emergent vegetation (V2 ) . When such vegetation is

8
Habitat variable Component

Vi Percentage of area with water

10-23 cm deep.

V2 Percentage of submerged or

emergent vegetation cover

in zone 10-23 cm deep.

Food HSI

(Feeding)

V3 Percentage of island covered by Cover

woody vegetation ~1 m in height. (Island site)

V4 Mean water depth in wooded - _ wetlands.

V6 Distance to road or dwelling.

site)

Disturbance

Vs Mean height of woody vegetation.

V7 Distance to human disturbance

other than road or dwelling.

I.D

Figure 1. Relationships of habitat variables and components to the separate HSI models for great

egret feeding and nesting habitats.
1acki ng, the habitat has a low value for feedi ng great egrets because small

fi sh may use unvegetated water that is too shallow for thei r 1arger aquatic

predators.

Nesting HSI model. The suitability of potential nesting sites for great

egrets is determined by two factors: cover and disturbance (Figure 1). In

this model, ground nesting is not considered because it involves few

individuals, occurs in proximity to "normal" colonies, and may reflect a

response to overcrowding rather than site preference.

Cover for nesting great egrets depends on vegetation characteristics and

the presence of water barri ers. On is 1ands surrounded by deep or wide water

barri ers, great egrets nest ina wide vari ety of habi tats rangi ng from low

(1 m, or 3.3. ft) shrubs or grasses with dense canopies, to tall trees.

Therefore, cover suitabil ity of nesting habitat on islands is assumed to be

related to the percentage of the island area having woody vegetation equal to

or exceeding 1 m (3.3 ft) in height (V3 ) . Optimal habitat is present when 60%

or more of the island supports woody vegetation equal to or exceeding 1 m in

height. For the application of this model, islands are defined as sites less

than 5 ha (12.4 acres) and completely surrounded by open water. Islands can be

either along the coast or located inland in freshwater habitats.

In any given area, some or all of the great egret population may nest in

non-island sites even though island habitats with suitable cover types are

available. Non-island nest sites are found in shrubs or trees in seasonally

(duri ng the great egret nesting season) or permanently flooded areas such as

the Estuarine Intertidal and Palustrine Scrub-Shrub and Forested wetlands

(Cowardin et al. 1979). For such areas, the model assumes that nesting

suitability varies with water depth (V4 ) and that a water depth of 0.6 m (2 ft)

or more reduces access by potential predators and represents optimal conditions

for nesting. The mean height of woody vegetation in non-island sites must

exceed 7 m (23 ft) to be optimal for nesting by great egrets (Vs).

Great egrets are sens i t i ve to di sturbance from humans and predators,

especially during the breeding season. Boating and other water activities do

not disturb nesting great egrets if they occur 50 m (164 ft) or more from the

colony, the noise level is normal (no horns or other loud noises), and no

humans walk in or near the colony. No colonies are known to occur within

0.5 km (0.3 mi) of a roadway or human dwelling. The model assumes that as the

distance from human disturbance increases, the suitability of a site also

increases. Sites 0.5 km (0.3 mi) or closer to a roadway or dwelling (V6 ) are

unsuitable for nesting by great egrets. Optimal sites must be at least 50 m

(164 ft) from a channel or other potential source of human disturbance (V7 ) .

Suitability Index (SI) Graphs for Model Variables

This section provides graphic representation of the relationship between

habitat variables and habitat suitability for the great egret in wetland (see

Table 2 for abbreviations) and upland (U) cover types. The SI values are read

directly from the graph (1.0 = optimal suitability, 0.0 = no suitability) for

each variable. Assumptions used in developing the SI graph for each variable

appear in Table 3.

10
Table 3. Data sources and assumptions for great egret suitability indices.

Variable and source

VI Willard 1977

Vz Willard 1977

V3 Chaney et al. 1978

Portnoy 1978

V4 Meanley 1955

Wiese 1976

Vs Pratt 1972

McCrimmon 1978

Wiese 1978b

Beaver et al. 1980

Assumption

Prey is most accessible in water

depths of 10-23 cm (4-9 inches).

Substrates with 40%-60% coverage of

emergent or submerged vegetation

provide the optimum balance between

cover for prey species and

vulnerability of prey to capture by

great egrets.

Suitability of nesting/roosting

habitat on islands is positively

related to the percentage canopy cover

of woody vegetation > 1 m (3.3 ft)

tall. -

Optimal nesting habitat for non-island

sites is found when mean water depth

beneath the woody vegetation is equal

to or deeper than 0.6 m (2 ft).

Suitability of nesting/roosting

habitat on non-island sites increases

with vegetation canopy height; optimum

mean height equals or exceeds 7 m

(23 ft).

Human disturbance is detrimental to

great egret nesting/roosting. Optimal

habi tat occurs where the nearest road

or dwelling is 0.5 km (0.3 mile) or

farther from the site.

The optimal distance from potential

nesting/roosting sites to disturbance

other than roads or dwe 11 i ngs exceeds

50 m.

aThese variables are not discussed in the literature on the great egret; they

were derived from general discussions in Thompson (1979) and Rodgers and Burger

(1982), from personal observations, and from results of other colonial seabird

studies.

11

,
The SI graphs are based on the assumption that the sui tabi 1i ty of a

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

surface. Although there may be interdependencies and correlations between many

habitat variables, the model assumes that each variable operates independently

over the range of other variables under consideration. Habitat abbreviations

are defined in Table 2.

Habitat Variable Sui tabil ity Graph

1.0 All habitats VI Percentage of study

in Table 2. area with water 10-

23 cm deep. In tidal )( 0.8

use depth at CD

areas, "C

mean low tide. In c

- 0.6 nontidal areas, use >.

average summer condi- :=

tions. ,g 0.4 tU

::

::::l

en 0.2

0.0

0 20 40 60 80 100

%

20 40 60 80 100

%

0.0 -,.....,..."T"""'..,......--r--.,.....~--.,.....,......'""

o

V2 Percentage of sub- 1.0

strate in zone 10-

23 cm deep covered )( 0.8

CD by submerged or emer- "C

gent vegetation. c

- 0.6 >.

::

-,g 0.4 tU

::

::::l

en 0.2

All habitats

in Table 2.

12
Habitat Variable Suitability Graph

)( 0.8

CD

'tJ

C

- 0.6

>-

0.4

en 0.2

0.0

0 20 40 60 80 100

%

1.

)(

CD

'0

C

>- :!::

-

J:l 0.4 as

-

en 0.2

0.0

0 0.2 0.4 0.6 0.8 1.0

Depth (rn)

Percentage of island

covered by woody vegetation 1 m in height.

V4 Mean water depth

in wooded wetlands.

E2SS,E2FO,U

E2SS, E2FO,

PSS, PFO

E2SS, E2FO,

PSS, PFO

Vs Mean height of woody

vegetation. )(

CD

'0

C

-

O.O....._...,.....--p--p--p-.,...,-.,-.,..

o 2.0 4.0 6.0 8.0 10.0

Height (rn)

13
Habitat Variable Suitability Graph

E2SS, E2FO,

PSS, PFO, U

Ve Distance to road or

dwelling.

)(

Q)

"

c

-

0.8

0.6

0.4

0.2

0.4 0.6 0.8 1.0 1.2

Distance (km)

O.O-t-.,......""""'-.,.....,..__~..,-~

0.2

0.6

0.8

1.CMI--l--............_ .......- .........

O.(HI-...-----.,...........-..,..

)(

Q)

"

.5

>-

:!::

-.a 0.4

as

'5-

rn 0.2

Distance to human

disturbance other than

road or dwelling.

E2SS,E2FO,

PSS, PFO, U

o 10 20 30 40 50 60

Distance (m)

Component Index Equations and HSI Determination

The following equations are suggested for combining individual variable SI

values into component indices and for obtaining the final HSI value. The HSI

for feeding (or nesting) habitat is set at 0 if no cover type suitable for

nesting (or feeding) can be located within 36 km (22.4 mi) of the study area.

14
Feeding HSI.

Component

Food (F)

HSI=F

Nesting HSI

Component

Cover, islands (C.) 1

Equation

SIV1 + SIV2

2

Cover, non-islands (C ) (SI v X SIV5)~ n 4

Disturbance (D) (SI V6 x SIV7)~

HSI (Islands) = C. or 0, whichever is lower.

1

HSI (Non-islands) = C or 0, whichever is lower.

n

Data representing three hypothetical

to calculate sample HSI values (Table 4).

to refl ect the potential of the areas

egrets.

Field Use of Models

study areas for great egret were used

The HSI values obtained are believed

to support feedi ng or nesting great

The level of detail needed for application of these models will depend on

time, money, and accuracy constraints. Detailed field sampling of all

variables will provide the most reliable and replicable HSI values. Any or all

variables can be estimated to reduce the amount of time or money required to

apply the models. Increased use of the subjective estimates decreases

reliability and replicability, and these estimates should be accompanied by

appropri ate documentation to insure that deci s i onmakers understand both the

method of HSI determination and quality of data used in the model. Techniques

for measuring habitat variables included in the great egret HSI models are

suggested in Table 5.

A project area may contain both potential feeding and nesting habitat. To

decrease the cost and time necessary to evaluate the area, assume that food is , not limiting and apply only the nesting HSI model. This recommendation is based

upon the following assumptions: (1) in most coastal areas of Texas and

Louisiana, aquatic habitats suitable for feeding are abundant and are,

therefore, less of a limiting factor to great egrets than are suitable nesting

sites; and (2) nesting value is easier and more accurately estimated by using

subjective methods than is food value. The variables used to measure food

15
Table 4. Calculations of suitability indices (5I), component indices, and

habitat suitability indices (H5I) for three sample data sets using habitat

variable (V) measurements and the great egret H51 model equations.

Model

element

Data set 1

Data 51

Data set 2

Data 51

Data set 3

Data 51

Variables

VI 60% 0.60

V2 90% 0.25

V3 75% 1.0

V4 0.27 m 0.45

Vs 6 m 0.86

V6 1.0 km 1.0 0.75 km 0.5

V7 25 m 0.38 50 m 1.0

Component indices

F 0.43

C. 1.0

1

C 0.62

n

D 0.61 0.71

H51

Feeding 0.43

Nesting 0.61 0.62

value are more indirect than those

reflects the difficulties involved

distribution, and prey accessibility.

used

with

to measure nesting value.

measuring prey abundance,

This

prey

A major assumption of the nesting H51 model is that all habitat areas with

appropriate cover types have some potential value to great egrets. However, it

is difficult to assess this potential because of two factors: (1) traditional

16
use of past colony sites, and (2) the enhancement of a site by the presence of

other herons. These two factors are usually, but not always, interrelated.

Great egrets tend to use the same colony site in successive years until the

site is degraded, and the site may i ncl ude great blue herons. When applyi ng

the HSI model, the user should be aware that an area known to be used by great

egrets (or great blue herons) is more likely to be used in future years than an

area with an equal HSI value not known to have a history as a colony site.

Table 5. Suggested measurement techniques for habitat variables used in the

great egret HSI models.

Variable Suggested technique

The percentage of the area with water 10-23 cm (4-9 inches)

deep can be determined by line transect sampling of water

depth.

in the 10-23 cm (4-9 inches)

by submerged or emergent

from available cover maps,

transect sampling.

The percentage of substrate

water depth zone covered

vegetati on can be determi ned

aerial photographs, or by line

The percentage of an is1and covered by woody vegetation

> 1 m (3.3 ft) in height can be determined by measuring the

height of randomly selected vegetation with a hypsometer or

altimeter (Hays et al. 1981).

Mean water depth beneath woody vegetation on non- i s1and

sites can be determined by line transect sampling of water

depth.

Mean height of woody vegetation on non-island sites can be

measured by using a hypsometer or altimeter (Hays et al.

1981) on randomly selected vegetation.

Distance to disturbance can be measured on maps or aerial

photographs. The disturbance location should be marked and

the straight line distance measured from the disturbance to

the middle of the potential nest/roost site.

If two or more distinct units of either potential feeding or nesting

habitat are present within the project evaluation area, a single feeding or

nesting index value for the project may be obtained by weighting the HSI of

each unit by its area. When a weighted HSI is desired, the following equation

should be applied:

,

17
n

HSI =L

i=1

HSI.A.

, ,

LA.

,

where n=number of distinct units of habitat

HSI.=HSI of unit.

A.=Area of unit.'

, ,

Interpreting Model Output

The HSI value obtained by applying the great egret models may have no

relationship to actual population levels. Great egret population levels may be

determined by nonhabitat factors, such as competition and predation, excluded

from the models. Model outputs can be used, however, to compare the potential

of two areas to support feeding or nesting great egrets at a single point in

time. HSI values can also be used to compare the potential of a single area to

support great egrets at future points in time.

ADDITIONAL HABITAT MODELS

No other habitat model for the great egret was located.

18
REFERENCES

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Baynard, O.E. 1912. Food of herons and ibises. Wilson Bull. 24:167-169.

Beaver, D.S., R.G. Osborn, and T.W. Custer. 1980. Nest-site and colony characteristics of wading birds in selected Atlantic coast colonies. Wilson

Bull. 92:200-220.

Bent, A.C. 1926. Life histories of North American marsh birds. U.S. Natl.

Mus. Bull. 135:1-392.

Blacklock, G.W., D.R. Blankinship, S. Kennedy, K.A. King, R. T. Paul, R.D.

Slack, J.C. Smith and R.C. Telfair II. 1978. Texas colonial waterbird

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108 pp.

Burger, J. 1978. Pattern and mechanism of nesting in mixed-species heronries.

Pages 45-58 A. Sprunt IV, J.C. Ogden, and S. Winckler, eds. Wading

birds. Natl. Audubon Soc. Res. Rep. 7.

Byrd, M.A. 1978. Dispersal and movements of six North American Ciconiiformes.

Pages 161-185 in A. Sprunt IV, J.C. Ogden, and S. Winckler, eds. Wading

birds. Natl. Audubon Soc. Res. Rep. 7.

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

wading birds in Texas. Tech. Rep. 0-78-8. U.S. Army Engineer Waterways

Exp. Stn., Vicksburg, Miss. 170 pp.

Chapman, B. R. 1980. Current status of colonial waterbird populations on the

Texas coast. Pages 14-18 in J.F. Parnell, and R.F. Soots, Jr., eds.

Management of colonial waterbirds. Univ. N.C. Sea Grant Publ. UNC-SG80-06, Wilmington.

Coffey, B.B., Jr. 1948. Southward migration of herons. Bird-Banding 19:1-5.

Cowardin, L.M., V. Carter, F.C. Go1et, and E. T. LaRoe. 1979. Classification

of wetlands and deepwater habitats of the United States. U.S. Fish Wi1d1.

Servo Bio1. Servo Program FWS/OBS-79/31. 103 pp.

Custer, T.W., and R.G. Osborn. 1977. Wading birds as biological indicators:

1975 colony survey. U.S. Fish Wildl. Servo Spec. Sci. Rep. Wildl. 206.

28 pp.

Custer, T.W., and R.G. Osborn. 1978a. Feeding habitat use by co l on i al lybreeding herons, egrets, and ibises in North Carolina. Auk 95:733-743.

Custer, T.W., and R.G. Osborn. 1978b. Feeding-site description of three heron

19
species near Beaufort, North Carolina. Pages 355-360 in A. Sprunt IV,

J.C. Ogden, and S. Winckler, eds. Wading birds. Natl. Audubon Soc. Res.

Rep. 7.

Custer, T.W., R.G. Osborn, and W.F. Stout. 1980. Distribution, species abundance, and nesting site use of Atlantic coast colonies of herons and their

allies. Auk 97:591-600.

Cypert, E. 1958. The relation of water level to populations of common egrets

in the Okefenokee Swamp. Oriole 23:9.

Dusi, J. L., and R. T. Dusi. 1968. Ecological factors contributing to nesting

failure in a heron colony. Wilson Bull. 80:458-466.

Dusi, J.L., R.T. Dusi, and D.L. Bateman. 1971. Water relations of wading

birds. Pages 99-106 in J.L. Dusi, R.T. Dusi, D.L. Bateman, C.A. McDonald,

J.J. Stuart, and J.F.-Desmukes, eds. Ecologic impacts of wading birds on

the aquatic environment. Water Resour. Res. Inst. Bull. 5, Auburn Univ.,

Auburn, Ala.

Erwin, R.M., and C.E. Korschgen. 1979. Coastal waterbird colonies: Maine to

Virginia, 1977. An atlas showing colony locations and species

composition. U.S. Fish Wildl. Servo Biol. Servo Program FWS/OBS-79/08.

647 pp.

Genelly, R.E. 1964. Common egret preys on meadowlark. Condor 66:247.

Girard, G. T. 1976. Reproductive parameters for nine avian species at Moore

Creek, Merritt Island National Wildlife Refuge. Kennedy Space Cent. Tech.

Rep. 3:1-59.

Goering, O.K., and R. Cherry. 1971. Nestling mortality in a Texas heronry.

Wilson Bull. 83:303-305.

Hays, R.L., C.

variables.

pp.

Summers, and W. Seitz. 1981. Estimating wildlife habitat

U.S. Fish Wildl. Servo Biol. Servo Program FWS/OBS-81/47. III

Hoffman, R.D. 1978. The diets of herons and egrets in southwestern Lake

Erie. Pages 365-369 in A. Sprunt IV, J.C. Ogden, and S. Winckler, eds.

Wading birds. Natl. Audubon Soc. Res. Rep. 7.

Hom, C.W. 1983.

sa It marsh.

Foraging ecology of herons in a southern San Francisco Bay

Colonial Waterbirds 6:37-44.

Hunsaker, D., II. 1959. Stomach contents of the American Egret, Casmerodius

albus, in Travis County, Tex. Tex. J. Sci. 11:454.

Kushlan, J.A.

86-94.

1976a. Feedi ng behavi or of North Ameri can herons. Auk 93:

Kushlan, J.A. 1976b. Wading bird predation in a seasonally fluctuating pond.

Auk 93:464-476.

20
Kushlan, J.A. 1978a. Nonrigorous foraging by robbing egrets. Ecology 59:

649-653.

Kushlan, J.A. 1978b. Feeding ecology of wading birds. Pages 249-297 in A.

Sprunt IV, J.C. Ogden, and S. Winckler, eds. Wading birds. Natl. Audubon

Soc. Res. Rep. 7.

Lowery, G. H., Jr.

Baton Rouge.

1974. Loui s i ana bi rds.

651 pp.

Louisiana State University Press,

Maxwell, G.R., II, and H.W. Kale II. 1977. Breeding biology of five species

of herons in coastal Florida. Auk 94:689-700.

McCrimmon, D.A., Jr. 1978. Nest-site characteristics among five species of

herons on the North Carolina coast. Auk 95:267-280.

Meanl ey, B.

Arkansas.

1955. A nesting study of

Wilson Bull. 67:84-99.

the little blue heron in eastern

Mendoza, C.H., and R. Ortiz. 1974. Anatomical and vegetational features of

spoi 1 banks vs. thei r uti 1i zati on by bi rds: Upper Laguna Madre of Texas.

Unpubl. M.S. Thesis. Texas A&I University, Kingsville. 180 pp.

Meyerriecks, A.J. 1960. Comparative breeding behavior of four species of

North American herons. Publ. Natl. Ornithol. Club 2.

Meyerri ecks, A.J. 1962. Diversity typifi es heron feedi ng. Nat. Hist. 71:

45-59.

Morrison, M. L., and E. Shanley, Jr. 1978. Breeding success of great egrets

on a dredged material island in Texas. Bull. Tex. Ornithol. Soc. 11:17-18.

/

Nesbitt, S.A., J.C. Ogden, H.W. Kale, II, B.W. Patty, and L.A. Rowse. 1982.

Florida atlas of breed-ing sites for herons and their allies: 1976-78.

U.S. Fish Wildl. Servo Biol. Servo Program. FWS/OBS-81/49. 449 pp.

Oberholser, H.C., and E.B. Kincaid, Jr. 1974. The bird life of Texas. Vol.

1. University Texas Press, Austin.

Ogden, J.C. 1978. Recent population trends of colonial wad-ing birds on the

Atlantic and Gulf Coastal Plains. Pages 137-153 in A. Sprunt IV, J.C.

Ogden, and S. Winckler, eds. Wading birds. Natl. Audubon Soc. Res. Rep. 7.

Palmer, R.S. 1962.

Press, London.

Handbook of North American birds.

567 pp.

Vol. 1. Yale Univ.

Portnoy, J.W. 1977. Nesting colonies of seabirds and wading birds - coastal

Louisiana, Mississippi, and Alabama. U. S. Fish Wi ldl. Servo Biol. Servo

Program. FWS/OBS-77/07. 126 pp.

Portnoy, J.W. 1978. Colonial waterbird population status and management on

the north Gulf of Mexico coast. Proc. 1977 Conf. Colon. Waterbird Group

1:38-43.

21
Pratt, H.M. 1972. Nesting success of common egrets and great blue herons in

the San Francisco Bay region. Condor 74:447-453.

Reese, J.C.

egrets.

1973. Unusual feeding behavior of great blue herons and common

Condor 75:352.

Rodgers, J.A., Jr. 1974. Aerial feeding by snowy and great egrets in

Louisiana waters. Wilson Bull. 86:70-71.

Rodgers, J.A., Jr. 1975. Additional observation on hover-feeding by North

American ardeids. Wilson Bull. 87:420.

Rodgers, J.A., Jr., and J. Burger.

human disturbance and colonial

1981. Concluding remarks: symposium on

waterbirds. Colon. Waterbirds 4:69-70.

Schlorff, R.W. 1978. Predatory ecology of the great egret at Humboldt Bay,

California. Pages 347-353 in A. Sprunt IV, J.C. Ogden, and S. Winckler,

eds. Wading birds. Natl. Audubon Soc. Res. Rep. 7.

Simmons, E.M.

egrets.

1959. Observations on effects of cold weather on nesting common

Auk 76:239-241.

Taylor, R.J., and E.D. Michael. 1971. Predation on an inland heronry in

eastern Texas. Wilson Bull. 83:172-177.

Teal, J.M. 1965. Nesting success of egrets and herons in Georgia. Wilson

Bull. 77:257-263.

Terres, J.K. 1980. The Audubon Society encyclopedia of North American birds.

Alfred A. Knopf, New York. 1109 pp.

Thompson, D.H. 1979a. Declines in populations of great blue herons and great

egrets in five Midwestern States. Proc. 1978 Conf. Colon. Waterbird Group

2:114-127.

Thompson, D.H. 1979b. Feeding areas of great blue herons and great egrets

nesting within the floodplain of the upper Mississippi River. Proc. 1978

Conf. Colon. Waterbird Group 2:202-213.

Wiese, J.H. 1976. Courtship and pair formation in the great egret. Auk 93:

709-724.

Wiese, J.H. 1978a. A study of the reproductive biology of herons, egrets,

and ibis nesting on Pea Patch Island, Delaware. Manomet Bird Observatory,

Manomet, Mass. 156 pp.

Wiese, J.H. 1978b. Heron nest-site selection and its ecologic effects. Pages

27-34 in A. Sprunt IV, J.G. Ogden, and S. Winckler, eds. Wading birds.

Natl. Audubon Soc. Res. Rep. 7.

Wiese, J.H. 1979. A study of the reproductive biology of herons, egrets and

ibis nesting on Pea Patch Island, Delaware. Delmarva Power and Light Co.,

Wilmington, Del. 255 pp.

22
Weise, J.H.,

ardeids.

and R.L. Crawford.

Auk 91:836-837.

1974. Joint 11 eap-froggi ng I feeding by

Willard, D.E. 1977. The feeding ecology and behavior of five species of

herons in southeastern New Jersey. Condor 79: 462-470.

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

ESM102 U.S. Fish and Wildlife Service, Washington, D.C. n.p.

23
, 2.

S. Re po rt : ate

'Gi

(C)

10. Proe c- r es..... nark Un,t '0.

3. Reclp,e--: s ~C-":."!."is'on r'6." I

--~ ___ ~~£.~~m~er l~§..4- j 6.

REPORT DOCUMENTATIO~ Ie-REPORT NO.

PAGE F~?(0_~S-~?110. 7?-

[ 4. T;;'''-~;;:O S JCi,,,.

17:~:~.t,at.~u~~a~~~itY Index MOd~l_s:_Great Egret

I B.R. Chapmans and R.J. Howard b

r9. AuThorTsJFffTHaHon's--- . -----.--

I Dept. of Biologya U.S. Fish and Wildlife Serviceb

, Corpus Christi State Univ. National Coastal Ecosystems Team

Corpus Christi, TX 78412 1010 Gause Blvd.

Slidell, LA 70458

-_._-----------------

1010 Gause Blvd.

Slidell, LA 70458

- --_._-~----- -- ----_.._---

I

, 12U ~~o:.,.o~~~ ~'g~n~Z~l,~ ir;a~~ ;~deAd~;~'V ice

Division of Biological Services

I National Coastal Ecosystems Team

11S. s~~p.-;n;nta;;-N,;",~,-

I

j

!

.-

13. Type -:' Re oo-t Pence ~.o"er~:

14.

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

great egret (Casmerodius albus). The model is scaled to produce an index of habitat

suitability between 0 (unsuitable habitat) and 1.0 (optimally suitable habitat) for

coastal wetland areas along the gulf and Atlantic coasts. Habitat suitability indices

are designed for use with the habitat evaluation procedures previously developed by the

U.S. Fish and Wildlife Service. Guidelines for great egret model applications and

techniques for measuring model variables are described.

j 15. A.~s.~ra:~ (:...~rTl;t: 2'.}J woeds i

I

I

] 7. Document ItnalY'iois a. De scr.otors

Mathematical models

Wildlife

Birds

b. ldent.f.e rs zOpe n-Ende d Terms

Habitat

Habitat suitability index

Impact assessment

Great egret

Common egret

Casmerodius albus

c. COSATI F:eld/Group

20. Sec crity Class (This P.J!te) ZZ. Price

Unclassified

18. k ..euae.t.tv Statement

Unlimited

19. Se currt y Cf ass nh.s Report)

Unclassified :::1. No. of :Ltf.'~S

vi + 23

(See ANSI-Z3~.:Sl OPi;ONAL F~-;~'" 27: 4-71\

rF::-'~erly !\l- s-z s

De c a rtment c- ::)m,.....t~·.:e
;

-,-, ... ..

'.

'* Headquarters , 01'''1510n of Biological

Services. Wasnongton . DC

)( Eastern Energy and Land Use Team

Leetown. WV

* Naloonal Coastal Ecosys tems Team

Slidell LA

• Western Energy aM LaM Use Team

FI Collins. CO

• Locations 01 Regional otnces

UGION 1

Rqiunlll Director

O.S. Fish lind Wildlife Service

Uoyd Five Hundred Building, Suite 1692

500N.E. Multnomah Street

Portland, Oregon 97232

REGION 4

Regional Director

U.S. Fish and Wildlife Service

Richard B. Russell Building

75 Spring Street, S.W.

Atlanta, Georgia 30303

,

,

J __r----

61

,----- L-, J_

1

I : ,--- I I

I

REGION 2

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 1306

Albuquerque, New Mexico 87103

REGION S

Regional Director

U.S. Fish and Wildlife Service

One Gateway Center

Newton Comer, Massachusetts 02158

REGION 7

Regional Director

U.S. Fish and Wildlife Service

10II E. Tudor Road

Anchorage, Alaska 99503

.--. . - 0 ,--,0.

Vtlg,n ISlands

REGION 3

Regional Director

U.S. Fish and Wildlife Service

Federal Building, Fort Snelling

Twin Cities, Minnesota 5SIII

REGION 6

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 25486

Denver Federal Center

Denver, Colorado 80225
DEPARTMENT OF THE INTERIOR

u.s. FISH AND WILDLIFE SERVICE

As the Nation's principal conservation asency, the Department of the Interior has responsibility for most of our .nationally owned public landl and n.tural resources. This includes

fosterins the wisest use of our land and water resources, protectins our fish and wildlife,

preservins th.environmental and cUltural'varun of our national parks and historical places,

and providing for the enjoyment of life throulh outdoor recreation. The Department aisesses our energy and mineral resources and works to assure that their development is in

the best interests of all our people. The Department also has a major responsibility for

American Indian reservation communities and for people who live in island territories under

U.S. administration.