Library

Natio a1 Wetland Researc Center

U. S. 1"1 hand Wlidllfc ervlce

7UO CaJ ndomc Bouleva d

Lafayette. La. 70506

BIOLOGICAL REPORT 82(10.100)

SEPTEMBER 1985

HABITAT SUITABILITY INDEX MODELS:

GADWALL (BREEDING)

,h and Wildlife Service

). Department of the Interior SK

361

. U54

no. 82-

10.100
This is one of the first reports to be published in the new "Bi ol ogi ca l

Report" series. This technical report series, published by the Rese arch

and Development branch of the U.S. Fish and Wlldlife Service, replaces

the IlFWS/OBS" series published from 1976 to September 1984 . The Biolog -

ical Report series is designed for the rapid publication of reports with

an application orientation, and it continues the focus of the FWS/OBS

series on resource management issues and fish and wlldlife needs.
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. However, it is impossible to develop a model that

performs equally well in all situations. Assistance from users and

researchers is an important part of the model improvement process. Each

model is published individually to facilitate updating and reprinting as

new information becomes available. User feedback on model performance

will assist in improving habitat models for future applications. Please

complete this form 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 also would appreciate information on

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

models or test results. Please return this form to:

Habitat Evaluation Procedures Group

U.S. Fish and Wildlife Service

2627 Redwing Road, Creekside One

Fort Collins, CO 80526-2899

Thank you for your assistance.

Species

Geographic

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 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(10.100)

September 1985

HABITAT SUITABILITY INDEX MODELS: GADWALL (BREEDING)

by

Patrick J. Sousa

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

Drake Creekside Building One

2627 Redwing Road

Fort Collins, CO 80526-2899

Western Energy and Land Use 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:

Sousa, P. J. 1985. Habitat suitability index models: Gadwall (breeding).

U.S. Fish Wildl. Servo Biol. Rep. 82(10.100). 35 pp.
PREFACE

This document is part of the Habitat Suitability Index (HSI) Model Series

[Biological Report 82(10)J which provides habitat information useful for impact

assessment and habitat management. Several types of habitat information are

provided. The Habitat Use Information Section is largely constrained to those

data that can be used to derive quantitative relationships between key environmental variables and habitat suitability. This information provides the

foundation for the HSI model and may be useful in the development of other

models more appropriate to specific assessment or evaluation needs.

The HSI Model Section documents the habitat model and includes information

pertinent to its application. The model synthesizes the habitat use information into a framework appropriate for field application and is scaled to

produce an index value between 0.0 (unsuitable habitat) and 1.0 (optimum

habitat). The HSI Model Section includes information about the geographic

range and seasonal application of the model, its current verification status,

and a list of the model variables with recommended measurement techniques for

each variable.

The model is a formalized synthesis of biological and habitat information

published in the scientific literature and may include unpublished information

reflecting the opinions of identified experts. Habitat information about

wi ldl ife species frequently is represented by scattered data sets collected

during different seasons and years and from different sites throughout the

range of a speci es. The model presents thi s broad data base ina formal,

logical, and simplified manner. The assumptions necessary for organizing and

synthesizing the species-habitat information into the model are discussed.

The model should be regarded as a hypothesis of species-habitat relationships

and not as a statement of proven cause and effect relationships. The model

may have merit in planning wildlife habitat research studies about a species,

as well as in providing an estimate of the relative suitability of habitat for

that speci es. User feedback concern i ng model improvements and other suggestions that may increase the utility and effectiveness of this habitat-based

approach to fish and wildlife planning are encouraged. Please send suggestions

to:

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

2627 Redwing Road

Ft. Collins, CO 80526-2899

iii
;v
CONTENTS

Page

PREFACE iii

FIGURES vi

ACKNOWLEDGMENTS vi i

HABITAT USE INFORMATION 1

Genera 1 1

Food 1

Water 2

Cover 2

Reproduction -. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Interspersion..................................................... 7

Special Considerations............................................ 8

HABITAT SUITABILITY INDEX (HSI) MODEL 8

Model Applicability 8

Model Description................................................. 11

Appl ication of the Model 24

SOURCES OF OTHER MODELS 31

REFERENCES 31

v
•

Number

1

2

3

4

5

6

7

8

FIGURES

Geographic applicability of the gadwall HSI model within

the United States .

The relationships between values of variables used to

evaluate gadwall pair habitat and suitability indices

for the variables .

The relationships between values of variables used to

evaluate gadwall nesting habitat in a given field and

suitability indices for the variables .

The relationship between the equivalent optimum area of

gadwwall nesting habitat and an overall nesting habitat

sui tabi 1i ty index .

The relationships between values of variables used to

evaluate gadwall brood habitat and suitability indices

for the variables .

Summary of equations used in the gadwall HSI model .

The relationships between habitat variables, derived

variables, life requisites, and an HSI for the gadwall

Definitions of variables and suggested measurement

techni ques .

vi

8

16

18

20

23

25

26

27

ACKNOWLEDGMENTS

The basis for this HSI model was developed in a workshop that included

the following waterfowl biologists from the Northern Prairie Wildlife Research

Center, Jamestown, NO: Leo Kirsch (retired), John Lokemoen, and George

Swanson. These individuals contributed freely of their experience so that we

could develop the most reasonable HSI model possible with the current knowledge

of waterfowl habitat requirements. L. Kirsch, J. Lokemoen, and G. Swanson

also reviewed the model that resulted from the workshop. L. Kirsch provided

unpublished data that were used in the development of the nesting component

portion of this model.

In addition to the waterfowl authorities, the following potential users

of the models participated in the workshop and/or the model review: Michael

McEnroe and Steven Young (U.S. Fish and Wildlife Service, Bismarck, NO);

Richard McCabe and Robert Schultz (U.S. Bureau of Reclamation, Bismarck, NO);

and Fred Ryckman and Terry Steinwand (North Dakota Department of Game and

Fish, Bismarck, NO). The inputs of all these individuals contributed to the

content of this model. Michael Armbruster and Arthur Allen (U.S. Fish and

Wildlife Service, Ft. Collins, CO) served as facilitators for the modeling

workshop.

The cover of this document was illustrated by Jennifer Shoemaker. Word

processing was provided by Carolyn Gulzow, Dora Ibarra, and Elizabeth Graf.

Kay Lindgren assisted with literature searches.

Funding for the development of this model was provided to the U.S. Fish

and Wildlife Service's Habitat Resources Program (Region 6) by the U.S. Bureau

of Reclamation under the provisions of the Fish and Wildlife Coordination Act.

Additional funds were provided by the U.S. Bureau of Reclamation's Program

Related Engineering and Scientific Studies (Environmental, Evaluation, and

Planning Project).

vi i
GADWALL (Anas strepera)

HABITAT USE INFORMATION

General

Extensive breeding populations of the gadwall (Anas strepera) in the

United States are limited to the northern prairies and to the marshes of the

intermountain valleys of the western United States (Bellrose 1979). Isolated

breeding populations exist along the Atlantic and Alaskan coasts and in other

inland locations (Bellrose 1976). The largest numbers of breeding gadwalls

occur in the mixed-grass prairies of the Dakotas and the Prairie Provinces of

Canada. Range expansion to the eastern United States has apparently resulted

from creation of suitable habitat in the form of impoundments on Federal

refuges and state management areas (Henny and Holgersen 1974). Recent range

expansion also has been noted west of the Cascades in the northwestern United

States (Canning and Herman 1983).

Food

The diet of gadwalls during fall and winter is predominantly vegetation

(Gates 1957; Landers et al. 1976; Paulus 1982). Vegetative material accounted

for over 95% of the diet of gadwalls on a Louisiana wintering area and included

algae, dwarf spikerush (Eleocharis parvula), common widgeongrass (Ruppia

maritima), spi ked waterm"il foil (Myriophyll urn spi catum), and baby pondweed

(Potamogeton pusillus) (Paulus 1982). The two most prominent plants in the

diet of gadwalls in South Carolina were fragrant flatsedge (Cyperus odoratus)

and Carolina redroot (Lachnanthes caroliniana) (Landers et al. 1976). Important food plants in Utah during the fall were sago pondweed (E. pectinatus),

widgeongrass, and inland saltgrass (Distichlis stricta) (Gates 1957).

Animal food makes up a larger proportion of the spring and summer diet of

adult gadwalls (Serie and Swanson 1976) and ducklings (Sugden 1973). From

early spring through late August, animal material accounted for 46% of the

diet of adult gadwalls on saline wetlands in North Dakota (Serie and Swanson

1976). Crustaceans, especi ally those bel ongi ng to the order Anostraca, and

insects, especially adult and larval chironomids (Chironomidae), were prominent

in the animal portion of the diet. Hens ate more crustaceans and dipterans,

and 1ess vegetation, than thei r mates duri ng the egg-l ayi ng peri od. The

proportion of animal food in the diet reached a peak of 72% in females during

the egg-l ayi ng peri od. Important plants duri ng the spri ng and summer peri od

were filamentous algae, widgeongrass, muskgrass (Chara spp.), and sago pondweed. ----

Recently hatched gadwall ducklings in Alberta predominantly fed on

invertebrates, but were essentially herbivorous by 3 weeks of age (Sugden

1973). Major animal foods included adult and larval chironomids, water boatmen

1
(Corixidae), beetles (Coleoptera), and cladocerans (Cladocera). Important

plants in the ducklings' diet were baby pondweed, green algae (Cladophoracea),

duckweed (Lemna minor), and seeds of American sloughgrass (Beckmannia syzigachne)-.---- -----

Gadwalls typically feed by dabbling, tipping, surface picking, and

filtering (Serie and Swanson 1976). Gadwalls in North Dakota concentrated

their feeding activities in the littoral zones of deeper, permanent wetlands

and throughout the entire basin of shallow wetlands (Serie and Swanson 1976).

Ephemeral ponds may be important sources of accessible and abundant planktonic

crustaceans early in the breeding season. Wintering gadwalls in Louisiana fed

96.7% of the time in water 15 to 67 cm deep (Paulus 1982). Broods in Alberta

also avoided feeding in areas < 15 cm deep and concentrated their feeding

activities in water 17 to 46 cm deep (Sugden 1973). Broods fed 86% of the

time over areas of submerged vegetation.

Water

The distribution and density of waterfowl is influenced to a large degree

by water permanence in available wetlands (Kantrud and Stewart 1977); wetlands

are considered to be the primary factor in waterfowl production (Higgins

1977) .

The use of stock ponds by gadwall broods in South Dakota primari ly was

i nfl uenced by the amount of open water (Mack and Flake 1980). Open water

sites are preferred by gadwa 11 s for 1oafi ng (Duebbert 1966) and as escape

cover by broods (Evans and Bl ack 1956, ci ted by Mack and Flake 1980) and

adults (Flake et al. 1977, cited by Mack and Flake 1980). Gadwall pairs in

Mani toba were found in greatest abundance when the ratio of open water to

emergent vegetation was approximately 50:50, compared to wetlands with 30:70

and 70:30 ratios (Kaminski and Prince 1981). However, preference of gadwall

pairs for habitats with a 50:50 ratio of water to cover was significant for

only 1 year of the 2-year study. Trauger (1967) recommended that open water

compose at least 40% of a wetland for brood use by dabbling ducks. Murkin

et al. (1982) concluded that a water to vegetation ratio of 50:50 resulted in

the maximum density of waterfowl pairs.

Cover

Gadwalls wintering in Texas utilized fresh water habitats and use was

concentrated in the deeper waters (maximum water depth was 2.5 m; average

water depth was 1.5 m) with abundant aquatic vegetation and sparse emergent

vegetation (White and James 1978). In contrast, late summer molting cover in

Manitoba was characterized by dense stands of cattail (~ spp.) or bulrush

(Scirpus spp.) (Oring 1969).

Reproduction

Pair habitat. Seasonal and semipermanent wetlands accounted for 54.6 and

35.4%, respectively, of use by gadwall pairs in North Dakota (Kantrud and

Stewart 1977) and 33 and 18%, respectively, of the wetland area (Stewart and

Kantrud 1973). A 2-year average of 61% of gadwall pairs in South Dakota used

2
natural basin wetlands, which accounted for 75% of the area of all wetland

basins (Ruwaldt et al. 1979). Semipermanent and seasonal wetlands accounted

for a 2-year average of 43.7 and 13.0%, respectively, of gadwall pair use and

32.1 and 13.4% of the wetland area. Constructed wetlands (dugouts and stock

ponds) in the same study accounted for a 2-year average of 15% of the wetland

area and 32.4% of use by gadwall pairs. Gadwall pairs in South Dakota used

ponds with scattered dense patches of emergents and avoided ponds with no

emergent vegetation (Flake and Vohs 1979). Use of wetlands by gadwall pairs

also was positively correlated with the presence of round-stem bulrushes

(Scirpus spp.) and shoreline irregularity.

Numbers of gadwall pairs over a 15-year period in Saskatchewan were

positively correlated with the number of pairs present in the previous year

(third-order, Spearman-rank, partial correlation coefficient =0.31; P < 0.10)

(Leitch and Kaminski 1985). Pair numbers were not significantly correlated

with the number of wetlands available in Mayor in August of the previous

year.

Nesting habitat. Gadwalls nest on islands, on dikes in marshes, and in

fields and meadows, but rarely nest over water (Bellrose 1976). Fields of

seeded native grasses supported the highest number of initiated nests of

gadwalls in North Dakota, followed by seeded introduced grasses and unplowed

prairie (Klett et al. 1984). Seeded native and seeded introduced grasses

supported about the same number of initiated nests in South Dakota, followed

by unplowed prairie. Ninety-five percent of gadwall nests in an intensively

farmed area of North Dakota were in untilled uplands, with the remaining 5% in

growing grains (Higgins 1977); summer fallow areas, mulched stubble, and

standing stubble were not used. Nests may be placed in herbaceous vegetation

(Duebbert and Lokemoen 1976; Kirsch et al. 1978) or on the ground under shrub

clumps (Duebbert et al. 1983; J. T. Lokemoen, U.S. Fish and Wildlife Service,

Northern Prairie Wildlife Research Center, Jamestown, NO; pers. comm.). Most

nests are located on the driest sites available (Miller and Collins 1954;

Gates 1962; Oring 1969).

Presence of residual herbaceous vegetation may be an important habitat

factor in nest site selection (Duebbert and Lokemoen 1976; Kirsch et al. 1978;

Voorhees and Cassel 1980), although new growth may partially compensate for

the lack of residual herbaceous vegetation (Martz 1967) because gadwalls begin

nesting after new plant growth has begun (Kirsch et al. 1978; Giroux 1981b;

Hines and Mitchell 1983). Data (Table 1) provided by L. M. Kirsch (U.S. Fish

and Wildlife Service, retired, Woodworth, NO; unpubl.) revealed an increase in

gadwall nesting density with an increase in the average height and density of

residual herbaceous vegetation as evaluated by a visual obstruction measurement (the hei ght at whi ch a round po 1e 3 x 150 cm is totally obscured by

vegetation when viewed from a distance of 4.0 m) (Robel et al. 1970). Linear

regression analysis of the data in Table 1 resulted in a regression equation

of Y = 1.68 + 2.42x, a correlation coefficient (r) of 0.77 (P < 0.05), and a

coefficient of determination (r 2 ) of 0.59. Nesting densities within a given

class of visual obstruction measurements of residual vegetation varied widely,

however, as indicated by the ranges in nesting density in Table 1, suggesting

that other factors also had a major influence on nest density. One reason for

the high variability in nest densities within given visual obstruction classes

3
Table 1. Gadwall nesting densities by classes of residual

vegetation for fields on the Woodworth Study Area, North

Dakota, 1974-1978 (data provided by L. Kirsch).

Xvisual obstruction Number of Mean number of

measurement (range) observation s. gadwall nests/40.5 ha

in dm in class ( range)

0.17 (0.12-0.24) 7 3.28 (0.00-14.28)

0.30 (0.25-0.34) 7 2.66 (0.00-8.33)

0.42 (0.35-0.49) 7 1. 36 (0.00-3.33)

0.55 (0.50-0.60) 7 1. 52 (0.00-5.00)

0.66 (0.62-0.72) 7 3.99 (0.00-9.09)

0.78 (0.74-0.83) 7 3.12 (0.00-8.33)

0.91 (0.86-0.98) 7 3.38 (0.00-8.33)

1.06 (1.01-1.14) 7 1. 36 (0.00-4.21)

1. 32 ( 1. 18-1. 44) 7 10.07 (0.00-25.00)

1. 52 ( 1. 45-1. 60) 7 6.85 (0.00-16.67)

1. 73 (1.62-1. 91) 7 4.85 (0.00-13.04)

2.31 (2.01-2.86) 7 6.51 (2.17-13.04)

3.70 (3.18-4.22) 2 10.36 (6.06-14.67)

4
is that some nests may have been initiated after new growth had begun (Kirsch,

pers. comm.), resulting in situations where residual vegetation was not a cue

used in nest site selection. Kirsch (pers. comm.) and Lokemoen (pers. comm.)

indicated that a field with an average visual obstruction measurement of

residual vegetation 2.5 dm would be ideal nesting habitat for gadwalls.

A study on 15 areas in North Dakota, Saskatchewan, and Manitoba suggested

a direct but weak (P = 0.06) relationship between gadwall nest density and

average visual obstruction measurement of vegetation in late spring (late

May-early June) (Shaffer et al. 1985). Visual obstruction measurements, study

area effects, and number of pairs explained 26% of the total variation in

gadwall nest density. Shaffer et al. (1985) suggested the following model to

determine the number of gadwall nests (N) to be found in a given field:

N =(0.0052 + 0.0045 x X) x P x A

where X =the average visual obstruction measurement in late spring

P =the estimated number of gadwall pairs within 0.6 km of the center

of a field

A = the area of the field

The above model was considered to be most useful when comparing fields that

shared similar study area effects, i.e., fields that were located in close

proximity to each other (Shaffer et al. 1985).

High densities of gadwalls nested on a North Dakota island where

herbaceous vegetation averaged 15 to 25 cm tall at the initiation of nesting

but 1.5 to 1.8 m tall during the late incubation stages (Duebbert 1966).

Fifty-one percent of nests in North Dakota nesting fields were in herbaceous

cover from 30 to 60 cm tall, while 47% were in cover> 60 cm tall (Duebbert

and Lokemoen 1980). No nests were found in herbaceous cover < 15 cm tall.

Most gadwall nests in a California study were in vegetation 33 to 91 cm tall

that provided concealment on all sides, as well as from above (Miller and

Collins 1954). Canopy cover at gadwall nests in Saskatchewan exceeded 25% in

vegetation> 30 cm tall that provided lateral concealment on three or four

sides (Hines and Mitchell 1983). Vegetation < 20.3 cm tall is considered too

short for nest concealment (Martz 1967). Kirsch et al. (1978:492) stated that

they II ••• have not found grassland vegetation that was too tall and dense for

use by nesting ducks nor have ... [they] found evidence that such conditions

exist in the pr-at r l e s ." Duebbert (1982:236) concluded that gadwall nesting

cover on islands II ••• can consist of brush, forbs, or grasses if the

vegetative structure is tall and den se ."

The highest nesting densities of gadwalls have been reported from island

habitats (Hammond and Mann 1956; Duebbert 1966, 1982; Giroux 1981a; Duebbert

et al. 1983; Hines and Mitchell 1983), in response to the lack of mammalian

predation on the islands (Duebbert et al. 1983). Nest success on islands is

higher than reported for nests in mainland habitats (Duebbert et al. 1983;

Hines and Mitchell 1983). For example, nest success on islands and isolated

5
ditch banks in Saskatchewan was 65%, while no nests were successful on uplands

(Hines and Mitchell 1983). High densities and success of gadwall nests in

mainland habitat in North Dakota resulted from intensive control of mammalian

predators (Duebbert and Lokemoen 1980).

Gadwa 11 nests on islands inA1berta were in forbs and grass-forbs cover

(Giroux 1981a), and gadwall nests on Miller Lake (North Dakota) islands were

concentrated in patches of western snowberry (Symphori carpos occi denta1is) -

woods rose (Rosa woodsii) (Duebbert et al. 1983). Gadwall nests on a 2.2 ha

island in Saskatchewan were concentrated in patches of western snowberry and

slim nettle (Urtica gracilis) (Hines and Mitchell 1983). Shrub clumps in

nonisland habitats are readily used for nesting cover by gadwalls (Lokemoen,

pers. comm.).

Preferred nesting cover is eliminated by activities such as grazing

(Kirsch 1969) or mowing (Martz 1967; Kirsch 1969; Voorhees and Cassel 1980).

Although a strong relationship has been demonstrated between duck nesting

densities and undisturbed cover (Kirsch et al. 1978), mowing may be useful for

maintaining vegetative cover in earlier, more productive successional stages

(Voorhees and Cassel 1980). Duebbert et al. (1981) recommended periodic

disturbance to native and introduced grassland nesting habitat to maintain

optimum conditions, although annual mowing or grazing was not recommended.

Brood habitat. Preferred escape cover for gadwall broods is large areas

of open water, rather than water with emergents (Evans and Black 1956, cited

by Mack and Flake 1980). Gadwall broods in Utah used deep-water marshes and

the edges of large impoundments (Gates 1962); broods in Washington used large

alkaline lakes with steep walls, as well as other wetlands (Yocom and Hansen

1960). Sixty-one percent of 1,073 gadwall broods observed over a 20-year

period in North and South Dakota were in semipermanent wetlands (Class IV of

Stewart and Kantrud 1971), 18% were in seasonal wetlands (Class II!), and 9%

were in permanent wetlands (Class V) (Duebbert and Frank 1984). The proportion

of total wetland area accounted for by these wetland types in North Dakota in

1967 was 18% semipermanent, 36% seasonal (including 3% in tilled condition),

and 3% permanent (Stewart and Kantrud 1973). However, wetland availability

figures were for 1 year only, apparently reflected the availability of wetlands

to pairs, and may not be a valid estimate of the wetland distribution available

to broods. Amount of open water and number of wetland basins/0.65 km 2 plot

were the primary factors that determined use of stock ponds by gadwalls in

South Dakota (Mack and Flake 1980). The mean open water area on stock ponds

used by gadwall broods was 1.4 ha, compared to a mean of 0.6 ha on ponds not

used by gadwall broods. Wetland basins averaged 5.0/0.65 km 2 on study plots

used by gadwall broods, but only 2.8/0.65 km 2 on plots not used by broods.

Gadwall broods over a IS-year period in Saskatchewan were positively correlated

with the number of pairs in the preceding spring (second-order, Spearman-rank,

partial correlation coefficient = 0.56; P < 0.01) and the number of wetlands

containing water in August (second-order, Spearman-rank, partial correlation

coefficient = 0.43; P < 0.05) (Leitch and Kaminski 1985).

The gadwall is the primary waterfowl speci es in North Dakota that uses

saline lakes for brood-rearing (Swanson et al. 1984). Brood use is closely

tied to the presence of freshwater seeps or areas of lower salt content.

6
These areas provide fresh water for drinking and support dense emergent

vegetation which provides cover for broods.

Interspersion

The average distance from nest sites to water was < 45.8 m in several

studies of gadwalls (Miller and Collins 1954; Gates 1962; Vermeer 1970).

Gadwall nests in North Dakota averaged 351 m from water (Duebbert and Lokemoen

1976), including some nests in fields up to 2.4 km from water (Duebbert and

Lokemoen 1980). G. A. Swanson (U.S. Fish and Wildlife Service, Northern

Prairie Wildlife Research Center, Jamestown, ND; pers. comm.) suggested that

selection of nesting habitat by gadwalls is based on proximity to pair feeding

habitat rather than on proximity to brood-rearing habitat.

Gadwall hens in Utah moved their broods an average of 0.9 km and a maximum

of 1.85 km from the nest to brood habi tat (Gates 1962). Hens and broods in

South Dakota dispersed into wetlands 1.6 to 3.2 km from the nests (Duebbert

and Lokemoen 1980). Breeding home ranges for five hens in Utah averaged 27.1

ha (range 13.8 to 35.2 ha) and included at least one feeding pond and a ditch

or channel used for loafing (Gates 1962).

Island-nesting gadwalls may reach high densities, for example: 494 nests/

ha on a 0.32 ha island in North Dakota (Hammond and Mann 1956); 139 to 237

nests/ha in preferred island habitat in North Dakota (Duebbert et al. 1983);

and 74 nests/ha on a 2.2 ha island in Saskatchewan (Hines and Mitchell 1983).

Nest density in the latter study was 284 nests/ha in two patches of snowberry,

totalling 0.5 ha (Hines and Mitchell 1984). Nest parasitism by other gadwalls

(31 of 355 nests) at this high nest density reduced nesting success from 76 to

54%, egg success from 74 to 45%, and hatchabi 1i ty of eggs from 97 to 91%

(Hines and Mitchell 1984). Parasitism by lesser scaup (Aythya affinis) reduced

egg success from 74 to 67%. Nest densities elsewhere on the island ranged

from 13 to 52 nests/ha; only one parasitized nest was found at the lower nest

densities.

Nest densities in nonisland habitats are generally much lower than those

observed on islands. For example, Kaiser et al. (1979) recorded only 0.67

nests/km" in native grasslands in South Dakota and 3.88 nest.s./km" in tame

grasslands. The estimated number of initiated gadwall nests in North Dakota

was 28/km2 in seeded native grasses, 14/km2 in seeded introduced grasses, and

4/km2 in unplowed prairie (Klett et al. 1984). Gadwalls in South Dakota

initiated an estimated 10 nests/km2 in seeded native grasses, 11/km2 in seeded

introduced grasses, and 0/km2 in unplowed prairie. Nest density in untilled

uplands and growing grains in North Dakota was 4.33 nests/km2 and 0.23 nests/

km2 , respectively (Higgins 1977). A density of 7.5 nests/km2 (61 nests on a

8.13 km 2 study area) was observed in an area of intensive predator control

(Duebbert and Lokemoen 1980).

Areas with diversified land uses are better for duck production than

large expanses of tilled grain monocultures (Duebbert and Lokemoen 1976).

7
Special Considerations

Island-nesting waterfowl, such as gadwalls, require suitable wetlands for

pair and brood-rearing habitat (Duebbert et al. 1983). Creation of nesting

habitat in the form of small islands should consider the carrying capacity of

surrounding wetlands for pairs (Hines and Mitchell 1983). Habitat management

for waterfowl production must involve both wetland and upland habitat (Leitch

and Kaminski 1985).

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Applicability

Geographic area. This HSI model was originally developed for use in

central and eastern North Dakota. It is considered applicable throughout the

Prairie Pothole Region, where the greatest breeding densities of gadwalls

occur (Fig. 1). Within the United States this region includes the mixed-grass

prairie of North and South Dakota; the tallgrass prairie in western Minnesota,

eastern North and South Dakota, and the sandhills of Nebraska; and the shortgrass prairie west of the Missouri River through Montana (Bellrose 1976,

1979). The model also should be applicable within the Prairie Provinces of

Canada and may be applicable in other portions of the breeding range of the

gadwa 11.

Figure 1. Geographic applicability of the gadwall HSI model within

the United States (corresponds to areas of highest gadwall breeding

densities, as shown in Bellrose 1976).

8
Season. This HSI model was developed to evaluate the quality of spring

and summer habitat for gadwalls.

Cover types. Duri ng the breedi ng season, gadwall s may use any of the

following upland cover types for nesting (terminology follows that of U.S.

Fish and Wildlife Service 1981): Cropland (C); Pasture and Hayland (P/H);

Grassland (G); Forbland (F); Deciduous Shrubland (OS); and Deciduous Shrub

Savanna (DSS). Gadwall pairs and broods use a variety of wetlands in the

Palustrine and Lacustrine systems (terminology from Cowardin et al. 1979).

The data that were used to develop the pair and brood components of thi s

model, however, were based on the classification of Stewart and Kantrud (1971).

Data presented in this model follow the Stewart and Kantrud (1971) classification used in the original references. The seven wetland classes defined by

Stewart and Kantrud (1971) generally correspond to the water regime modifiers

of Cowardin et al. (1979) (Table 2). The model can be used with either classification system using the relationships in Table 2. Definitions of the wetland

cl asses and water regime modifi ers 1i sted in Tabl e 2 are provided under

Application of the Model. Constructed wetlands (stockponds, dugouts, and

reservoirs) can be included in this model by classifying them into one of the

wetland classes or water regime modifiers, based on a comparison of their

physical and vegetational characteristics to the criteria used in the appropriate classification system.

Minimum habitat area. Minimum habitat area is defined as the minimum

amount of contiguous sui tab1e habi tat that is requi red before an area wi 11 be

occupied by a species. Specific information on the minimum habitat area for

the gadwall was not located in the literature.

Verification level. The critical habitat requirements and associated

habitat variables identified in this model resulted from a modeling workshop

held April 13-16, 1982, in Fort Collins, Colorado. The purpose of the workshop

was to develop habitat models for the blue-winged teal (Anas discor) and

gadwall, based on the available literature and the expertise of the following

individuals:

John Lokemoen, U.S. Fish and Wildlife Service, Northern Prairie Wildlife

Research Center, Jamestown, North Dakota.

George Swanson, U.S. Fish and Wildlife Service, Northern Prairie Wildlife

Research Center, Jamestown, North Dakota.

Leo Kirsch, U.S. Fish and Wildlife Service (retired), Woodworth, North

Dakota.

Michael McEnroe, U.S. Fish and Wildlife Service, Ecological Services,

Bismarck, North Dakota.

Richard McCabe, Bureau of Reclamation, Bismarck, North Dakota.

9
Table 2. Comparison of the wetland classes of Stewart and Kantrud (1971) with

the water regime modifiers of Cowardin et al. (1979). (Modified from Cowardin

et al. 1979)

Wetland class

(Stewart and Kantrud 1971)

I Ephemeral ponds

II Temporary ponds

III Seasonal ponds and lakes

IV Semipermanent ponds and lakes

Nonea

V Permanent ponds and lakes

VI Alkali ponds and lakes

VII Fen (alkaline bog) pondsb

Water regime modifier

(Cowardin et al. 1979)

None, not considered a

wetland

Temporarily flooded

Seasonally flooded

Semi permanently flooded

Intermittently exposed

Permanently flooded (with

mixohaline water)

Intermittently flooded

(with saline or hypersaline

water)

Saturated

aNo corresponding wetland class exists for the intermittently exposed flooding

regime.

bFens are not included in the gadwall HSI model.

10
A meeting was held May 25-28, 1982 in Bismarck, North Dakota, to review

the waterfowl models developed from the workshop. Results of the review

indicated that the pair and brood components of the model appeared to meet

perceptions of the participants about the suitability of several study areas

for which data were available. The reliability of the nesting component of

the model was questioned, but could not be evaluated with the available data.

Participants in the review meeting were J. Lokemoen, L. Kirsch, M. McEnroe,

R. McCabe, and F. Ryckman and M. Johnson (North Dakota Department of Game and

Fish, Bismarck, North Dakota).

A review of the assumptions and limitations of the model was conducted

May 24-25, 1983 in Jamestown, North Dakota. Participants in the review were

J. Lokemoen, L. Kirsch, R. McCabe, R. Schultz (Bureau of Reclamation, Bismarck,

North Dakota), S. Young (U.S. Fish and Wildlife Service, Ecological Services,

Bismarck, North Dakota), and T. Steinwand (North Dakota Department of Game and

Fish, Bismarck, North Dakota). Participants in the model review meeting

concluded that the HSI model was realistic and as good as could be expected

without further field testing and/or application. J. Lokemoen and L. Kirsch

concluded that the model would adequately predict potential habitat qual ity

for the gadwall in the Prairie Pothole Region. Data provided by L. Kirsch at

this meeting were used to develop the nesting component portion of the current

mode1.

A study was conducted on several study areas in North Dakota and Canada

to further evaluate the relationship between average vegetative conditions of

a field and attractiveness of the field to nesting gadwalls (Shaffer et al.

1985). Results of the study were used to modify the nesting component of the

current mode1.

Model Description

Overview. Breeding habitat suitability for the gadwall can be evaluated

by assessing the habitat requirements for pairs, nesting hens, and broods.

Habitat suitability for pairs and broods is a function of the availability and

distribution of wetlands; pairs are able to use all wetland types other than

ephemeral wetlands, but broods are restricted to more permanent wetlands that

will provide a reliable source of water during the brood-rearing period.

Suitability of nesting habitat is dependent on the abundance of dense, tall

herbaceous or shrub cover.

This HSI model provides a method to evaluate gadwall habitat sUitability

within the context of habitat suitabil ity for all species of upland nesting

waterfowl in the Prairie Pothole Region. The standards of comparison used in

the pair, nesting, and brood components of this model are based on the needs

of all upland nesting waterfowl. Evaluation of the habitat in a given area,

however, is modified by the specific habitat requirements of the gadwall to

obtain a Habitat Suitability Index for this species.

The following sections identify important habitat variables, describe

suitability levels of the variables, and describe the relationships between

variables.

11
Pair habitat component. Pairs use wetlands for feeding, loafing, and

courtship prior to nesting. Data on the use of basin wetlands in the Prairie

Pothole Region of North Dakota (Kantrud and Stewart 1977) and in South Dakota

(Ruwaldt et al. 1979) indicate that various classes of wetlands (as defined by

Stewart and Kantrud 1971) are used to different degrees by gadwall pairs. An

index of preference for wetland classes can be developed, based on pair use

compared to the availability of wetland classes. These indices are presented

in Table 3, based on North Dakota data (Stewart and Kantrud 1973; Kantrud and

Stewart 1977), and in Table 4, based on data from South Dakota wetlands

(Ruwaldt et al. 1979). Based on this analysis, the highest quality natural

wetlands for gadwall pairs in North Dakota are semipermanent wetlands, followed

by temporary and seasonal wetlands (Table 3). Gadwall pairs in South Dakota

also showed the highest preference for semipermanent wetlands, followed by

seasonal and permanent wetlands (Table 4). Use of preference indices assumes

that all wetlands within a given class have the same value for gadwall pairs.

Variability in other factors, such as available food, open water to vegetation

ratio, and shoreline length and irregularity, are not addressed in this model.

Although variability within a wetland class certainly occurs, this model is

intended for evaluation of large areas with numerous wetlands, rather than an

evaluation of each wetland.

Gadwall pairs will use constructed wetlands. However, preference indices

for gadwall pairs in constructed wetlands are not included in this model

because the classification of constructed wetlands is not based on ecological

characteristics. If constructed wetlands are to be included in an application

of this model, preference indices must be developed by the model users.

Similarities in water conditions and vegetation between constructed and natural

wetlands can be used to assign pair preference indices to constructed wetlands.

Data provided by Ruwaldt et al. (1979) for South Dakota may be useful in

developing pair preference indices for stock ponds and dugouts.

Optimum conditions for waterfowl pairs, including gadwalls, in the Prairie

Pothole Region are assumed to exist when a minimum of 150 optimum wetlands

account for a minimum of 64.8 ha/2S9 ha. This assumption was based on the

perceived need for a large number of small wetlands within a section (259 ha)

in order to support the maximum number of waterfowl pairs, while still providing potentially optimum nesting and brood habitat (discussed below). The

selection of ISO/section as the standard of comparison for the density of

optimum wetlands was based on the opinion of species experts in the modeling

workshop that this is an attainable figure that represents optimum conditions.

A complete lack of wetlands provides no pair suitability. The value of

wetlands to pairs is assumed to decrease in a linear relationship as the

number and area of wetlands approaches zero. Pair densities on smaller wetlands are usually greater than on larger wetlands, because larger wetlands

generally have large areas of open water that do not provide the required

isolation for pair use. The conditions described as optimum for pairs (150

wetlands totalling 64.8 ha/section) equate to an average wetland size of

0.43 ha. If it is assumed that a few large wetlands will be present, then

most of the wetlands will be < 0.4 ha, a condition considered by workshop

participants to be optimum for pairs.

12
Table 3. Determination of a wetland preference index for gadwall

pairs in the Prairie Pothole Region of North Dakota.

Ava il abil i ty of

Gadwall use wetland class

Wetland classa (% total b (% of total

Indexd distribution) wetland area)c Use/availability

Ephemeral (I) 0.0 1 0.00 0.00

Temporary (I! ) 3.6 3 1.20 0.61

Seasonal (II!) 54.6 36 1. 52 0.77

Semipermanent (IV) 35.4 18 1. 97 1.00

Permanent (V) 2.0 3 0.67 0.34

Alkali (VI) 0.4 6 0.07 0.04

aThe classification used is that of Stewart and Kantrud (1971), because data

on waterfowl use presented by Kantrud and Stewart (1977) were based on thi s

classification. See Table 2 and Application of the Model for qutde l i nes on

using other wetland classification systems.

bFr om Kantrud and Stewart (1977:247, Table 1).

c Fr om Stewart and Kantrud (1973:45, Table 2). The number represents the proportion of the total wetland acreage accounted for by the individual wetland

class. Total is 67% because only those wetland classes of the Stewart and

Kantrud (1971) classification system referred to in Kantrud and Stewart (1977)

were used. The remaining wetlands were undifferentiated tillage ponds (25%),

streams and oxbows (5%), and constructed wetlands (s 3%).

dDetermined by dividing the use/availability value by 1.97, the maximum use/

availability value.

13
Table 4. Determination of a wetland preference index for gadwall

pairs in South Dakota.

Availability of

Gadwall use wetland class

Wetland classa (% total b W~~l~~dt~~:~)c Use/availability Indexd distribution)

Ephemeral (I) 0.0 14 0.00 0.00

Temporary (I! ) 1.8 11 0.16 0.12

Seasonal (II!) 13.0 13 1.00 0.74

Semipermanent (IV) 43.7 32 1.36 1. 00

Permanent (V) 2.6 4 0.65 0.48

Alkali (VI)

aThe classification used here is that of Stewart and Kantrud (1971). See

Table 2 and Application of the Model for guidelines on using other wetland

classification systems.

bFrom Ruwaldt et al. (1979:378, Table 3). Figures represent the average of use

data provided for 1973 and 1974. Use of natural wetland basins equals 61.1%;

remaining use was on streams and constructed wetlands.

cFrom Ruwaldt et al. (1979:376, Table 1). Total of natural basin wetlands

equals 74%; remaining area was in streams and constructed wetlands.

dDetermined by dividing the use/availability value by 1.36, the maximum use/

availability value.

14
The number of wetlands on a study area can be converted to an equivalent

number of optimum wetlands by weighting the number of wetlands in each class

by the wetland preference indices for pairs (Table 3 or Table 4):

n

EONWP = t (w.p.)

i=1 "

(1)

where EONWP = number of equivalent optimum wetlands/259 ha (i.e., weighted

by preference indices) for pairs

n =the number of wetland classes available

wi =the number of wetlands of wetland class i/259 ha

Pi = preference index for gadwall pairs for wetland class

Equation 1 determines the sum of the number of wetlands per section

weighted by the quality of the wetland classes for gadwall pairs. The relationship between the number of equivalent optimum wetlands/259 ha and a suitability index (SIVl) for gadwall pairs is presented in Figure 2a.

A value for equivalent optimum area of available wetlands can be determined by:

where

n

EOAWP = t (aiPi)

i=1

EOAWP =the equivalent optimum area of wetlands/259 ha for pairs

ai = the area of wetlands of class i/259 ha

(2)

The resulting sum from Equation 2 is the total area of wetlands available

per section weighted by the quality of the available wetlands to gadwall

pairs. The relationship between this value and a suitability index (SIV2) for

gadwall pairs is shown in Figure 2b.

15
Fig. 2a Fig. 2b

1.0 1.0 r- (\J

>- >- ...... ...... (/') 0.8 (/') 0.8 -- x X OJ 0.6 OJ

"0 "0 0.6 l: l:

...... ......

0.4 0.4 .....

r- r-

..... ..... .D 0.2 .D 0.2 10 10 ...., ...., ..... ..... ::s ::s (/') 0.0 (/') 0.0

0 50 100 150 200+ 0 64.8 129.6+ ~ha~ 0 160 320+ vac

Equivalent optimum number

of wet1ands/259 ha Equivalent optimum area of (pairs) wet1ands/259 ha (pairs)

Figure 2. The relationships between values of variables used to evaluate

gadwall pair habitat and suitability indices for the variables.

Number and' area of wetlands are assumed to be of equal importance in

determining habitat suitability for gadwall pairs. These two variables are

not entirely independent. For example, an increase in the number of equivalent

optimum wetlands will likely result in an increase in equivalent optimum area

of wetlands for gadwall pairs. Although area and number of wetlands are not

independent, the variable with the lowest suitability level is considered to

have the greatest influence on the final index for pair habitat suitability

(SIP). This relationship is best expressed by a geometric mean of the suitability indices for the two variables:

SIP = (SIV1 x SIV2)1/2 (3)

Nesting habitat component. Gadwalls typically select the tallest,

densest, herbaceous or shrubby vegetation available in which to nest. As

vegetative cover (i .e., height and density of herbaceous vegetation or shrubs)

increases, the potential for nest establishment and success increases.

Residual vegetative cover (i .e., vegetation available before the growing

season begins) is a particularly important source of suitable nest cover.

Height and density of vegetation are assumed to be more important than vegetative composition.

16
A visual obstruction measurement that incorporates height and density

into a single descriptor of vegetative structure (Robel et al. 1970) can be

used to determi ne an index of the quantity and qua1i ty of residual vegetative

cover. A visual obstruction measurement is the height, to the nearest 0.5 dm,

at which a round pole 3 x 150 cm is totally obscured by vegetation when viewed

from a distance of 4 m (see Robel et al. 1970 for further details). Data

provided by Kirsch (unpubl.) can be used to determine the relationship of

nesting density to residual vegetation. Table 1 displays the mean gadwall

nesting density by visual obstruction classes from 1974 to 1978 on the

Woodworth Study Area, Woodworth, North Dakota. Workshop participants believed

that a field with a mean visual obstruction measurement of 2.5 dm represented

ideal nesting conditions. Therefore, a mean visual obstruction measurement of

2.5 dm is considered equal to optimum nesting quality (corresponding to a

1.0 Suitability Index). The regression equation determined for the data in

Table 1 is:

where

Y = 1. 68 + 2. 42x

Y = gadwall nests/40.5 ha

x = mean visual obstruction measurement (dm) for a given field

(4 )

Equat i on 4 can be modi fi ed to provi de an output that is an index of

nesting suitability of a habitat. This is accomplished by first assuming that

a mean visual obstruction measurement 2.5 dm equals a suitability index of

1.0. The nest density predicted from Equation 4 when the mean visual obstruction measurement (i.e., nest density equals 7.73) is used as the maximum nest

density. The predicted nest density of 1.97 nests/40.5 ha when the mean

visual obstruction measurement equals 0.12 (the lowest observed value reported

by Kirsch's data) can be compared to the maximum nest density. The resulting

index value for this predicted nest density is 0.25 [i .e., (1.97 nests/

40.5 ha)/(7.73 nests/40.5 ha)]. The same approach when the mean visual

obstruction measurement equals 0.0 yields a suitability index of 0.22.

However, sites that have a visual obstruction measurement of 0.0 throughout

the nesting season (e.g., summer fallow) will not provide any cover for nests.

Therefore, a mean visual obstruction measurement of 0.0 prior to spring vegetative growth is assigned a suitability index of 0.0. This approach may underestimate areas of growing grain or alfalfa that may provide some nesting cover

later in the nesting season. Gadwalls may benefit from these habitats because

they are relatively late nesters. Far fewer nests are placed in uplands

(Higgins 1977), however, and a total failure of nests in croplands has been

reported (Hines and Mitchell 1983). Therefore, assigning a 0.0 suitability to

areas with a mean of 0.0 for the visual obstruction measurement of residual

vegetation may not be a significant underestimate of habitat suitability for

the gadwall.

Gadwalls are late nesters and the average conditions of residual vegetat i on may not be as appropri ate a measure of the sui tabil ity of a fi e1d for

nesting as conditions later in the spring. Shaffer et al. (1985) developed a

17
model to predict the number of nests (N) that could be expected in a field

based on the availability of pairs (P), a late-spring average visual obstruction measurement (X), and field size (A), as follows:

N =(0.0052 + 0.0045 x X) x P x A (5)

The above model did not include an estimate of an optimum nesting density and,

therefore, cannot be used to estimate an index of habitat suitability.

The relationship between an average visual obstruction measurement in a

given field and a suitability index (SIV3) for nesting gadwalls is shown in

Figure 3a. The index obta i ned from Figure 3a represents the potential of a

cover type to support maximum gadwall densities based on residual vegetation

conditions.

Fig. 3a Fig. 3b

---. 1.0 1.0 M o:::t

:> :> 1-1 1-1

V) 0.8 V)

"'-" 0.8

x x

Q) Q)

""C 0.6 ""C 0.6 t: t: 1-1 1-1

t' 0.4 t' 0.4 ..... ..... r- ..... ..0 0.2 ..0

to to 0.2 +.> +.> ..... ..... V) 0.0 V) 0.0

a 1.0 2.0 3.0 4.0+ (dm) a 0.8 1.6 2.4 3.2+ (km)

a 3.9 7.9 11.3 15.7+ (i n) a 0.5 1.0 1.5 2.0+ (mi)

Mean visual obstruction Mean distance to a wetland

measurement (other than ephemeral wetlands)

Figure 3. The relationships between values of variables used to evaluate

gadwall nesting habitat in a given field and suitability indices for the

variables.

18
Distance from nesting cover to suitable wetland habitat may influence the

va1ue of potential nesting cover. Gadwall hens and broods have been reported

to disperse 1.6 to 3.2 km from the nest. However, selection of nesting habitat

is influenced more by proximity of wetlands available to pairs than by proximityof larger brood wetlands (Swanson, pers. comm.). It is assumed that

nesting cover within 1.6 km of any wetland other than ephemeral wetlands (if

the classification system of Stewart and Kantrud 1971 is used) is optimally

interspersed with suitable wetlands. Potential nesting cover located> 3.2 km

from a nonephemeral wetland is considered unavailable to nesting gadwall hens.

The relationship between these assumptions and a suitability index (SIV4) is

depicted in Figure 3b. Virtually all the nesting cover in the Prairie Pothole

Region is within 1.6 km of a suitable wetland; therefore, interspersion is not

a factor in many habitat evaluations for the gadwall. It is included in this

model, however, for use in situations where interspersion of suitable wetlands

and nesting cover is less suitable as the result of major land use changes or

from naturally existing conditions.

The physical suitability of a cover type for gadwall nesting is based on

the suitability index obtained for the mean visual obstruction measurement for

the cover type (SIV3). This suitability is directly modified by the availability of the cover type for nesting, which is evaluated by the suitability

index obtained for distance to a wetland from the cover type (SIV4) to yield a

suitability index for nesting (SIN). This relationship is expressed as:

SIN i = SIV3 i x SIV4 i (6)

Equation 6 determines the nesting suitability of a given cover type. In

order to be directly comparable to the suitability of pair habitat and brood

habitat (discussed below), the suitability values for each cover type must be

combined to obtain an overall estimate of the suitability of available nesting

habitat. The contribution of each cover type to nesting quality is a function

of qual ity and area. The sum of the products of qual ity and area across all

cover types yields an estimate of the equivalent optimum area of nesting

habitat available to the gadwall (EGAN). This value can be determined by:

(7)

where EGAN = the equivalent optimum area of gadwall nesting habitat/259 ha

S =size of the total study area in hectares

m =the number of cover types potentially providing gadwall

nesting cover

Ai = the area of cover type i

= the nesting suitability index for cover type

Equation 6)

19

(from
The estimate obtained from Equation 7 must be compared to an optimum

condition in order to obtain an overall index of nesting habitat quality for

the area being evaluated. Optimum wetland habitat for pairs and broods is

considered in this model to equal 64.8 ha of optimum wetlands1259 ha. Under

the best wetland conditions, therefore, 194.2 ha of the section could be

managed for optimum nesting habitat for gadwall. Optimum conditions, therefore, are assumed to exist if there are 64.8 ha of optimum wetlands and

194.2 ha of optimum nesting habitat/259 ha. If no nesting habitat is available, the index of nesting quality is 0.0. A linear relationship is assumed

to exist for values of equivalent optimum area of nesting habitat per section

between 0 and 194.2 ha (Fig. 4). The estimated equivalent optimum nesting

habitat per section from Equation 7 can be compared to Figure 4 to obtain an

index of nesting habitat quality (SIN) for a given study area.

Brood habitat component. Habitat suitabil i ty for gadwall broods is a

function of wetland availability and distribution. Workshop participants

believed that different wetland classes have different potentials to support

gadwall broods. Indices that indicate the relative value of each wetland

class to gadwall broods were assigned (Table 5). These values were based on

the experiences of workshop participants and considered several factors,

including water permanence, vegetative cover, water depth, potential food

resources, and observed brood usage of the wetland cl asses. Semi permanent

wetlands were considered the preferred natural wetland type for gadwall broods,

1.0

z:

.....

U) 0.8

x

<lJ

-0 0.6 .....

>,

...... 0.4

'r-

.0

to 0.2 ......

:::::l

U) 0.0

a 64.8 129.6 194.2 259.2 (ha)

a 160 320 480 640 (ac)

Equivalent optimum area of

nesting habitat/259 ha

Figure 4. The relationship between the equivalent optimum area of gadwall

gadwall nesting habitat and an overall nesting habitat suitability index.

20

•
Table 5. Wetland preference indices for gadwall broods in the

Prairie Pothole Region.

Preference index b Preference index c

Wetland classa (expert opinion) (use/availability)

Ephemeral ( I) 0.00

Temporary (I! ) 0.00 < 0.001

Seasonal (II!) 0.25 0.15

Semipermanent (IV) 1. 00 1.00

Permanent (V) 0.80 0.89

A1ka1i (VI) 0.40 < 0.001

aTerminology from Stewart and Kantrud (1971). See Table 2 and Applic~tion of

the Model for gUidelines on using other classification systems.

bBased on input from model workshop participants; recommended for use in model

applications.

cBased on brood use data from Duebbert and Frank (1984) and wetland availability

data from Stewart and Kantrud (1973).

21
followed by permanent, alkaline, and seasonal wetlands. Ephemeral and

temporary wetlands were assigned a value of 0.0 because they typically are

unavailable during the brood-rearing period. Support for these wetland rankings for gadwall brood use is presented in Table 5, based on observed gadwall

brood use (Duebbert and Frank 1984) and the availability of wetlands (Stewart

and Kantrud 1973). The only major discrepancy in the two brood indices is for

alkali wetlands. This difference may be due to the low number of observed

broods (n =5 over 20 years; Duebbert and Frank 1984) that may have a function

of the large size of alkali wetlands (6% of wetland area, < 0.05% of wetland

numbers; Stewart and Kantrud 1973). Swanson et al. (1984) found that gadwalls

were the most prevalent waterfowl species using saline lakes for brood habitat;

use was concentrated in areas of freshwater seeps that supported emergent

vegetation for cover and provided fresh water for drinking. The indices

proposed by workshop participants are recommended for use in model applications

because the use/availability indices are based on a limited amount of wetland

data.

Gadwall broods also use constructed wetlands (e.g., stockponds and dugouts). Preference indices for gadwall broods in constructed wetlands are not

included in this model because the classification of constructed wetlands is

not based on ecological characteristics. If constructed wetlands are considered in a given application of this model, preference indices must be developed

by model users. Similarities in water conditions and vegetation between

constructed and natural wetlands can be used to assign brood preference indices

to constructed wetlands.

Use of preference indices (Table 5) implies that all wetlands within a

given wetland class or water regime are of the same value to gadwall broods.

While differences in sUitability for broods exist between wetlands of a given

class or water regime, this model is intended for evaluation of large areas

with numerous wetlands, rather than an evaluation of each wetland.

Optimum habitat conditions for waterfowl broods, including gadwalls, are

assumed to exist when at least 20 ha of equivalent optimum wetlands and at

least six equivalent optimum wetlands are present per section. A total lack

of wetlands provides no brood suitability. The value of wetlands to waterfowl

broods is assumed to decrease in a linear relationship from optimum conditions

as the number and area of optimum wetlands approaches zero. The selection of

20 ha as the standard of comparison was based on the opinion of Lokemoen

(pers. comm.) that 100 waterfowl broods/259 ha was an attainable production

level. Further, he assumed that a semipermanent wetland (optimum brood habitat

as defined by this model) could support 2 broods/0.4 ha. Therefore, 20 ha of

opti mum wet1ands cou1d support the max i mum production of 100 brood s/259 ha.

The selection of a minimum of six optimum wetlands/259 ha is based on the

experiences of workshop participants.

The number of wetlands on a study area can be converted to the number of

equivalent optimum wetlands available for brood-rearing by weighting the

number of wetlands in each class by the preference indices for broods

(Table 5), as follows:

22
where

n

EONWB = t (wibi) i=l

EONWB = equivalent optimum number of wet1ands/259 ha

available for gadwall brood rearing

n = the number of wetland classes available

(8)

wi = the number of wetlands of class i/259 ha

bi =preference index for gadwall broods for wetland class i (from

Table 5)

Equation 8 determi nes the sum of the number of wetlands per section

weighted by the quality of the classes of wetlands available for gadwall

broods. The relationship between the number of equivalent optimum wetlands

per section and a suitability index (SIV6) for gadwall broods is presented in

Figure 5a.

Fig. 5a Fig. 5b

1.0 ....... 1.0 \D r-,

> > ...... ...... V') 0.8 V') 0.8

>< ><

Q) Q)

"'0 0.6 "'0 c c 0.6 ...... ......

..... 0.4 ..... 0.4 ..... ..... ..... ..c ..c rtl 0.2 rtl +) +) 0.2 ..... ..... V') 0.0 V') 0.0

0 2 4 6 8+ 0 5.1 10.1 15.2 20.2+ (ha)

0 12.5 25.0 37.5 50.0+ (ac)

Equivalent optimum number

of wet1ands/259 ha (broods) Equivalent optimum area of

wet1ands/259 ha (broods)

Figure 5. The relationships between values of variables used to evaluate

gadwall brood habitat and suitability indices for the variables.

23
A value for the equivalent optimum area of wetlands/259 ha for

broods can be determined by:

n

EOAWB = L

i=l

(a.b.) 1 1 (9)

where EOAWB = equivalent optimum area of wetlands/259 ha available for

gadwall brood-rearing

a. = the area of wetlands in wetland class i/259 ha

1

Equation 9 determines the sum of the area of wetlands per section weighted

by the quality of the wetlands available for gadwall broods. The relationship

between this value and a suitability index (SIV7) for gadwall broods is shown

in Figure 5b.

The two variables selected for evaluating brood cover are not entirely

independent. For example, an increase in the number of equivalent optimum

wetlands will likely result in an increase in equivalent optimum area for

gadwall broods. Although the variables are not independent, the variable with

the lowest suitability level will have the greatest influence on the final

value for brood-rearing habitat suitability (SIB). This relationship is best

expressed by a geometric mean of the suitability indices for the two variables:

SIB = (SIV6 x SIV7)1/2 (10)

HSI determination. The calculation of life requisite values considered

composition and interspersion needs. The production of gadwalls on a

particular area is assumed to be ultimately determined by the component with

the lowest potential to support the needs of the species. The Habitat

Suitability Index is based on the limiting factor theory and equals the lowest

of the suitability indices determined for pair (SIP), nesting (SIN), or brood

habitat (SIB).

Application of the Model

Summary of model variables and equations. A number of habitat variables

and equations are used in this model to evaluate pair, nesting, and broodrearing habitat for the gadwall (Fig. 6). The relationships between the

habitat variables, derived variables, and life requisites used in this model

and an HSI for the gadwall are summarized in Figure 7. The definitions and

suggested measurement techniques for the variables used in this model are

listed in Figure 8.

24
Pair Component Page

n

Equatt on (1) EONWP = I (w;p;) 15

;=1

n

(2) EOAWP = I (a;p;) 15

;=1

(3) SIP = (SIVI x SIV2)1/2 16

Nest;ng Component Page

Equat;on (4) Y = 1. 68 + 2.42x 17

(6) SIN; = SIV3; x SIV4. 19

1

m

(7) EOAN = 259 I (A;SIN;) 19

S ;=1

Brood Component Page

n

Equati on (8) EONWB = I (w;b;) 23

;=1

n

(9) EOAWB = I (a;b;) 24

;=1

(10) SIB = (SIV6 x SIV7)1/2 24

F;gure 6. Summary of equations used ;n the gadwall HSI model (equation

variables are defined on the pages indicated).

25
Ha b i ta t va ria b Ie Derived variables Li fe requ isite

Number of wetlands/259 ha Equivalent optimum number

by wetland class or water regim~ of wetlands for pairs

Preference index of each wetland

c I... f'o r pa l r s ]

Area of wetlands/259 ha by L-----Equivalent optimum area

wetland class or water regime of wetlands for pairs

Mean visual obstruction measurement type

I Pa i r ha b i ta t

Area of cover types potentiallY~ Equivalent optimum area Nesting habitat I HSI

used for nesting of nesting habitat

Mean distance to a wetland (other

than ephemeral wetlands)

N

m Number of wetlands/259 ha by ] Equivalent optimum number

wetland class or water regime of wetlands for broods

Preference index of each wetland

c I a s s ro r "mod, ]

Area of wetlands/259 ha by '-----Equivalent optimum area

wetland class or water regime of wetlands for broods

I Brood habitat

Figure 7. The relationship between habitat variables, derived variables,

life requisites, and an HSI for the gadwall.
Variable (definition)

Number of wetlands/

259 ha (640 ac) by

wetland class or water

regime.

Area of wetlands/

259 ha (640 ac) by

wetland class or water

regime.

Mean visual obstruction

measurement of residual

vegetation [an estimate

of the amount of cover

provided by residual

vegetation within a cover

type; measured as 'the height

at which a pole is totally

obscured by vegetation when

viewed from a distance of

4 m (13.1 ft)].

Mean distance to a wetland

(other than ephemeral

wetlands) (an average of

the distances from randomly

selected points within a

cover type to the edge of

the nearest wetland, other

than ephemeral wetlands).

Cover types

Entire study

area

Entire study

area

C,P/H,G,F,

DS,DSS

C,P/H,G,F,

DS,DSS

Suggested technique

Classify wetlands and

tally number within

each wetland class

(Stewart and Kantrud

1971) or water regime

(Cowardin et al.

1979); convert

density for each

class or water regime

to number/259 ha.

Classify wetlands and

determine area of

each wetland; sum the

areas of all wetlands

in each wetland

class or water

regime; convert

total area of each

class or water

regime to ha/259 ha.

Transect.

Aerial photographs,

ruler.

Figure 8. Definitions of variables and suggested measurement techniques.

27
Data collection and analysis. Values for habitat variables used to evaluate pair and brood habitat can be estimated through wetland classification

and measurement using aerial photographs. Evaluation of nesting cover with

this model requires the collection of field data for visual obstruction

measurements, as well as determination of the average distance to the nearest

wetland for each cover type that provides potential nesting habitat. Indices

for these two variables are combined with the area of each nesting cover type

to determine an equivalent optimum area of nesting habitat per section

(Equation 7).

Although the variables used in this model

259 ha, any size study area can be evaluated

estimates are standardized to 259 ha before

determined.

are based on an area of

as long as all variable

suitability indices are

Use of other wetland classification systems. In order to use this model

without modification, wetlands on a study area must be classified according to

the system developed by Stewart and Kantrud (1971). Other classifications

that are generally available include those of Shaw and Fredine (1956) and

Cowardin et al. (1979). When this model is used where wetlands are classified

by a system other than that of Stewart and Kantrud (1971), the terminology of

the classification system used must be equated to that used in this model.

Guidel ines relating the system of Shaw and Fredine (1956) to Stewart and

Kantrud (1971) are provided in the latter publication. Equivalent terminology

from Cowardin et al. (1979) is presented in Table 2 (p.10). The primary discrepancy between the two systems is that ephemeral wetlands of Stewart and

Kantrud (1971) are not considered wetlands by Cowardin et al . (1979). In

practice, this will not have any impact on the outputs of this model because

emphemeral wetlands have preference indices of 0.0 for pairs (Tables 3 and 4)

and broods (Table 5).

The wetland classes of Stewart and Kantrud (1971) are II ••• distinguished

by the vegetational zone occurring in the central or deeper part and occupying

5 percent or more of the total wetland area being c l as st f t ed'' (Stewart and

Kantrud 1971:7). Definitions of the wetland classes are as follows (Stewart

and Kantrud 1971:7-8):

Class I - emphemeral ponds. The wetland-low-prairie zone dominates

the deepest part of the pond basin.

Cl ass II - temporary ponds. The wet-meadow zone domi nates the

deepest part of the wetland area. A peripheral low-prairie zone is

usually present.

Class III seasonal ponds and lakes. The shallow-marsh zone

dominates the deepest part of the wetland area. Peripheral wetmeadow and low-prairie zones are usually present.

Cl ass IV - semi permanent ponds and 1akes. The deep-marsh zone

dominates the deepest part of the wetland area. Shallow-marsh,

wet-meadow, and low-prairie zones are usually present, and isolated

marginal pockets of fen zones occasionally occur.

28
Class V - permanent ponds and lakes. The permanent-open-water zone

domi nates the deepest part of the wetland area. Peri phera 1 deepmarsh, shallow-marsh, wet-meadow, and low-prairie zones are often

present, and isolated marginal pockets of fen zone occasionally

occur.

Cl ass VI - a ] ka1i ponds and 1akes. The i ntermi ttent-a °1 ka 1i zone

dominates the deepest part of the wetland area. Peripheral shallowmarsh, wet-meadow, and 1ow-pra i ri e zones are usua lly present. A

deep-marsh zone is normally absent except occasionally for isolated

patches near marginal seepage areas. A few isolated pockets of fen

zone are normally present along the margins.

Class VII - fen (alkaline bog) ponds. The fen zone dominates the

deepest part of the wetland area. Peri phera 1 wet-meadow and lowprairie zones are often present.

The water regime modifiers of Cowardin et al. (1979) that are used in

this model (Table 2) are described below (Cowardin et al. 1979:24);

Permanent ly Flooded. Water covers the 1and surface throughout the

year in all years. Vegetation is composed of obligate hydrophytes.

Intermi ttently Exposed. Surface water is present throughout the

year except in years of extreme drought.

Semi permanently Flooded. Surface water persists throughout the

growing season in most years. When surface water is absent, the

water table is usually at or very near the land surface.

Seasona lly Flooded. Surface water is present for extended peri ods

especially early in the growing season, but is absent by the end of

the season in most years. When surface water is absent, the water

table is often near the land surface.

Saturated. The substrate is saturated to the surface for extended

periods during the growing season, but surface water is seldom

present.

Temper-ar-t ly Flooded. Surface water is present for bri ef peri ods

duri ng the growi ng season, but the water table usua lly 1i es well

be low the soi 1 surface for most of the season. Pl ants that grow

both in uplands and wetlands are characteristic of the temporarily

flooded regime.

Intermittently Flooded. The substrate is usually exposed, but

surface water is present for vari abl e peri ods without detectable

seasona 1 peri odi ci ty. Weeks, months, or even years may intervene

between periods of inundation. The dominant plant communities under

this regime may change as soil moisture conditions change.

29
Use of the model on islands. Gadwalls reach much higher nesting densities

on islands than in nonisland habitats. These high densities apparently result

from an absence of predation which results in high success rates, and

subsequent homing to island habitats. However, the available literature does

not indicate any significant differences between island and mainland nesting

habitat. In both island and nonisland habitat, gadwalls select tall, dense

herbaceous vegetation or shrub cover. This model does not consider factors

such as predation, nesting success, or homing; therefore, island nesting

habitat is evaluated in the same way as nonisland nesting habitat. The model

user should be aware, however, that in terms of absolute production, significantly more gadwalls can potentially be produced on islands. This difference

should be considered when management recommendations or mitigation plans are

being developed.

Model assumptions. An abundance of literature exists on waterfowl in the

Prairie Pothole Region. The majority of published literature concentrates on

individual facets of waterfowl ecology, such as feeding habitat or nesting

habitat, and very few studies attempt to evaluate the total habitat needs of

individual species. As a result, numerous unifying assumptions were necessary

to develop a model that at'tempts to evaluate overall habitat suitability.

Another reason numerous assumptions were necessary in model development is

that the purpose of the HSI model is to provide a tool that can be used to

evaluate large areas of habitat in relatively short periods of time with

limited data. As a result, simplification was necessary to develop a practical

model to be used in habitat evaluation by field biologists. Increased

practicality is generally made at the expense of complexity and predictive

capabilities.

The major assumptions in this HSI model are:

1. Semipermanent wetlands (semipermanently flooded water regime) provide

optimal habitat for pairs and broods and can be used as a standard

against which to measure the suitability of other wetlands.

2. All wetlands within a given wetland class or water regime have the

same level of suitability for gadwall pairs and broods.

3. Under optimal conditions for pair, nesting, and brood habitat, the

optimal ratio of upland to wetland habitat is 3 to 1.

4. Average conditions for the height and density of residual vegetation

in upland habitat are suitable for predicting the quality of nesting

habitat for gadwalls.

5. The distance between potential nesting habitat and brood habitat is

less of a factor in nest site selection than the distance between

potential nesting habitat and pair habitat.

30
Regarding the assumption that average conditions of residual vegetation

in an upland cover type are adequate predictors of nesting suitability,

evidence suggests that the predictive capability of average vegetative conditions is limited (Shaffer et al. 1985). This most likely results from

simplifying complex relationships to produce a practical model. Other

variables potentially important in nest site selection and nesting density

include II ••• the quality of nearby wetlands, the attractiveness of competing

nesting habitats, and the extent to which homing influences nest density in a

particular f i e l d" (Shaffer et al. 1985:16). Other factors are probably involved in determining the overall attractiveness of a given habitat to nesting

hens.

SOURCES OF OTHER MODELS

No other habitat models to predict overall habitat suitability for the

gadwall were located in the literature. Hines and Mitchell (1983) developed

the following regression equation to predict the number of gadwall nests

expected on 10.0 by 30.0 m artificial islands in Waterhen Marsh, Saskatchewan:

Y = 0.085C + 0.177H - 1.813

where Y= predicted number of nests on an island

C = % forb canopy cover

H = vegetation height (cm)

The multiple correlation coefficient (R) was 0.96 (p < 0.01) with an RZ

value of 0.92.

Johnson (1982, cited by Shaffer et al. 1985) developed a model to predict

pair densities from wetland area. Density of gadwall pairs was estimated to

be 0.128 pairs/ha of wetland. Shaffer et al. (1985) proposed a model to

predict nest density based on available pairs, field size, and a visual

obstructi on measurement under 1ate spri ng condi t ions (Equation 5). Both of

these models estimate specific population parameters; neither attempts to

estimate overall habitat suitability.

REFERENCES

Bellrose, F. C. 1976. Ducks, geese, and swans of North America. Stackpole

Books, Harrisburg, PA. 544 pp.

1979. Species distribution, habitats, and characteristics of

breeding dabbling ducks in North America. Pages 1-15 in T. A. Bookhout,

ed. Waterfowl and wetlands - an integrated revi ew. Proc. 1977 Symp.,

Madison, WI, North Central Sect., The Wildl. Soc.

31
Canning, D. J., and S. G. Herman. 1983. Gadwall breeding range expansion

into western Washington. Murrelet 64(1):27-31.

Cowardin, L. M., V. Carter, F. C. Golet, and E. T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. U.S. Fish

Wild1. Serv. FWS/OBS-79/3l. 103 pp.

Duebbert, H. F. 1966. Island nesting of the gadwall in North Dakota. Wilson

Bull. 78(1):12-25.

1982. Nesting of waterfowl on islands in Lake Audubon, NO.

Wildl. Soc. Bull. 10(3):232-237.

Duebbert, H. F., and A. M. Frank. 1984. Value of prairie wetlands to duck

broods. Wildl. Soc. Bull. 12(1):27-34.

Duebbert, H. F., and J. T. Lokemoen. 1976. Duck nesting in fields of undisturbed grass-legume cover. J. Wildl. Manage. 40(1):39-49.

1980. High duck nesting success in a predator-reduced

environment. J. Wildl. Manage. 44(2):428-437.

Duebbert, H. F., J. T. Lokemoen, and D. E. Sharp. 1983. Concentrated nest i ng

of mallards and gadwalls on Miller Lake Island, NO. J. Wildl. Manage.

47(3):729-740.

Duebbert, H. F., E. T. Jacobson, K. F. Higgins, and E. B. Podoll. 1981.

Establishment of seeded grasslands for wildlife habitat in the Prairie

Pothole Region. U.S. Fish Wildl. Servo Spec. Sci. Rep. Wildl. 234.

21 pp.

Evans, C. D., and K. E. Black. 1956. Duck production studies on the pra t r i e

potholes of South Dakota. U.S. Fish Wildl. Servo Spec. Sci. Rep. Wildl.

32. 59 pp. Cited by Mack and Flake 1980.

Flake, L. D., and P. A. Vohs, Jr. 1979.

production and to non-game bird

B-045-SDAK. Wildl . and Fi sh Dept.,

50 pp.

Importance of wetland types to duck

populations. Completion Rep. Proj.

South Dakota State Univ., Brookings.

Flake, L. D., G. L. Petersen, and W. L. Tucker. 1977. Habitat relationships

of breeding waterfowl on stock ponds in northwestern South Dakota. Proc.

South Dakota Acad. Sci. 56:135-151. Cited by Mack and Flake 1980.

Gates, J. M. 1957. Autumn food habi ts of the gadwa11 in northern Utah.

Proc. Utah Acad. Sci. 34:69-71.

1962. Breeding biology of the gadwall in northern Utah. Wilson

Bull. 74(1):43-67.

32
Giroux, J. F. 1981a. Use of artificial islands by nesting waterfowl in

southeastern Alberta. J. Wildl. Manage. 45(3):669-679.

1981b. Ducks nesting on artificial islands during drought. J.

Wildl. Manage. 45(3):783-786.

Hammond, M. C., and G. E. Mann. 1956. Waterfowl nesting islands. J. Wildl.

Manage. 20(4):345-352.

Henny, C. J., and N. E. Holgersen. 1974. Range expansion and population

increase of the gadwall in eastern North America. Wildfowl 25:95-101.

Higgins, K. F. 1977. Duck nesting in intensively farmed areas of North

Dakota. J. Wildl. Manage. 41(2):232-242.

Hines, J. L, and G. J. Mitchell. 1983. Gadwall nest-site selection and

nesting success. J. Wildl. Manage. 47(4):1063-1071 .

. 1984. Parasitic laying in nests of gadwalls. Can. J. Zool.

-----:;6'""""""2,--;(4"")--::6=-;:;"27 - 630 .

Johnson, D. H. 1982. Procedures for developing regression equations to

predict duck numbers from wetland information. Unpublished report.

12 pp. Cited by Shaffer et al. 1985.

Kaiser, P. H., S. S. Berlinger, and L. H. Fredrickson. 1979. Response of

blue-winged teal to range management on waterfowl production areas in

southeastern South Dakota. J. Range Manage. 32(4):295-298.

Kaminski, R. M., and H. H. Prince. 1981. Dabbling duck and aquatic macroinvertebrate responses to manipulated wetland habitat. J. Wildl. Manage.

45(1):1-15.

Kantrud, H. A., and R. E. Stewart. 1977. Use of natural basin wetlands by

breeding waterfowl in North Dakota. J. Wildl. Manage. 41(2):243-253.

Kirsch, L. M. 1969. Waterfowl production in relation to grazing. J. Wildl.

Manage. 33(4):821-828.

Kirsch, L. M., H. F. Duebbert, and A. D. Kruse. 1978. Grazing and haying

effects on habitats of upland nesting birds. Trans. N. Am. Wildl. Nat.

Resour. Conf. 43:486-497.

Klett, A. L., H. F. Duebbert, and G. L.

native grasses as nesting cover

12(2): 134-138.

Heismeyer.

by ducks.

1984. Use of seeded

Wildl. Soc. Bull.

Landers, J. L., A. S. Johnson, P. H. Morgan, and W. P. Baldwin. 1976. Duck

foods in managed tidal impoundments in South Carolina. J. Wildl. Manage.

40(4):721-728.

33
Leitch. W. G.• and R. M. Kaminski. 1985. Long-term wetland-waterfowl trends

in Saskatchewan grassland. J. Wildl. Manage. 49(1):212-222.

Mack. G. D.• and L. D. Flake. 1980. Habitat relationships of waterfowl

broods on South Dakota stock ponds. J. Wildl. Manage. 44(3):695-700.

Martz. G. F. 1967. Effects of nesting cover removal on breeding puddle

ducks. J. Wildl. Manage. 31(2):236-247.

Miller. A. W.• and B. D. Collins. 1954. A nesting study of ducks and coots

on Tule Lake and Lower Klamath National Wildlife Refuge. California Fish

and Game 40:17-37.

Murkin. H. R.• R. M. Kaminski, and R. D. Titman. 1982. Responses by dabbling

ducks and aquatic invertebrates to an experimentally manipulated cattail

marsh. Can. J. Zool. 60:2324-2332.

Dring. L. W. 1969. Summer biology of the gadwall at Delta, Manitoba. Wilson

Bull. 81(1):44-54.

Paulus. S. L. 1982. Feeding ecology of gadwalls in Louisiana in winter. J.

Wildl. Manage. 46(1):71-79.

Robel. R. J .• J. N. Briggs. A. D. Dayton, and L. C. Hulbert. 1970. Relationships between visual obstruction measurements and weight of grassland

vegetation. J. Range Manage. 23(4):295-297.

Ruwa l dt , J. J .• Jr.• L. D. Flake. and J. M. Gates. 1979.

of natural and man-made wetlands in South Dakota.

43(2):375-383.

Waterfowl pair use

J. Wildl. Manage.

Ser i e , J. R.• and G. A. Swanson. 1976. Feeding ecology of breeding gadwalls

on saline wetlands. J. Wildl. Manage. 40(1):69-81.

Shaffer. T. L.• J. T. l.okemoen , and D. H. Johnson. 1985. Relations between

blue-winged teal and gadwall nest densities and habitat features. U.S.

Fish Wildl. Serv., Northern Prairie Wildl. Res. Center. report to Western

Energy and Land Use Team. Ft. Collins. CO. 23 pp.

Shaw. S. P.• and C. G. Fredine. 1956. Wetlands of the United States. U.S.

Fish Wildl. Servo Circ. 39. 67 pp.

Stewart. R. E.• and H. A. Kantrud. 1971. Classification of natural ponds and

lakes in the glaciated prairie region. U.S. Bur. Sport Fish. Wildl.

Resour. Publ. 92. 57 pp.

__--,----,-_------:,-------;. 1973. Eco1ogi ca 1 di stri buti on of breedi ng waterfowl popul ations in North Dakota. J. Wildl. Manage. 37(1):39-50.

Sugden. L. G. 1973. Feeding ecology of pintail. gadwall. American widgeon.

and lesser scaup ducklings. Can. Wildl. Servo Rep. 24. 45 pp.

34
Swanson, G. A., V. A. Adomaitis, F. B. Lee, J. R. Serie, and J. A. Shoesmith.

1984. Limnological conditions influencing duckling use of saline lakes

in south-central North Dakota. J. Wildl. Manage. 48(2):340-349.

Trauger, D. L. 1967. Habitat factors influencing duck brood use of semipermanent and permanent prairie potholes in North Dakota. M.S. Thesis,

Iowa State Univ., Ames. 229 pp.

U.S. Fish and Wildlife Service. 1981. Standards for the development of

Habitat Suitability Index models. 103 ESM. U.S. Fish Wildl. Serv., Div.

Ecol. Serv., Washington, DC. n.p.

Vermeer, K. 1970. Some aspects of the nesting of ducks on islands in Lake

Newell, Alberta. J. Wildl. Manage. 34(1):126-129.

Voorhees, L. D., and J. F. Cassel. 1980. Highway right-of-way: mowing versus

succession as related to duck nesting. J. Wildl. Manage. 44(1):155-163.

White, D. H., and D. James.

ronments by winteri ng

90(1):99-111.

1978. Differentia 1 use of fresh water enviwaterfowl of coastal Texas. Wi 1son Bull.

Yocom, C. F., and H. A. Hansen. 1960. Population studies of waterfowl in

eastern Washington. J. Wildl. Manage. 24:237-250.

35
50272-101

REPORT DOCUMENTATION 11. REPORT NO.

PAGE Biological Report 82(10.100)

3. Reclpl.nt'. Acceulon No.

.. Tltl••nd SUbtltl.

Habitat Suitability Index Models: Gadwall (Breeding)

7. Author(.)

Patrick J. Sousa

9....rfonnln.0,..nl~.tlonN.m••nd Addr••• Western Energy and Land Use Team

U.S. Fish and Wildlife Service

Drake Creekside Building One

2627 Redwing Road

Fort Collins, CO 80526-2899

5. R.port D.t.

September 1985 ..

a. Perfonnln. O,..nlzetlon R.pt. No.

10. ProjectlT..k/Work Unit No.

11. COntl'Kt(C) or Grant(G) No.

(C)

(G)

12. Sponaorln.O....nIZlltlonN.m••ndAddr... Western Energy and Land Use Team 13. TypeofR.port&"'rlodCOv.,.d

Division of Biological Services

Research and Development

Fish and Wildlife Service 1~

Department of the Interior~ Washinoto DC 20240

15. SUppl.m.nt.ry Not.. The Biological Report series was initiated in October 1984 to replace the

FWS/OBS series published since 1976. The report number assigned to this model follows

consecutively from the Habitat Suitability Index models published as FWS/OBS-82/10. , now

numbered as Bioloqical Report 82(10. ). - 1.. Abstract (limit: 200 _Fda)

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

Suitability Index (HSI) model for the gadwall (Anas strepera). The model

consolidates habitat use information into a framework appropriate for field

application, and is scaled to produce an index between 0.0 (unsuitable habitat)

and 1.0 (optimum habitat). HSI models are designed to be used with Habitat

Evaluation Procedures previously developed by the U.S. Fish and Wildlife Service.

17. Document An.lysl. •• Descriptors

Birds

Wildlife

Habitabil ity

Mathematical models

b. ldentlfiera/Open·Endlld T.rm.

Gadwall

Anas strepera

Habitat suitability

Co COSATI FI.ld/Group

*u.s. GOVERNMENT PRINTING OFFICE: 1985--580-640/25404

1.. Av.lI.blllty Stat.m.nt

Release unlimited

(5. ANS!-Z3lUI)

19. Security CI... (Thl. R.port)

Unclassified

20. Security CIa.. (Thl. Pa••)

Unclassified

See InstructIon. on R.v.,..

21. No. of ......

35

22. PrIce

OPTIONAL fORM 272 (4-77)

(Formerly NTI$-35)

Dep.rtment ofCom_