FWStOBS·8211 0.43

JULY 1983

HABITAT SUITABILITY INDEX MODELS:

WOOD DUCK

SK

361

.U54

no. 82-

10.43

, 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.
FWS/OBS-82/10.43

July 1983

HABITAT SUITABILITY INDEX MODELS: WOOD DUCK

by

Patrick J. Sousa

and

Adrian H. Farmer

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., and A. H. Farmer. 1983. Habitat suitability index models:

Wood duck. U.S. Dept. Int., Fish Wildl. Servo FWS/OBS-82/10.43. 27 pp.
PREFACE

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

(FWS/OBS-821l0), 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. The habitat use information provides

the foundation for HSI models that follow. In addition, this same information

may be useful in the development of other models more appropriate to specific

assessment or evaluation needs.

The HSI Model Section documents a habitat model and 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 application information includes descriptions of the geographic ranges and seasonal

application of the model, its current verification status, and a listing of

model variables with recommended measurement techniques for each variable.

In essence, the model presented herein is a hypothesis of species-habitat

relationships and not a statement of proven cause and effect relationships.

Results of model performance tests, when available, are referenced. However,

models that have demonstrated reliability in specific situations may prove

unreliable in others. For this reason, feedback is encouraged from users of

this model concerning improvements and other suggestions that may increase the

utility and effectiveness of this habitat-based approach to fish and wildlife

planning. 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
iv
CONTENTS

Page

PREFACE iii

ACKNOWLEDGMENTS vi

HABITAT USE INFORMATION 1

Genera 1 1

Food 1

Water 2

Cover 3

Reproduct ion 4

Interspersion..................................................... 6

Special Considerations............................................ 7

HABITAT SUITABILITY INDEX (HSI) MODELS 8

Model Applicability 8

Model Description - Breeding...................................... 9

Model Description - Winter........................................ 15

Model Relationships - Breeding and Winter 16

Application of the Models 20

SOURCES OF OTHER MODELS 21

REFERENCES 24

v
ACKNOWLEDGMENTS

Earl ier versions of an HSI model for the wood duck were reviewed by

Drs. Leigh Fredrickson, Frank Bellrose, and Frank McGilvrey. Dr. Fredrickson

commented on two earlier drafts and his comments were very valuable in helping

to describe the relationships between wood ducks and their habitat. The

comments and suggestions of all three reviewers have added considerably to the

quality and value of this model, and their input is very gratefully

acknowledged.

The development of this HSI model was partially funded by the U.S. Army

Corps of Engi neers through thei r Waterways Experiment Station in Vi cksburg,

Mississippi. The participating work unit is Testing of Habitat Evaluation

Methods within the Environmental Impact Research Program.

Word processing of this document was provided by Carolyn Gulzow and Dora

Ibarra. The cover illustration was drawn by Jennifer Shoemaker.

vi
WOOD DUCK (Aix sponsa)

HABITAT USE INFORMATION

General

Wood ducks (Aix sponsa) inhabit creeks, rivers, floodplain lakes, swamps,

and beaver ponds (Bellrose 1976). The major breeding range of the wood duck

is in the eastern United States, from Florida and east Texas north to Maine

and North Dakota, and north into the eastern Canadi an provi nces. A Pacifi c

population breeds from British Columbia south to California and east to

Montana. The major wintering range occurs south of Maryland in the Atlantic

and Gulf coast States, as well as Arkansas and Tennessee. The majority of the

Pacific population winters in the Sacramento Valley. Wood ducks are permanent

residents in the southern half of their breeding range.

Food

Wood ducks have been referred to as primarily herbivorous (Landers et al.

1977) although recent studi es have i ndicated that invertebrates make up a

significant part of the annual diet (Drobney and Fredrickson 1979). Wood

ducks forage on the ground or in water at depths up to 46 cm (18 inches)

(McGilvrey 1968). In Missouri, they foraged primarily in flooded timber

duri ng spri ng and fall (Drobney and Fredri ckson 1979). The daily foragi ng

radius in the southeastern United States may be as much as 40 to 48 km (25 to

30 mi) (U.S. Forest Service 1971). Food items include mast and fruits, aquatic

plants and seeds, insects, and aquatic invertebrates. Acorns and other mast

are important fall and winter foods (Landers et al. 1977). When acorns are

lacking, other important foods include the seeds of baldcypress (Taxodium

distichum), hickories (Carya spp.), buttonbush (Cephalanthus occidental is),

arrowarum (Peltandra virginica), and burreed (Sparganium spp.) (Bell rose

1976). In South Carolina, McGilvrey (1966) found that greater than 98% of the

stomach contents of 108 wood ducks shot by hunters were fruits and seeds of

water oak (Quercus ni gra), pi n oak (Q. pal ustri s), ba1dcypre ss , sweetgum

(Liquidambar styraciflua), water hickory (f. aquatica), and corn (Zea mays).

Important fall foods of wood ducks in Maine were pondweeds (Potamogeton spp.),

burreeds, water bulrush (Scirpus subterminalis), oaks, and wild rice (Zizania

aquatica) (Coulter 1957). Wood ducks prefer to forage for mast in areas of

shallow water, although they may also forage on the forest floor (Brakhage

1966; Bell rose 1976) and even on tree limbs before the mast has fallen

(Brakhage 1966). Important foods during the breeding season include persistent

1
overwintering fruits; corn and other domestic grain; seeds and fruits from

bottomland hardwood trees, shrubs, and aquatic herbaceous plants; early spring

plants; and invertebrates (McGilvrey 1968).

Female wood ducks have high protein and calcium requirements in the

spring and feed heavily on aquatic invertebrates (Landers et al. 1977). They

satisfy their protein requirements for egg laying through their diet rather

than through internal stores (Drobney 1980). Invertebrates made up about 82%

by volume of the diet of wood duck hens in Missouri during the laying period

(Drobney 1980). During incubation, when protein requirements were reduced,

58.5% of the diet of the hens was plant foods. Drakes did not exhibit the

same pattern of invertebrate use, indicating that hens fed selectively on

invertebrates during the egg laying period. The abundance and availability of

macroinvertebrates to wood duck hens during the pre-breeding period is critical

to successful reproduction (Fredrickson, pers. comm.). Invertebrates made up

about one-third of the fall diet of drakes and hens, and the spring diet of

drakes (Drobney and Fredrickson 1979).

Ducklings less than 1 week old are dependent on animal foods (primarily

insects) and forage in areas where both food and some protective cover are

present (Hocutt and Dimmick 1971). The diet of ducklings is similar to that

of adul ts by 6 weeks of age.

Water

No information on dietary water needs of the wood duck was found in the

literature. However, water needs are likely satisfied in wetland habitats

used by the wood duck. The remainder of this section describes those water

characteristics that influence habitat use by wood ducks.

Water depth affects the quantity, variety, and distribution of cover and

food, and wood duck needs are generally met between the shoreline and a water

depth of 1.8 m (6 ft) (McGilvrey 1968). However, even when wood ducks feed in

deeper water, the actual feeding depth is generally restricted to the top

30 cm (12 inches) of water (Fredrickson, pers. comm.). Water is critical in

wood duck breeding and brood-rearing habitat from mid-January to late September

in the southern Uni ted States and from mi d-April to 1ate September in the

northern portions of the range. Water in most of the breeding habitat should

be from 7.5 to 45 cm (3 to 18 inches) deep, still or slow-moving, and sheltered

from the wind. Areas with water less than 30 cm (12 inches) deep are

especi ally important in provi di ng invertebrate foods for breeding wood ducks

(Drobney and Fredrickson 1979). A water current of 4.8 km/hr (3 mph) has been

estimated as the maximum tolerable stream flow for breeding wood ducks,

although broods seldom use areas with currents greater than 1.6 km/hr (1 mph)

(McGilvrey 1968).

Isolated wetlands much less than 4 ha (10 acres) in size are considered

marginal brood rearing habitat (McGilvrey 1968). The more shoreline per unit

area of water, the more sui tab1e the habi tat, provi ded the di stance between

opposite shores is at least 30 m (100 ft).

2
Cover

Suitable cover for wood ducks may be provided by trees or shrubs overhangi ng water, flooded woody vegetation, or a combi nat i on of these two types

(McGilvrey 1968). A ratio of 50 to 75% cover to 25 to 50% open water is

preferred in breeding and brood rearing habitat. Adult molting habitat is

similar to brood habitat (Palmer 1976), although molting adults make greater

use of herbaceous wetlands domi nated by cattail sand bul rushes (Bell rose,

pers. comm.).

An abundance of downed timber provides suitable year-round cover (Webster

and McGilvrey 1966). Young trees and mature shrubs with low overhead and

lateral growth provide optimal cover for breeding adults (McGilvrey 1968).

Ideal shrub cover is provided by shrubs that form a dense canopy about 0.6 m

(2 ft) above the water surface. The deciduous forested types used by breeding

wood ducks vary throughout their range, although wooded areas that are flooded

in early spring are the most suitable nesting habitat. McGilvrey (1968) lists

the following as the most important habitats for nesting wood ducks: Southern

floodplain forests; red maple (Acer rubrum) swamps; Central floodplain forests;

temporari ly flooded oak-hi ckory forests; and Northern bottoml and hardwoods.

Buttonbush is an important source of cover for wood ducks throughout much of

their range (Webster and McGilvrey 1966; McGilvrey 1968).

Winter-persistent emergents that have a life form similar to shrubs, such

as cattail (Typha spp.), soft rush (Juncus effusus), bulrush (Scirpus spp.),

burreed, purple loosestrife (Lythrum salicaria), and phragmites (Phragmites

communis), may satisfy cover requirements where more desirable shrubs and

trees are not available (McGilvrey 1968).

Wood duck brood cover is provided by a combination of downfall and woody

and herbaceous emergent plants, well interspersed wi th small, open water

channels (Webster and McGilvrey 1966; Palmer 1976). In the Mississippi

Alluvial Valley, broods less than 2 weeks old typically use flooded lowland

forests in order to satisfy their requirements for invertebrate foods

(Fredrickson, pers. comm.). Wood ducks older than 2 weeks of age use habitats

domi nated by buttonbush. Wood duck broods in Massachusetts preferred areas

with dense cover interspersed with small open ~ools, clumps of buttonbush, and

muskrat houses (Grice and Rogers 1965). Buttonbush clumps and muskrat houses

provided loafing sites out of the water. Optimal composition in brood habitat

consists of 30 to 50% shrubs, 40 to 70% herbaceous emergents, 0 to 10% trees,

and 25% open water (McGilvrey 1968). Eight wood duck broods in Florida concentrated their activities in a shrub wetland community with shrub cover

greater than 76%, dominated by mature Carolina willow (Salix caroliniana)

(Wenner and Ma ri on 1981). Shrubs and/or clumped herbaceous vegetation may

provide cover in areas where downed timber is not available (Webster and

McGilvrey 1966). South Carolina beaver ponds that provided both shrubby and

herbaceous cover received greater use by wood duck broods than ponds dominated

by either shrubs or herbaceous vegetation (Hepp and Hair 1977). Shrubs provide

cover, security, and loafing sites, while herbaceous vegetation provides cover

and habitat for invertebrates that make up a major portion of the diet of

duckl ings. Emergent herbaceous vegetation that does not provide any early

3
spring cover, especially in pure stands, does not provide much suitable brood

cover (Webster and McGilvrey 1966). An abundance of downed trees in shallow

water [up to 0.9 m (3 ft) deep] provides excellent brood rearing cover and

" ... is particularly important for early broods hatching before leaves appear

on trees and shrubs and before the appearance of emergent plants" (McGi 1vrey

1968:11).

Emergent plants used for brood cover vary with latitude but include

smartweeds (Polygonum spp.), American lotus (Nelumbo lutea), pickerelweed

(Pontederia cordata), bluejoint (Calamagrostis canadensis), arrowheads

(Sagittaria spp.), soft rush, spatterdock (Nuphar luteum), arrowarum, and

clump sedges (Carex spp.) (McGilvrey 1968). Other important herbaceous plants

are water primrose (Jussiaea spp.), reed canarygrass (Phalaris arundinacea),

cattail, burreed, swamp loosestrife, and grasses.

Wood duck broods and breeding pairs require loafing sites scattered

throughout their habitat for preening and sunning (McGilvrey 1968). The best

loafing sites are surrounded by water, have good visibility, and are near

escape cover. Loafing sites should be at least 45 by 45 em (18 by 18 inches)

in size and 5 to 15 cm (2 to 6 inches) above water. Optimal habitat contains

10 to 20 loafing sites (muskrat mounds, stumps, logs, small islands, and

tussocks) per 0.4 ha (1 acre). Shorelines and points of land that are relatively bare of vegetation are marginal substitutes for more optimal loafing

sites. The lack of suitable loafing sites may be a limiting factor in brood

use (Beard 1964).

Wood duck broods in South Carolina used small ponds (0.03 to 0.50 ha;

0.07 to 1.2 acres) significantly more often than larger ponds (1.51 to 3.80 ha;

3.7 to 9.4 acres) (Hepp and Hair 1977).

Shrub swamps dominated by buttonbush were preferred as fall roost sites

in southern III i noi s over fl ooded forested habitats and open water (Parr

et al. 1979). One such roost of 200 ha (494 acres) consisted of 60% buttonbush cover and 40% open water. Another fall roost site was dominated by

American lotus, and another one was dominated by water willow (Decodon

verticil latus).

Ideal winter habitat consists of a complex of wetlands centered on a

permanent wetland (Fredrickson, pers. comm.). Optimum winter habitat includes

scrub/shrub wetlands, emergent wetlands, dead timber, and flooded forests.

Reproduction

The distribution of breeding populations of wood ducks is closely related

to ..... bottomland hardwood forest with trees of sufficient size to contain

usable nest cavities and water areas that satisfy food and cover requirements"

(McGilvrey 1968:3). Important limiting factors include the availability of

suitabl e nesting cavities (McGil vrey 1968), and the avail abil ity of protei n

foods for pre-breedi ng females (Fredri ckson, pers. comm). Hens are most

easily able to satisfy their protein requirements in flooded lowland forests,

where flooding dynamics create a highly productive invertebrate food base. In

4
the Mississippi Alluvial Valley, 1 ha (2.47 acres) of properly flooded forest

can provide enough protein foods to support 800 wood ducks for 1 day

(Fredrickson, pers. comm.). If it is assumed that a hen will use a flooded

forest habitat for 60 days during the pre-breeding and nesting periods, then

1 ha (2.47 acres) of properly flooded forest can support about 13 hens (or 5

hens/0.4 ha [1.0 acre]) during the 60-day use period. A ratio of 8 ha

(20 acres) of nesting habitat to every 0.4 ha (1 acre) of brood habitat is

recommended for maximum production in areas where natural cavities provide the

only potential nest sites (McGilvrey 1968). However, this ratio is based on:

(1) the presence of at least 1 suitable cavity/2 ha (5 acres); and (2) the

carrying capacity of each 0.4 ha (1.0 acre) of brood habitat being sufficient

to accommodate broods produced by four nest cavities.

The closer the nest cavity to water, particularly to suitable brood

habitat, the better (McGilvrey 1968). Cavities in trees in or near the water

are preferred. Most wood duck nests in tree cavities in Massachusetts were

located within 183 m (200 yds) of w~ter (Grice and Rogers 1965). Wood ducks

nesting in tree cavities in Minnesota selected cavities that were significantly

closer to water and to canopy openings than were randomly sampled trees (Gilmer

et al. 1978). Nest trees ranged from 0 to 350 m (0 to 383 yds) from water and

averaged 80 m(87.5 yds). Twenty-one of 31 nest trees selected by radio-marked

hens were within 0.5 km (0.31 mi) of permanent water, while eight nests were

farther than 1.0 km (0.62 mi) from permanent water. Artificial nest sites in

wooded areas are best located within 0.4 km (0.25 mi) of water, but nest boxes

located up to 1.6 km (1 mi) from water may also receive use (Bellrose 1976).

Nest boxes placed within 1.4 km (0.86 mi) of brood habitat in a Florida study

area received significantly greater use than those placed further away (Wenner

and Marion 1981).

Wood ducks generally nest in tree species that have a mature size of at

1east 35 to 40 cm (14 to 16 inches) dbh and along 1ife expectancy (Hansen

1966). The minimum-sized tree used for nesting in Minnesota was 28 cm

(11 inches) dbh (Gilmer et al. 1978). Overmature and decadent trees usually

contai n the 1argest number of sui tabl e cavities (McGil vrey 1968). Conifers

(Hansen 1966) and dead trees, other than cypress ,rarely provide suitable

cavities (McGilvrey 1968). The most suitable cavity trees range from 60 to

90 cm (24 to 36 inches) dbh. Natural cavities used for nesting by wood ducks

in Massachusetts ranged from 33.0 to 91. 4 cm (13 to 36 inches) dbh, with a

mean dbh of 68.6 cm (27 inches) (Grice and Rogers 1965).

Acceptable nest cavities in trees are at least 2 m (6 ft) above ground,

have an entrance size of 9 to 30.5 cm (3.5 to 12 inches) in diameter, and a

depth of 15 to 120 cm (6 to 48 inches) (McGilvrey 1968). Bellrose (pers.

comm.) considered the minimum entrance dimensions to be 7.6 by 10.0 cm (3.0 by

4.0 inches); smaller entrances restrict many wood ducks. Optimal tree

cavities, according to McGilvrey (1968) have an entrance size of 10 cm

(4 inches) in di ameter, a di ameter at the bottom of 25 to 27.5 cm (10 to

11 inches), a cavity depth of 60 cm (24 inches), and are 6 to 15 m (20 to

50 ft) above ground. Fredrickson (pers. comm.) suggested that the optimum

cavity height of 6 to 15 m, as defined by McGilvrey (1968), is simply where

most suitable cavities form in trees rather than an expressed preference by

5
nesting wood ducks. However, Bellrose et al. (1964) found an increasing index

of use (i .e., use compared to availability) with increasing cavity height. A

suitable cavity must drain well and preferably has its entrance protected from

the weather (McGi 1vrey 1968). Cavi ty trees in southeastern Mi ssouri were

defined as all trees at least 24.1 cm (9.5 inches) dbh that contained at least

one cavity with an entrance size of at least 6.4 by 8.9 cm (2.5 by 3.5 inches)

(Weier 1966). Suitable cavities were those of adequate dimensions that did

not have adverse features, such as water or excessive debris in the cavity or

open tops above the cavity. A total of 109 cavity trees were found in three

cover types, and 17 were judged to contain suitable cavities for wood ducks, a

ratio of 1 suitable cavity to 6.4 cavity trees. A suitable cavity on two

study areas in Massachusetts was defined as having a minimum entrance size of

6.4 by 8.9 cm (2.5 by 3.5 inches) and being within 0.8 km (0.5 mi) of water

(Grice and Rogers 1965). Results were 1 suitable cavity/5.3 cavity trees (13

suitable out of 69 caVities) on one study area and 1 suitable cavity/4 cavity

trees (9 suitable out of 36 cavities) on the second area.

The density of suitable cavities on two Massachusetts study areas was

2.5/2.59 km 2 (1 mF) and 0.6/2.59 km 2 (1 mF), although the estimates were

based on total study area size rather than on timbered area only (Grice and

Rogers 1965). The density of suitable cavities in timbered bottomland in Iowa

was 1/9.7 ha (24 acres) (Dreis and Hendrickson 1952, cited by Grice and Rogers

1965). In Illinois, suitable cavities were defined as those with an entrance

diameter of at least 8.9 cm (3.5 inches) and that were free of water or debris

(Bellrose et al. 1964). One suitable cavity/5.3 ha (13 acres) was found in

bottomland forests, and 1 suitable cavity/2.0 ha (5 acres) was found in upland

woodlots. The density of suitable cavities (defined above) in three timber

types in Missouri ranged from 1/1.4 ha (3.4 acres) to 1/4.2 ha (10.3 acres),

and averaged 1/2.1 ha (5.2 acres) of forested habitat (Weier 1966). The

highest reported density of suitable cavities [defined by an entrance diameter

of at least 10 cm (3.9 inches)] was 4/ha (1.6/acre) in mature northern hardwood

and mature aspen forests in Minnesota (Gilmer et al. 1978).

Interspersion

The best wood duck habitat is characterized by nest sites in close proximi ty to brood habitat (McGil vrey 1968). However, wood duck broods in North

Carolina moved 2.4 km (1.5 mi) from a nesting pond to a shrub thicket marsh

for brood rearing (Hardister et al. 1962). Although most of the movement was

along a water course, overland travel of 0.16 km (0.1 mi) was requi red from

the nesting pond to the river used for the major part of the movement. Wood

duck hens and broods in Minnesota travelled overland up to 3.9 km (2.4 mi)

from nest site to brood habitat (Ball 1973, cited by Gilmer et a l . 1978).

Wood duck broods in eastcentra 1 Texas moved up to 11. 7 km (7.7 mi) to brood

habitat from nest sites located in areas without brood habitat, although

overall brood survival was only 8% (Ridlehuber 1980). Management of forests

for wood duck nesting cavities greater than 0.8 km (0.5 mi) from brood habitat

is generally not recommended (McGilvrey 1968). Ball et al. (1975:778) found

II ••• a significant negative linear correlation ... between distance of overland

moves completed prior to 2 weeks of age and number of surviving ducklings in

6
broods of radio-marked hens" (21 wood duck hens, 8 mallard [Anas platyrhyncos]

hens). Broods that moved less than 0.8 km (0.5 mi) averaged 8.5 ducklings

compared to an average of 6.8 ducklings in broods that moved greater distances.

The maximum reported brood density is 17 broods on a 5.7 ha (14 acres) impoundmentin Maryland (McGilvrey n.d., cited by McGilvrey 1968). In North Carolina,

a 16.2 ha (40 acres) brood-rearing area supported a minimum of 27 wood duck

broods in 1966 and 17 broods in 1967 (Vance 1968). Also in North Carolina,

duckling density averaged about 2.0/0.4 ha (1.0 acre) of suitable brood rearing

habitat and ranged from 1.6 to 2.3 ducklings/0.4 ha (1.0 acre) (Baines 1971).

McGil vrey (1969) reported a survi va1 rate of hatched duck1i ngs to fl i ght

stage of 53% (9.8 ducklings/brood at hatch; 5.2 ducklings/brood reaching

flight stage). Ball et al. (1975) accounted for the loss of total broods, and

concluded that wood duck hens successfully raised 41% of the total ducklings

hatched.

Wood ducks do not maintain stable home ranges, and both the size and

shape of their home ranges are flexible (Bellrose 1976). The total home range

utilized by broods in South Carolina varied from 0.77 to 29.6 ha (1.9 to

73.1 acres) (Hepp and Hair 1977). Movements from fall roosts in III i noi s

ranged up to 10 km (6.2 mi), although most movements were wi thi n 2.2 km

(1.4 mi) of the roosts (Parr et al. 1979). Areas of activity during the fall

ranged from 23.9 to 186.2 ha (59 to 460 acres) and averaged 90.6 ha

(224 acres). Most activity of nesting hens in Minnesota was within 1.0 km

(0.6 mi) of the nest site, suggesting that a pair may use an area of approximately 3.0 km 2 (1.6 mP) (Gilmer et al. 1978).

Special Considerations

In areas where natural cavities are lacking or limiting, artificial nest

boxes can be used to increase breeding populations (Bell rose et al. 1964).

The most important factors limiting wood duck breeding populations are availability of and competition for suitable cavities, predators (McGilvrey 1968),

and food (Fredrickson, pers. comm.). A nest box program that provides

predator-proof nesting cavities can minimize the effects of the first two of

these factors. In Massachusetts, Grice and Rogers (1965) found strong evidence

that natural nest cavities were in short supply and concluded that (p. 87) " ... wood ducks can be maintained at a higher level of abundance with [nest

boxes] than without them". Other studies have also reported increases in

breeding populations due to the use of nest boxes (Bellrose et al. 1964; Jones

and Leopold 1967; Strange et al. 1971; Alexander 1977). However, some evidence

exists to suggest that an excessive number of nest boxes may be detrimental to

wood duck production. In California, a breeding population of wood ducks

increased faster than the number of available nest sites (Jones and Leopold

1967). Over the course of the 9-year study, nest sites were gradually

increased from 3 to 16 on a 11.3 ha (28 acres) marsh; an increase of breeding

pairs from 3 to 35-40 occurred during the same period. At the higher levels

of pair density, the population became essentially self-limiting due to intraspecific competition for nest cavities, an increase in nest desertion and dump

nesting (i.e., instances in which several hens lay eggs in the same nest

site), and a resultant decrease in the production of young per pair. Nest

7
interference is also common on sites with extensive habitat where food is

abundant and nest sites are limited (Fredrickson, pers. comm.). However,

severa 1 researchers have reported that dump-nesting resulted in a greater

production of young (Morse and Wight 1969; Clawson et al. 1979; Heusmann

et al. 1980). Strader et al. (1978) cautioned that crowded nesting conditions

could be detrimental to wood duck production; they observed a wood duck hen

call a brood from an adjacent nest box mounted on the same support pole and

abandon incubation of her own clutch.

McGilvrey (1968) recommended that nest boxes be placed in clusters of 5

to 10 spaced 15 to 30 m (50 to 100 ft) apart within clusters. Bellrose (1976)

recommended that nest boxes be placed in groups of 2 to 4/0.4 ha (1.0 acre).

Bellrose et al. (1964) recommended a nest box density of 2 to 3/0.4 ha

(1.0 acre) in "high-quality habitat", although criteria to determine highquality habitat were not presented. This level of nest boxes was recommended

for woodlots where nesting in natural cavities was 1 pair/4.0 ha (10 acres).

Additional guidelines for nest box placement are available in Bellrose et al.

(1964), Bellrose (1976), and McGilvrey (1968). None of these references,

however, contain information on a possible saturation level of nest boxes

beyond which production would either remain constant or decrease. All of the

above references note that nest boxes are effective only if they are predatorproof and regularly maintained.

Clearing of bottomland hardwoods has adversely affected wood duck populations because bottomland hardwood sites provide habitat for nesting, brood

rearing, and wintering (Bellrose 1976).

HABITAT SUITABILITY INDEX (HSI) MODELS

Model Applicability

Geographic area. The two HSI models contained here have been developed

for application within the breeding and wintering range of the wood duck

(Fig. 1).

Season. These HSI models may be used to evaluate breeding (spring and

summer) habitat and/or winter (fall and winter) habitat, depending on the

residency status of the wood duck in the area to be evaluated.

Cover types. These models may be used to evaluate habitat in the following cover types (terminology follows that of U.S. Fish and Wildlife Service

1981): Deciduous Forest (OF); Deci duous Forested Wetl and (DFW); Deciduous

Scrub-Shrub Wetland (OSW); Herbaceous Wetland (HW); and Riverine (R). Use of

unflooded deciduous forests is restricted to the breeding season model and

should not be included when using the winter habitat model; however, flooded

lowland deciduous forests should be included as winter habitat. Evaluation of

wetlands should be restricted to those with water present during either the

nesti ng/brood-reari ng peri od or duri ng the wi nter peri od, dependi ng on the

model(s) being used.

8
Breeding range

Wintering range

Figure 1. Geographic applicability of the wood duck HSI

models within the United States (ranges from Bellrose 1976).

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

amount of contiguous habitat that is required before an area will be occupied

by a speci es. The minimum habitat area for broods is estimated to be 4 ha

(10 acres) of any of the wetland cover types listed above. Potential brood

habitat may exist either as an isolated wetland of at least 4 ha or as smaller

wetlands separated by less than 46 m (50 yds) of land where the total area of

potential brood habitat equals at least 4 ha. In stream or riverine habitat,

small brood units should be within 0.4 km (0.25 mi) of each other. Minimum

habitat area for habitat components other than brood habitat is unknown.

Verification level. These models have not been tested against habitats

of known quality. Earlier drafts were reviewed by Drs. Leigh Fredrickson,

Frank Bellrose, and Frank McGilvrey. Their review comments have been incorporated into the models.

Model Description - Breeding

Overview. The breeding season HSI model for the wood duck considers

nesting and brood-rearing needs as critical components of breeding habitat.

An HSI value for the breeding season considers the quality, composition, and

juxtaposition of nesting and brood rearing resources. Food (vegetable and

invertebrate) is considered to be correlated with vegetative cover, and the

variable used to evaluate brood cover in this model is assumed to serve as a

surrogate measure of food sui tabi 1i ty. Factors other than vegetative cover

9
(e.g., water quality, current, depth, permanence) may affect food suitability

for wood ducks, but are not included in this model due to the difficulty of

establishing relationships between the variables and a measure of food

suitability. This is particularly difficult for highly dynamic variables,

such as flooding periodicity. The assumption that food suitability can be

estimated by considering vegetative cover only is the major limitation of this

mode 1.

The following sections identify important habitat variables, describe

suitability levels of the variables, and describe the relationships between

variables. The relationship between habitat variables, life requisites, and

cover types used in this model and an HSI value for the wood duck during the

breeding season is shown in Figure 2.

Nesting component. The quality of nesting habitat is a function of the

availability of nesting sites. Potential nesting sites may be either naturally

occurring tree cavities or artificial nest sites in the form of nest boxes.

However, the presence of natural (including those in live trees and snags)

and/or artificial nest cavities does not guarantee an equivalent number of

successful nests. The proportion of observed potential nesting sites that are

actually suitable for wood duck nesting and the proportion of suitable nesting

sites that can be expected to support successful nests are important criteria

determining the number of ducklings produced in a specified area.

Grice and Rogers (1965) tallied all cavities on two study areas but

defined as suitable those cavities with minimum entrance dimensions of 6.4 by

8.9 cm (2.5 by 3.5 inches) and that were located within 0.8 km (0.5 mi) of

water. Only 22 of 105 cavities (20.9%) met the minimum criteria. Weier

(1966) tallied all cavities within 0.8 km (0.5 mi) of water that had a minimum

entrance dimension of 6.4 by 8.9 cm (2.5 by 3.5 inches), a nesting platform of

at least 12.7 by 17.8 cm (5 by 7 inches), and that were located in trees with

a minimum dbh of 24 cm (9.5 inches). Suitable cavities met those criteria,

did not contain water or debris, and were not open-topped. Seventeen of 109

cavities (15.6%) meeting minimum criteria were classed as suitable. In order

to most easily evaluate natural cavities with this model, it is assumed that a

cavity is potentially useful if it has a minimum entrance size of 7.6 by

10.0 cm (3.0 by 4.0 inches) (Bellrose, pers. comm.). Based on the information

presented above, it is also assumed that only 18% of observed cavities meeting

this minimum criterion will actually be suitable for wood duck use. All

artificial nest sites are assumed to be suitable if they are predator-proof

and cleaned and repaired annually.

The second major criterion determining the number of successful nests on

a given area is the proportion of suitable cavities that can be expected to

produce successful nests. Bellrose et al. (1964) found that of 631 natural

cavities available and structurally suitable (i.e., minimum entrance dimensions

as described above and free of water or debris), 235 (37%) were used by wood

ducks. Data from numerous studies summarized by Bellrose (1976) indicate that

the average use of artificial nest sites is 41% (46,761 house years; 19,108

nests). However, these data for both natural and artificial sites do not take

into account whether factors other than the avail abi 1i ty of nest sites were

limiting the nesting population; for example, poor quality brood-rearing

10
Percent of the water

surface covered by Brood-rearing-------- potential brood cover.

I-'

I-'

Habitat variables

Number of potentially suitable tree cavities/

0.4 ha (1.0 acre)

[cavities/0.4 ha

(1.0 acre) with minimum

entrance dimensions of

7.6 by 10.0 cm (3.0 by 4.0 inches); cavities

may be in I ive trees or snags].

Number of nest boxes/

0.4 ha (1.0 acre)

that are predator-proof and maintained.

Life requisites

Density of potential Nesting- _

nest sites/0.4 ha - ( 1.0 ac re 1•

Cover types

Deciduous forest

Deciduous forested

wetland

Dec i duous sc rub/

shrub wetland

Herbaceous wetland

Riverine

Deciduous forested

wetland

Dec iduous sc rub/

sh rub wet Iand

Herbaceous wetland

River i ne

HSI

(breeding

season)

Figure 2. The relationship of habitat variables, life requisites, and cover types

to an HSI value for the wood duck during the breeding season.
habitat may have limited recruitment of hens into the breeding population, or

poor pre-breeding habitat may have limited the number of hens able to successfully nest. For the purposes of this model, it is assumed that all potential

nest sites meeting the minimum criteria defined above may potentially be used.

If it is assumed that all suitable natural and artificial nest sites may

potentially be used, then the success rate of the initiated clutches will

determine the overall production of young from nest sites. The success rate

of nests in natural cavities in Illinois was 49.1% (118 nests, 58 successful)

from 1939-1940 and 39.9~6 (158 nests, 68 successful) from 1958-1961, with the

lower success rate due to an increase in predation (Bellrose et al. 1964).

However, the highest success rate in natural cavities reported in the literature is 52~~ (Prince 1965, cited by Bellrose 1976). It is assumed in this

model that 52% is the best success rate that can be expected for wood ducks

nesting in natural cavities.

Bellrose (1976) summarized the results of a number of studies of artificial nest sites for wood ducks. The average success rate, with individual

success rates weighted by the number of nests, was 71.6%. However, the two

highest reported success rates for wood ducks nesting in artificial cavities

are 95%, based on 341 nests in Arkansas (Brown 1973, cited by Bellrose 1976),

and 94%, based on 281 nests in Iowa (Leopold 1966, cited by Bellrose 1976).

Based on this information, it is assumed in this model that 95% is the best

success rate that can be expected for wood ducks nesting in nest boxes.

Based on the preceding discussion, the number of successful nests that

can be expected on a given area can be determined by the following equation:

# of potentially successful nests = (NT x PIT x P2T) + (NB x P1B x P2B) (1)

where: NT = the number of tree cavities with a minimum entrance size of 7.6

by 10.0 cm

PIT = the proportion of observed tree cavities that can be expected

to be suitable for nesting by wood ducks

P2T = the proportion of suitable cavities that can be expected to

produce successful nests

NB = the number of available nest boxes

= the proportion of nest boxes that are actually suitable for

nesting by wood ducks

the proportion of suitable nest boxes that can be expected to

produce successful nests

Substituting the values determined previously for PIT' P2T, P1B, and P2B

yields the following equation:

12
# of potential successful nests = (NT x .18 x .52) + (NB x 1.0 x .95)

= (NT x .09) + (NB x .95) (2)

The maximum reported density of successful nests appears to be about 5

successful nests/0.4 ha (1.0 acre) on a North Carolina study area (Hester

n.d., cited by McGilvrey 1968). Although this may not represent a stable

maximum density (Bellrose, pers. comm.), it is assumed in this model that 5

successful nests/0.4 ha (1.0 acre) represents the maximum density of successful

nests and therefore determines the maximum production of ducklings. Based on

equation (2), this maximum density can be achieved with either 55.6 natural

cavities/0.4 ha (1.0 acre) or 5.3 nest boxes/0.4 ha (1.0 acre), or by a

combination of the two types of nest sites. However, this nest site density

does not necessarily need to exist across an entire study area in order to

have optimal habitat. The relationship between optimal nesting habitat and

optimal brood-rearing habitat is discussed under the Interspersion Component

section. Although some evidence exists to suggest that wood duck nesting

populations can be so dense that overall production is adversely affected

(Jones and Leopold 1967; Strader et al. 1978), such a relationship has not

been documented to the point that a decrease in habitat suitability beyond a

certain density of nesting sites can be predicted.

Brood-rearing component. The quality of brood-rearing habitat is influenced by cover, water permanence, and wetland characteristics.

Cover for wood duck broods consists of dense cover in shallow wetlands

with water present throughout the peri od of brood occupancy. Cover can be

provided by emergent herbaceous vegetation, emergent shrubs and trees with

crowns within 1 m (3.3 ft) of the water surface, or woody downfall. Dense

cover that is well interspersed with small open water channels provides optimal

brood habitat. Optimal brood cover within a wetland is assumed to occur when

the proportion of total cover in the wetland ranges from 50 to 75 %. Other

factors that influence the suitability of brood habitat include water depth,

qua 1i ty, current, and permanence . All of these factors i nfl uence the amount

of cover and the macroi nvertebrate food base to a certain extent and may be

highly dynamic within a wetland. It is assumed in this model that cover

conditions are the reflection of the combined influence of these variables.

It is assumed, therefore, that the quality of wood duck brood habitat can be

evaluated solely on the basis of the amount of cover available in the wetland.

A major implication of this assumption is that the abundance and quality of

vegetative and invertebrate foods is indicated by the cover conditions

described above. This assumed relationship may not be valid in all conditions,

especially in flooded lowland forests, where an abundant detrital-based food

source may be present in the absence of low, dense cover.

Interspersion component. Nesting and brood-rearing needs can be met by

different cover types, and a consideration of the juxtaposition and composition of cover types providing the life requisites is necessary in order to

evaluate breeding habitat suitability.

13
Habitat suitabil ity is i nfl uenced by the juxtapositi on of nesting and

brood-rearing habitat. Optimal juxtaposition of nesting and brood-rearing

resources is assumed to exist when cover types providing these life requisites

are located within 0.8 km (0.5 mi) of each other. When potential nesting and

brood-rearing habitats are separated by more than 3.2 km (2 mi) of upland

habitats with no aquatic "trave l l ane s", it is assumed that the cover types

are too far apart to be used by wood ducks or that mortal ity of duck"lings

travelling from the nest to brood-rearing habitat will equal 100%.

Habitat suitability is also influenced by the proportion of habitat

(composition) providing nesting and brood-rearing resources. In order to

determine the optimal composition of nesting and brood-rearing habitat, it is

necessary to determine the number of young capable of reaching fl ight stage

per unit area of optimal brood-rearing habitat compared to the number of young

produced per unit area of optimal nesting habitat. The maximum reported

density of broods is 17 broods on a 5.7 ha (14 acres) impoundment in Maryland,

equivalent to 1.2 broods/0.4 ha (1.0 acre) (McGilvrey n.d., cited by McGilvrey

1968). The observed broods on a 54.7 ha (135 acres) area, including the

5.7 ha impoundment, averaged 9.8 ducklings at hatching and 5.2 ducklings

reaching flight stage, a survival rate of 53% (McGilvrey 1969). The 5.7 ha

impoundment, therefore, supported about 88 duckl ings (i .e., 17 broods x 5.2

ducklings/brood) to flight stage, an average of 6.2 ducklings/0.4 ha (1.0 acre)

of brood-rearing habitat. This level of production is considered to be the

potential of optimal brood-rearing habitat for the purposes of this model.

Optimal nesting habitat was described earlier as capable of producing 5

successful nests/0.4 ha (1.0 acre). If the average clutch size in normal

nests is assumed to be 12.2 (Bellrose 1976) and all eggs are assumed to hatch

successfully, then 0.4 ha (1.0 acre) of optimum nesting habitat can potentially

produce 61 ducklings (i .e., 12.2 ducklings/clutch x 5 clutches/0.4 ha) leaving

the nest sites. The highest survival rate of ducklings reported in the literature is 53% (McGilvrey 1969). It is assumed in this model that this is the

optimal survival rate of ducklings reaching brood-rearing habitat. If it is

further assumed that survival from the nest to brood-rearing habitat equals

100% (i .e., interspersion is optimal), and optimal brood-rearing habitat

exists, then an average of 32.3 ducklings (0.53 x 61) will survive to flight

stage from the 61 ducklings produced on 0.4 ha (1.0 acre) of optimal nesting

habitat. As described above, 0.4 ha (1.0 acre) of optimum brood-rearing

habitat can potentially support 6.2 ducklings to flight stage. Therefore, the

ratio of optimum brood-rearing habitat to optimum nesting habitat to support

maximum wood duck production is approximately 5.2:1 (i.e., 32.3/6.2 = 5.2).

The maximum potential production of wood ducks per unit area will occur if

optimal nesting and optimal brood-rearing conditions exist on all areas under

consideration. Therefore, the optimal composition of wood duck habitat is

approximately 19% optimal nesting habitat ([1/5.2J x 100 = 19%) and 100%

optimal brood-rearing habitat ([5.2/5.2J x 100 =100%).

The assumptions involved in determining optimal composition of nesting

and brood-rearing resources are summarized below:

1. Optimal nesting habitat will produce 5 successful nests/0.4 ha (1.0

acre).

14
2. Average clutch size in normal nests (i.e., non-dump nests) is 12.2,

and hatching success equals 100%.

3. Survival of ducklings from nests to brood-rearing habitat equals

100%, and survival to flight stage of ducklings reaching broodrearing habitat equals 53%.

4. Optimal brood-rearing habitat can support 6.2 ducklings/0.4 ha

(1.0 acre) to flight stage.

5. Optimal habitat conditions for wood duck production consist of

nesting habitat and brood-rearing habitat provided by the same cover

types (i .e., all cover types provide both nesting and brood-rearing

habitat).

Model Description - Winter

Overview. This winter HSI model for the wood duck considers cover as the

key life requisite determining winter habitat suitability. The measurement of

vegetative cover within wetlands is assumed to serve as a surrogate measure of

wi nter food suitabil ity , Other factors affect food sui tabil ity, but are not

included in this model. The assumption that a measure of vegetative cover can

be used to evaluate food suitability is a limitation of the model. The assumption may not be valid in some situations, such as when wood ducks are feeding

in fl ooded bottoml and forests, where food may be abundant in the absence of

low vegetative cover. The relationship between habitat variables, winter

cover, cover types, and an HSI for winter habitat of the wood duck is shown in

Figure 3.

Habitat variable

Life

requisites Cover types

Percent of the water forested

surface covered by ----- Cover ____ potential winter

cover

Deciduous

wetland

Deciduous

wetland

Herbaceous

Riverine

scrub/shrub -- HSI

(wi nter)

wetland

Figure 3. The relationship of habitat variables, life requisites, and

cover types to an HSI value for the wood duck during the winter.

Cover component. It is assumed in this model that winter habitat needs

of the wood duck are similar to habitat used during the brood-rearing period

(see p. 13). Optimal conditions are assumed to be present if the amount of

tota 1 cover (woody and/or herbaceous) ranges from 50-75%. Wi nter-persistent

herbaceous plants are the only type of herbaceous vegetation considered in an

eva1uat i on of wi nter habitat. Water depth, qual i ty , current, and permanence

are not treated as separate habitat variables for the reasons discussed in the

15
brood-rearing section of the breeding season model. Although acorns and other

mast are an important winter food source, wood ducks will use other foods if

necessary. It is assumed that food suitability will vary directly with cover

suitability, and is not considered as a separate winter life requisite in this

model.

Model Relationships - Breeding and Winter

Suitability Index (SI) graphs for habitat variables. This section contains suitability index graphs that illustrate the habitat relationships

described earlier. Suitability index graphs for both the breeding HSI model

and the winter HSI model are presented in this section.

Cover

OF, DFW,

DSW,HW,R

Variable

Density of potential

nest sites/0.4 ha (1.0

acre). Determined by

the equation:

(0.18 x VI) +

(0.95 x V2 ) where

VI =the number of

potentially suitable

tree cavities/0.4 ha,

and V2 =the number

of nest boxes/0.4 ha

(see Figure 4 for

complete definition of

VI and V2 ) .

1.

>(

CIJ

"0

c

...... 0.8

>,

+-'

..... 0.6 .....

.0

."

+-' 0.4 .....

;:,

(/)

0.2

o 2 3

No./0.4ha

4 5+

DFW ,DSW, V,. Percent of the water 1.0

HW,R surface covered by >(CIJ

potential brood cover

"0

c 0.8 (see Figure 4 for ......

>, definition). +-'

..... 0.6 .....

.0

."

.....

+-' 0.4 ;:,

(/)

0.2

0 25 50 75 100

%

16
DFW,DSW, Vs Percent of the water 1.0

HW,R surface covered by x

Q)

potential winter cover -0

s:: 0.8 (see Figure 4 for ......

definition). >,

+-> .....

r- 0.6 .....

.a

ttl

+-> 0.4 :::::l

V)

0.2

a 25 50

%

75 100

Suitability Index (51) graphs for interspersion variables. This section

contains suitability index graphs that illustrate the relationship between

interspersion variables and breeding habitat suitability for the wood duck.

The use of these graphs is explained under HSl determination.

Variable

V6 Distance between

cover types.

1.0

x

Q)

-0

......

s:: 0.8

>,

.....

+-> 0.6 r-

.....

.a

ttl

+-> 0.4 .....

:::::l

V) 0.2

a 0.8 1.6 2.4 3.2+(km)

a 0.5 1.0 1.5 2.0+(mi)

17
Percent of area

providing equivalent

optimum nesting

habitat.

Percent of area

providing equivalent

optimum broodrearing habitat.

x

Q)

-0

..s 0.8

b

..... 0.6 ,.....

.....

..Cl

..... 0.4

0.2

x

Q)

-0 0.8 .......

>-,

.~ 0.6

..Cl

0.4 .....

(/)

0.2

o

o

25

25

50

%

50

%

75

75

100

100

Determination of life requisite values. The determination of life

requisite suitability indices by cover type with this model involves simple

one-variable equations. The nesting value in all cover types equals the 51 of

V3 • Brood habitat suitability and winter habitat suitability in all cover

types except deciduous forest, equals the SI of V4 and Vs , respectively.

HSI determination - breeding HSI model. It is possible that some cover

types will provide nesting habitat but not brood-rearing habitat, or brood

habitat but not nesting habitat. In order to adequately evaluate breeding

habitat, juxtaposition and composition of resources must be considered.

Several steps and calculations are necessary in order to properly incorporate

interspersion variables into the HSI determination. They are as follows:

18
1. Compute the nesting and brood-rearing values for each cover type by

collecting field data for each habitat variable, entering this data

into the proper sui tabi 1i ty index curve, and using the resul t i ng

index values in the appropriate life requisite equations. If either

nesting or brood-rearing equal s zero in all cover types, then the

HSl will equal zero and no further calculations are necessary.

2. Determine the relative area (%) of each cover type within the study

area as follows:

Relative Area (%) for Cover Type A = Area of Cover Type A

Total Area of All

Cover Types used by

the Wood Duck

x 100

Consider only those cover types used by the wood duck in determining

this percentage.

3. Determi ne whi ch cover types are not provi di ng either nesting or

brood-rearing habitat. For each of these cover types, a suitability

index for juxtaposition of resources must be computed using V6 •

This is accomplished by selecting random points on a map in each

cover type missing a life requisite and measuring the distance to

the edge of the nearest other cover type that provides that life

requi site. Enter each di stance measurement into the SI graph for

V6 , record the individual interspersion indices, and calculate the

average interspersion index for each cover type. If both nesting

and brood-rearing habitat are provided within a specific cover type,

the interspersion index equals 1.0 for the cover type.

4. Modify the relative area (%) of each cover type missing a life

requisite by multiplying the relative area by the average interspersion index for that cover type. This determines the useable relative

area (%) of each cover type. For those cover types that provide all

life requisites the useable relative' area (%) is the same as the

relative area (%).

5. To determine the % area in optimum condition for any life requisite,

first multiply the useable area (%) for each cover type by the life

requi site values for that cover type (from 1 above). Sum the

products of this multiplication across all cover types for each life

requisite. The sum for each life requisite is the equivalent percent

area that provides that life requisite at optimal levels (this is

actually an equivalent figure, i.e., 100% of the area at a 0.5 value

is equal to 50% of the area at an optimal, 1.0 value).

6. To determine overall life requisite values enter the value determined in Step 5 for nesting into the SI graph for V7 , and the value

19
determined for brood-rearing into the SI graph for Va. The resulting

index value from V, is the overall nesting value, and the index

value from Va is the overall brood-rearing value.

7. The HSI is equal to the lowest of the overall life requisite values.

This-sTngle HSI value is considered to represent breeding sUitability

across the entire area evaluated.

HSI determination - winter HSI model. The winter HSI for the wood duck.

in a specified cover type equals the winter cover value (i.e., the SI for Vs )

determined for that cover type.

HSI determination for year-round use areas. The HSI models presented

here are designed to evaluate breeding and winter habitat separately. In

those areas where the wood duck is a resident species, it may be desirable to

assign one overall HSI to a study area. In order to do so, a weighted (by

cover type area) average HSI for winter habitat is determined and compared to

the single HSI determined for breeding habitat. Because wood ducks may move

between wi nter habi tat and breedi ng habitat, the HSI in areas of permanent

residency should equal the highest of the values determined for breeding and

winter habitat suitability.

Application of the Models

Model limitations. These models represent a relatively simple approach

to evaluating wood duck habitat suitability during the breeding season and

winter. The use of cover estimates as surrogate measures of food SUitability

is perhaps the most important limitation of this model. Other factors that

affect food suitability, such as wetland dynamics, and more direct food

measurements are not included in this model because of the lack of adequate

literature in these areas. Fredrickson (pers. comm.) indicates that current

studies have the potential to address the unknowns in these models and that it

should be possible to improve these models in the next few years. However,

until such information becomes available, users should be aware of the model1s

limitations, especially in regards to wetland dynamics. For example, flooded

1owl and forests potentia lly provi de an abundant source of macroi nvertebrates

to hens prior to nesting, also to broods during the first few weeks after

hatchi ng, and to wi nteri ng wood ducks. The quality of thi s habitat may be

high even in the absence of optimum cover conditions as depicted by Variables 4

and 5 in this model. However, means to accurately and directly address the

impacts of wetland dynamics on a macroinvertebrate food base are not currently

available. The major problem limiting the use of the winter HSI model is that

the model does not include an assessment of the importance of wetland complexes

to wintering wood ducks (Fredrickson, pers. comm.). Rather, each wetland type

is evaluated individually, since the means of evaluating a large variety of

arrangements of wetlands is not currently available. Users of this model

should use the Habitat Use Information section of this model, as well as local

information, to adapt this model to local conditions, if necessary.

20
Use of model variables. Although these models provide a relatively

simple means of evaluating the suitability of wood duck habitat, use of the

breeding HSI model requires an estimate of the number of potential nest

cavities in trees. Sampling of cavities in live trees is difficult and likely

to provide an underestimate. Several options, other than intensive sampling,

are available for estimating density of potential nest sites. In areas that

are managed for wood ducks with a nest box program, optimum conditions may be

provided by artificial sites alone. In cases where there are at least 5.3

nest boxes/0.4 ha (1.0 acre), optimum suitability levels have been reached,

and a survey of potential natural nest sites is unnecessary. Alternatively,

the potential for cavity production in various cover types can be estimated

based on species composition and size classes of trees.. McGilvrey (1968)

provides a list of desirable tree species for cavity production by geographic

region. The minimum dbh of a potential nest tree is 35 cm (14 inches),

although the most suitable cavity trees range from 60 to 90 cm (24 to

30 inches) dbh. Intensive sampling of a limited area may provide an adequate

estimate of cavity density, or an estimate may be interpolated from available

literature (e.g., Dreis and Hendrickson 1952; Bellrose et al. 1964; Weier

1966; Gilmer et al. 1978) or provided by local knowledge.

Definitions of habitat variables and suggested field measurement techniques (Hays et al. 1981) are provided in Figure 4.

SOURCES OF OTHER MODELS

Several other attempts have been made to develop habitat models for the

wood duck, including models developed for use with the Habitat Evaluation

Procedures in Missouri (Flood et al. 1977; Hallett and Fredrickson 1980; Urich

et al. 1983). The Missouri models provide a means of ranking habitat suitability based on habitat characteristics. Flood et al. (1977) includes the wood

duck and hooded merganser (Lophodytes cucullatus) in a model for waterfowl in

bottomland hardwood, upland hardwood, and riverine cover types. The model in

Ha 11 ett and Fredri ckson (1980) is intended for use in both bottoml and and

upland hardwood cover types and is a refinement of the model in Flood et al.

(1977). The model in Urich et al. (1983) is intended for use in bottomland

hardwoods and is a modification of the two previous Missouri models. The

Missouri models evaluate habitat suitability only in bottomland and/or upland

hardwood forests, and do not provide criteria for evaluating the suitability

of other wetland types for wood ducks. They are most useful, therefore, where

wood duck habitat is provided by upland hardwood forests and forested wetlands.

A major difference between the Missouri models and the breeding season HSI

model presented here is the method by which interspersion variables are

treated. The Missouri models consider the distance between the cover type

being evaluated and some critical resource (i.e., timbered habitat or permanent

water) as a habitat variable. In our model, we use the distance between a

cover type and a missing life requisite (i.e., nesting or brood-rearing

habitat) to modify the available habitat area and also use life requisite

composition suitability index curves to evaluate the balance of life requisites

provided by a given area. A final major difference between the Missouri

models and the breeding season HSI model presented here lies in the manner in

21
Variable (definition)

Number of potentially

suitable tree cavities/

0.4 ha (1.0 acre)

[tree cavities/0.4 ha

(1.0 acre) with minimum

entrance dimensions of

7.6 by 10.0 cm (3.0 by

4.0 inches); cavities

may be in live trees or

snags].

Number of nest boxes/

0.4 ha (1.0 acre)

(the number of artificial wood duck nest

sites/0.4 ha that

are predator-proof

and maintained).

Density of potential

nest sites/0.4 ha

(1.0 acre) (an estimate

of the density of natural

and artificial nest sites

available to wood ducks.

Determined by the

following equation:

where V1 and V2 are as

defined above).

Cover types

OF, OFW, DSW,

HW,R

OF, OFW ,DSW,

HW,R

OF, OFW, OSW,

HW,R

Suggested techniqu~

Quadrat

Quadrat

V.. Percent of the water

surface covered by

potential brood cover

[an estimate of the

proportion of a wetlandis water surface

area that is covered

by shrub cover, overhanging tree crowns

within 1 m (3.3 ft)

of the water surface,

woody downfall, and

herbaceous vegetation].

DFW,OSW,

HW,R

Remote sensing,

ocular estimation,

l t ne intercept

Figure 4. Definitions of variables and suggested measurement techniques.

22
Variable (definition)

Percent of the water

surface covered by

potential winter

cover (same as for

V4 except that only

winter persistent

species should be

considered in the

herbaceous vegetation component).

Cover types

Figure 4. (concluded).

23

Suggested technique

Remote sensing,

ocular estimation,

line intercept
which HSI values are determined. The former models result in one HSI value

for each cover type, while this model results in one HSI value for the aggregation of cover types used by the wood duck in a given area.

A simple approach to evaluating wood duck breeding habitat along streams

was developed by Burbank (1972). This approach is based on tree size and

subjective evaluation of general stand conditions. McGilvrey (1968) provides

criteria that can be used to develop a habitat model for the wood duck for

several geographic areas.

REFERENCES

Alexander, H. L., Jr. 1977. Mallard and wood duck brood survival at Remington

Farms, Chestertown, Maryland. Trans. Northeast Fish Wildl. Conf.

34:221-230.

Baines, A. S. 1971. Estimates of the population numbers of juvenile wood

ducks (Aix sponsa) on two brood rearing areas. M.S. Thesis, N.C. State

Univ., Raleigh. 56 pp .

Ball, 1. J., Jr. 1973. Ecology of duck broods in a forested region of northcentral Minnesota. Ph.D. Thesis, Univ. Minnesota, St. Paul. 67 pp.

Cited by Gilmer et al. 1978.

Beard, E. G. 1964. Duck brood behavior at the Seney National Wildlife Refuge.

J. Wildl. Manage. 28(3):492-521.

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

Books, Harrisburg, PA. 544 pp.

Personal communication (phone conversation record 17 February

1983). Illinois Natural History Survey. Box 599, Havana, IL 62644.

Bellrose, F. C., K. L. Johnson, and T. U. Meyers. 1964.

natura 1 cavi ties and nesting houses for wood ducks.

28(4):661-676.

Relative value of

J. Wildl. Manage.

Burbank, J. H. 1972.

habitat. Proc.

26:205-206.

A quick method to assess streamside wood duck breeding

Southeastern Assoc. Game and Fish Commissioners

Pages 75-80

a symposium.

Brakhage, G. K. 1966. Management of mast crops for wood ducks.

J. B. Trefethen, ed. Wood duck management and research:

Wildl. Manage. Inst., Washington, DC.

Brown, B. W. 1973. The Big Lake wood duck: a two-year study of its preflight mortality, nest population, growth and migration, 1970-1971.

Proc. Southeastern Assoc. Game and Fish Commissioners 26:195-291. Cited

by Bellrose 1976.

24
Clawson, R. L., G. W. Hartman, and L. H. Fredrickson. 1979. Dump nesting in

a Missouri wood duck population. J. Wildl. Manage. 43(2):347-353.

Coulter, M. W. 1957. Food of wood ducks in Maine. J. Wildl. Manage.

21(2):235-236.

Dreis, R. E., and G. O. Hendrickson. 1952. Wood duck production from nest

boxes and natural cavities on the Lake Odessa area, Iowa, in 1951. Iowa

Bird Life 22(2):18-22. Cited by Grice and Rogers 1965.

Drobney, R. D. 1980.

97(3):480-490.

Reproductive bioenergetics of wood ducks. Auk

Drobney, R. D., and L. H. Fredrickson. 1979. Food selection by wood ducks in

relation to breeding status. J. Wildl. Manage. 43(1):109-120.

Flood, B. S., M. E. Sangster, R. D. Sparrowe, and T. S. Baskett. 1977. A

handbook for habitat evaluation procedures. U.S. Dept. Int., Fish Wild.

Servo Resour. Publ. 132. 77 pp.

Fredrickson, L. H. Personal communication (letters dated 24 February 1983 and

11 April 1983). Director, Gaylord Memorial Laboratory, Univ. of Missouri,

Puxico, MO 63960.

Gilmer, D. S., I. J. Ball, L. M. Cowardin, J. E. Mathisen, and J. H. Riechmann.

1978. Natural cavities used by wood ducks in north-central Minnesota.

J. Wildl. Manage. 42(2):288-298.

Grice, D., and J. P. Rogers. 1965. The wood duck in Massachusetts. Final

Rep. Fed. Aid Proj. W-19-R, Mass. Div. Fish Game. 96 pp.

Hallett, D. L., and L. H. Fredrickson. 1980. Wood duck (Aix sponsa). Pages

106-114 in T. S. Baskett, D. A. Darrow, D. L. Hallett, M. J. Armbruster,

J. A. E1ITs, B. F. Sparrowe, and P. A. Korte (eds.). A handbook for

terrestrial habitat evaluation in central Missouri. U.S. Dept. Int.,

Fish Wildl. Servo Resour. Publ. 133. 155 pp.

Hansen, H. L. 1966. Silvical characteristics of tree species and decay

process as related to cavity production. Pages 65-69 in J. B. Trefethen,

ed. Wood duck management and research: a symposium. Wildl. Manage.

Inst., Washington, DC.

Hardister, J. P., Jr., F. E. Hester, and T. L. Quay. 1962. Movements of

juvenile wood ducks as measured by web-tagging. Proc. Southeastern

Assoc. Game and Fish Commissioners 16:70-75.

Hays, R. L., C. Summers, and W. Seitz. 1981. Estimating wildlife habitat

variables. U.S. Dept. Int., Fish Wildl. Servo FWS/OBS-81/47. 111 pp.

Hepp, G. R., and J. D. Hair. 1977. Wood duck brood mobility and utilization

of beaver pond habitats. Proc. Southeastern Assoc. Game and Fish

Commissioners 31:216-225.

25
Heusmann, H. W., R. Bellville, and R. G. Burrell. 1980. Further observations

on dump nesting by wood ducks. J. Wi1d1. Manage. 44(4):908-915.

Hocutt, G. E., and R. W. Dimmick. 1971. Summer food habits of juvenile wood

ducks in east Tennessee. J. Wi1d1. Manage. 35(2):286-292.

Jones, R. E., and A. S. Leopold. 1967. Nesting interference in a dense

population of wood ducks. J. Wi1d1. Manage. 31(2):221-228.

Landers, J. L., T. T. Findley, and A. S. Johnson. 1977. Feeding ecology of

wood ducks in South Carolina. J. Wi1d1. Manage. 41(1):118-127.

Leopold, F. 1966. Experiences with home-grown wood ducks. Pages 113-123 in

J. B. Trefethen, ed. Wood duck. management and research: a symposium.

Wi1d1. Manage. Inst., Washington, DC. Cited by Be11rose 1976.

McGi1vrey, F. B. 1966. Fall food habits of wood ducks from Lake Marion,

South Carolina. J. Wi1d1. Manage. 30(1):193-195.

McGilvrey, F. B. (compiler). 1968. A guide to wood duck production habitat

requirements. Bur. of Sport Fish. and Wi1d1., Resour. Pub1. 60. 32 pp.

McGi1vrey, F. 11. 1969. Survival in wood duck broods. J. Wild1. Manage.

33(1):73-76.

Morse, T. E., and H. M. Wight. 1969. Dump nesting and its effect on production in wood ducks. J. Wi1d1. Manage. 33(2):284-293.

Palmer, R. S. (ed.). 1976. Handbook of North American birds: Volume 3.

Yale Univ. Press, New Haven and London. pp. 252-277.

Parr, D. E., M. D. Scott, and D. D. Kennedy. 1979. Autumn movements and

habitat use by wood ducks in southern Illinois. J. Wild1. Manage.

43(1): 102-108.

Prince, H. H. 1965. The breeding ecology of wood duck (Aix sponsa L.) and

common goldeneye (Bucepha1a c1angu1a L.) in central New Brunswick. M.S.

Thesis, Univ. New Brunswick, Fredericton. 109 pp. Cited by Bellrose

1976.

Rid1ehuber, K. T. 1980. Wood duck production and habitat use. Ph.D. Diss.,

Texas A&M Univ., College Station. 52 pp. (Abstract only, from Dt ss :

Abstr. Int. B Sci. Eng. 41(6):1998.)

Strader, R. W., R. E. Murry, Sr., H. R. Perry, Jr., and R. B. Hamilton. 1978.

Hen wood duck calls brood from neighboring nest box. J. Wildl. Manage.

42(4):919-920.

26
Standards for the development of

103 ESM. U.S. Dept. Int., Fish

Strange, T. H., E. R. Cunningham, and J. W. Goertz. 1971. Use of nest boxes

by wood ducks in Mississippi. J. Wi1d1. Manage. 35(4):786-793.

Urich, D. L., J. P. Graham, and C. C. Cook (compilers). 1983. A handbook for

habitat evaluation in Missouri. Missouri Dept. Conserv., U.S. Soil

Conserv. Servo (Columbia), and U.S. Fish Wildl. Servo (Columbia). 148 pp.

U.S. Fish and Wildlife Service. 1981.

Habitat Suitability Index models.

Wild1. Serv., Div. Eco1. Servo n.p.

U.S. Forest Service. 1971. Wildlife habitat management handbook. Southern

Region. U.S. Dept. Agric., For. Servo FSH 2609.23 R. n.p.

Vance, J. F., Jr. 1968. A characterization of a brood-rearing area of the

wood duck (Aix sponsa Linneaus). M.S. Thesis, N.C. State Univ., Raleigh.

51 pp.

Webster, C. G., and F. B. McGilvrey. 1Q66. Providing brood habitat for wood

ducks. Pages 70-75 in J. B. Trefethen, ed. Wood duck management and

research: a symposium:- Wi1d1. Manage. Inst., Washington, DC.

Weier, R. W. 1966. A survey of wood duck nest sites on Mingo National Wildlife Refuge in southeast Missouri. Pages 91-108 in J. B. Trefethen, ed.

Wood duck management and research: a symposium. Wild1. Manage. Inst.,

Washington, DC.

Wenner, K. C., and W. R. Marion. 1981. Wood duck production on a northern

Florida phosphate mine. J. Wi1d1. Manage. 45(4):1037-1042.

27
$027'% -, 01

REPORT DOCUMENTATION 1.1._REJIOlIT i'fO.

PAGE FWS/OBS-82/10.43

Habitat Suitability Index Models: Wood duck

I~

S. "-'t 0••

July 1983

7. AVUIorfs' •

Patrlck J. Sousa and Adrian H. Farmer

Habitat Evaluation Procedures Group

U.S. Fish and Wildlife Service

Western Energy and Land Use Team

Creekside One Building 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

Washlngton, DC 20240

(C)

(G)

14.

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

breeding and wintering habitats for the wood duck (Aix spon(a). The models are

scaled to produce indices of habitat suitability between 0 unsuitable habitat)

and 1 (optimally suitable habitat). Habitat suitability indices are designed for

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

Wildlife Service.

17. Oocu",ent An••"". •• O...,rilltO"

Mathematical models

Birds

Wildlife

Habitabil ity

Wood duck

A1X sponsa

Habitat Suitability Index

:. COSATl Fleld/GrouD

:80 ,a,.....I.Dility Staftfn_

Release unlimited

U.S. Government Printing Office 1984 - 681-459/515 Reg. 8

19. See:unty Cla.. (This ReOO!'!1

Unclassifjed 20. Security Class (This ;Oseel

Unclassifi

27

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REGION 1

Regional Director

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REGION S

eJional Director

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u.s. FISH AND WILDLIFE SERVICE

..s. ..·........ W1J.lKI...: st:RVK·.:

:\ .' .... .' "''''' ''' ,,,,- .-

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