American Black Duck Habitat Model
go to: USFWS Gulf of Maine Watershed Habitat Analysis
go to: Species Table
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Draft Date:
June 2001

Species:
American black duck, Anas rubripes

Use of Study Area Resources:
Reproductive (breeding pair, brood-rearing), migration, and winter foraging. The following narrative describes how habitat components for each of these 'uses' were mapped, and then combined.

Habitat Requirements - Reproduction:
Breeding Pair. Habitat for breeding pairs include: (1) nesting cover and substrate (Reed 1970), and (2) high quality foraging areas (USFWS 1988). Since nest sites may be located in any of a wide range of upland to lowland cover types, we assumed that hens could always find suitable nest sites in the vicinity of any suitable inland foraging area. Such areas include land (where seeds and tubers may be consumed), or wetlands and shallow water (where they feed on invertebrates and vegetation).

A variety of wetlands types are used by breeding black ducks. In inland Maine, wetlands selected for pre-laying, laying, and incubation periods, in order of preference, were: palustrine emergent, broad-leaved deciduous forested, and broad-leaved deciduous scrub-shrub types (Ringelman et al. 1982), while ephemeral pools served as important foraging sites. Streams with sandy or stony bottoms interspersed with invertebrate-rich detrital patches also were preferred (Ringelman et al. 1982). In northeastern Nova Scotia, black ducks commonly nest along streams and contiguous freshwater marshes (Seymour 1984). Black ducks nesting in coastal salt marsh in Nova Scotia foraged in the tidal marsh (Reed and Moisan 1971). Seymour and Jackson (1996) also found that a relatively large proportion of birds nesting throughout a Nova Scotia coastal watershed foraged in the downstream estuarine tidal marsh.

Brood-rearing. Following successful hatching of the eggs, hens move their broods to rearing wetlands, often considerable distances from the nest site (Ringelman and Longcore 1982). In Maine, hens and broods traveled as far as 3.3 km from the nest to an inland rearing wetland (Ringelman and Longcore 1982). In Nova Scotia hens moved broods up to 12 km from inland palustrine wetlands to a tidal marsh (Seymour and Jackson 1996). Streams serve as travel corridors to rearing wetlands (Ringelman and Longcore 1982, Seymour 1984, Seymour and Jackson 1996). Small (<0.02 ha) ephemeral pools are often used by broods en route to rearing wetlands (Ringelman and Longcore 1982).

Habitat requirements for brood-rearing include: (1) cover from predators and weather, and (2) invertebrate-rich foraging sites (USFWS 1988). Palustrine emergent, scrub-shrub and deciduous forested wetlands provide optimal cover and forage conditions (Ringelman and Longcore 1982). In Maine black ducks preferred emergent wetlands over evergreen scrub-shrub wetlands; however, dead scrub-shrub, unconsolidated bottom, and aquatic bed wetlands were not used by broods (Ringelman and Longcore 1982). Wetlands with large areas of open water, submerged aquatic vegetation, or ericaceous shrub vegetation were rarely used by broods (Ringelman and Longcore 1982).

In an estuarine environment along the St. Lawrence River in Nova Scotia, newly hatched black duck broods foraged in widgeon grass (Ruppia maritima) pools within the Juncus and Spartina patens zones of the upper marsh and, as they got older, used portions of the Spartina alterniflora zone in the lower marsh. Black ducks may associate with these vegetative and physiognomic features because of a combination of edge, cover, and invertebrate abundance (Reed and Moisan 1971). Seymour and Jackson (1996) noted that a relatively large proportion of ducks nesting throughout the watershed of a tidal estuary took broods to estuarine marsh for rearing. However, mortality of ducklings was significantly higher in broods using tidal compared to interior habitats. This likely is associated with greater predator densities in tidal marshes (Seymour 1984, Ringelman and Longcore 1982). Suitable tidal habitats are limited to depths of one meter or less.

Mapping: The above studies indicated that wetlands were important to black duck reproduction for two general purposes: cover (availability of protective structure), and abundance and accessibility of forage organisms. Therefore scored the relative suitability of various wetland types for each aspect. The "optimum" score, 1.0, was reserved for areas where black ducks had actually been observed. Otherwise, high (.7), medium (0.4) and null (0) scores were assigned on the basis of the relative height and density of structure, and the diversity, abundance and accessibility of forage organisms. The cover and forage scores were averaged to calculate suitability index values (see following table). Our scores were based on the available literature. However, because of differences in wetland classifications, and incomplete information on value of some NWI types for black ducks, we had to interpret factors underlying habitat suitability from the discussions by the authors.  In some cases our cover classes included a relatively wide range of conditions, varying in suitability (e.g. palustrine forest encompassing different depths and duration of flooding, wide variety of plant types within the palustrine shrub type), and so these general classes were assigned lower scores than a specific subset might have merited. and the values were checked for conformance with the relative preferences indicated in the literature.

Habitat Requirements - Migration
Black ducks migrate into and through the study area from southern wintering habitats around March through mid-April. During the Fall migration they pass back through from northern areas around October through November.

North of Chesapeake Bay, black ducks feed on tidal flats and use emergent wetlands, ice-free bays, rivers, and coastal reservoirs as rest areas. Eelgrass, widgeon grass, and smooth cordgrass are important plant food items, while snails, mussels, and clams are important animal foods in coastal bays and marshes (Lewis and Garrison 1984). Migrating birds have greater flexibility in use of resources than do brooding birds; they can use forage and cover separated by relatively great distances. Cover is widely available in the form of emergent, forested, or scrub/shrub wetlands, or even large water bodies, and so only feeding areas were mapped for migration use.

Mapping: We scored interior wetlands as feeding areas base on the NWI types (see below). Scores were assigned to reflect the probable abundance and accessibility of forage organisms. Coastal wetlands were scored on the basis of forage resources, water depth, and known level of use (see 'Mapping of Coastal Migration and Wintering Habitats', below).

General Wetland Suitabilities for Black Ducks (0 - 1 scale)

NWI Designations
(wetlands only)
Cover Types

reproduction:
structure

reproduction:
forage

reproduction:
(average of structure, forage values)
migration
(forage value)
winter
(surveys, forage value)
PEM, L2EM Lake/pond, emergent vegetation

0.7

0.7

0.7 0.7
PFOcon Palustrine forest, conifer

0.7

0

0.3
PFOdec Palustrine forest, deciduous

0.7

0.7

0.7 0.4
PSSdec Palustrine scrub shrub, deciduous

0.7

0.7

0.7 0.4
PSScon Palustrine scrub shrub, conifer

0.7

0

0.3
PAB, L2AB Lake/pond, aquatic vegetation

0

0.7

0.3 0.4
L1UB, PUB Lake/pond, unconsolidated bottom

0

0.4

0.2
L2US Lake, unconsolidated shore

0

0.7

0.3 0.4
L2RS Lake, rocky shore

0

0.5

0.2
R1UB Riverine subtidal unconsolidated

-

-

* *
Rper Riverine perennial

0.4

0.4

0.4 0.4
R1US Riverine intertidal unconsolidated shore

0

0.7

0.3

0.7

*

E1AB Estuarine subtidal vegetated

-

-

* *
E1UB Estuarine subtidal unconsolidated bottom

-

-

* *
E2AB Estuarine intertidal algae

0

0.7

0.4 * *
E2EM Estuarine intertidal emergent

0.4

0.4

0.4 0.7 *
E2RS, R1RS Estuarine, tidal river rocky shore

-

-

* *
E2SS Estuarine intertidal shrub

-

-

E2US Estuarine intertidal unconsolidated shore

0

0.7

0.4 0.4 *
M1AB Marine subtidal vegetated

-

-

* *
M1UB Marine subtidal unconsolidated bottom

-

-

* *
M2AB Marine intertidal algae

-

-

* *
M2RS Marine intertidal rocky shore

-

-

* *
M2US Marine intertidal unconsolidated shore

-

-

*

*

NOTES * Of use, but scored on basis of forage resources, depth, and observed level of use; see table, below

- not used


Habitat Requirements - Wintering
Food availability, freedom from disturbance, protection from severe weather, and presence of large bodies of open water are interrelated factors that appear to affect habitat use by black ducks in winter (Lewis and Garrison 1984). North of Chesapeake Bay, black ducks forage primarily on tidal flats and rest in emergent wetlands, or remaining ice-free bays, rivers, and coastal reservoirs (Lewis and Garrison 1984). During winter storms and in response to hunting pressure, ducks use estuarine emergent wetlands, and estuarine and marine open waters. The southeast side of islands and peninsulas are used as loafing and feeding areas to achieve thermal advantages from maximum sunlight exposure and protection from the wind (Albright 1981 in Lewis and Garrison 1984). The lee side of land forms was used during low temperatures (< 0 degrees C) and by flocks of > 50 black ducks in coastal Maine during a severe winter (Longcore and Gibbs 1988).

Animal foods make up between 65 and 96% of the winter diet (Mendall 1949, Hartman 1963, Grandy 1972, Jorde and Owen 1990), with snails, amphipods, blue mussels, and clams as the primary foods (based on volume). For the purposes of this analysis, we assume cover requirements are met where suitable food resources occur (Hartman 1963).

Mapping of Coastal Migration and Wintering Habitats:
Snow and ice cover most interior and some estuarine wetlands during winter, and so coastal habitats may be critical for survival of black ducks in the study area. Coastal foraging habitats can be delineated by the occurrence of shellfish beds, and utilization of open coastal habitats by black ducks is directly apparent from aerial surveys.

Shellfish availability. Beds of a variety of bivalve molluscs were identified using previously developed data for coastal New Hampshire (Banner and Hayes 1996), a Maine DMR shellfish coverage, and the NOAA 1995 National Shellfish Register coverage, characterizing shellfish growing areas by state. The latter described shellfish abundance within relatively large coastal segments, and so the information was of lower resolution and given less weight than the other sources.

Observed use areas. The data from annual USFWS mid-winter waterfowl surveys, 1985 through 1999 (through 1994 in Maine), were processed by taking the maximum counts per segment polygon (or sub-segment, where available), and calculating nominal number of birds per unit area. Maine's Coastal Wildlife Concentration Areas (Maine Department of Inland Fisheries and Wildlife) counts were similarly converted to birds per unit area, and used to supplement the mid-winter counts.

Foraging habitat (having suitable depth and shellfish) was scored 0.5 for higher resolution ("apparent foraging habitat") and 0.3 for lower resolution ("potential foraging habitat") shellfish data. Where these foraging habitats coincided with the occurrence of one or more black ducks per 10 ha, they were scored 1.0 and 0.6, respectively. Areas having black ducks and suitable depth, but without mapped forage were scored 0.5 (see table, below).
COASTAL HABITAT SUITABILITY SCORING forage not documented apparent foraging habitat potential foraging habitat
black ducks abundant 0.5 1.0 0.6
black ducks uncommon 0 0.5 0.3

Habitat Suitability Scoring:
Habitat suitability of interior wetlands was scored as the maximum of the NWI suitability values for reproduction and migration (first table, above). Scores for coastal habitats were based on use for migration and during the critical winter period (table just above).  The overall habitat suitability score was the maximum from the interior and coastal components.

Testing of Results (winter habitat): The winter habitat model was tested using 1999 winter waterfowl count point data for Maine. We created a bounding polygon encompassing all waterfowl observations for the survey, and created a randomly distributed set of 70 points within it. We then compared the presence of habitat near the random points to that for sites at which black ducks were observed. Of the 1001 sites with black ducks, 984 had mapped habitat, while only 47 out of the 70 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model does indicate localities useful to black ducks.

Testing of Results (breeding habitat): interior (breeding) habitat use was tested using year 2000 breeding transect counts across Maine (aerial flights by John Bidwell, USFWS). We compared the occurrence of habitat within 100 m of points having black ducks, along the aerial transects, to habitat occurrence at random sites. Of 38 sites with black ducks, 25 coincided with our mapped habitat, while 239 of 798 random sites had habitat. The difference in proportion (0.66 vs 0.30) was highly significant.

Testing of Results (overall habitat): Maine Department of Inland Fisheries and Wildlife marsh bird survey data (courtesy of T. Hodgman) also were used to test the habitat map. We compared the distribution of mapped habitat around a random set of 798 upland points to that for marsh bird survey stops at which black ducks were observed in 1998 through 2000. Of the 60 sites with birds, all had mapped habitat, while only 156 sites out of the 798 randomly distributed sites had habitat. The Chi-square was highly significant, indicating that the overall model predicts localities useful to bitterns.  Restricting the test to habitats scored above 0.4 gave even an higher probability of association, supporting our premise that more highly scored areas have a higher suitability for this species.

Sources:
Banner, A. and G. Hayes. 1996. Important Habitats of Coastal New Hampshire. USFWS Gulf of Maine Program, Falmouth, ME. 75 p.

Grandy, J.W. IV. 1972. Winter ecology of maritime black ducks (Anas rubripes) in Massachusetts with special reference to Nauset marsh, Orleans and Eastham. Ph.D. Dissertation. University of Massachusetts, Amherst, MA. 111 pp.

Hartman, F.E. 1963. Estuarine wintering habitat for black ducks. J. Wildl. Manage. 27(3):339-347.

Jorde, D.G. and R.B. Owen, Jr. 1990. Foods of black ducks, wintering in marine habitats of Maine. Can. Field Nat. 104:300-302.

Lewis, J.C. and R.L. Garrison. 1984. Habitat suitability index models: American black duck (wintering). USFWS. FWS/OBS-82/10.68. 16 pp.

Longcore, J.R. and J.P. Gibbs. 1988. Distribution and numbers of American black ducks along the Maine coast during the severe winter of 1980-1981. Edited by M.W. Weller, . Pp. 377-389. In: Waterfowl in Winter, University of Minnesota Press, Minneapolis, MN.

Mendall, H.L. 1949. Food habits in relation to black duck management in Maine. J. Wildl. Manage. 13(1):64-101.

Reed, A. 1970. The breeding ecology of the black duck in the St. Lawrence estuary. D.Sc. Thesis. University Laval, Quebec. 175 pp.

Reed, A. and G. Moisan. 1971. The Spartina tidal marshes of the St. Lawrence estuary and their importance to aquatic birds. Naturaliste Can. 98:905-922.

Ringelman, J.K. and J.R. Longcore. 1982. Movements and wetland selection by brood rearing black ducks. J. Wildl. Manage. 46(3):615-621.

Ringelman, J.K., J.R. Longcore and R.B. Owen, Jr. 1982. Breeding habitat selection and home range of radio-marked black ducks (Anas rubripes) in Maine. Can. J. Zool. 60:241-248.

Seymour, N.R. 1984. Activity of black ducks nesting along streams in northeastern Nova Scotia. Wildfowl 35:143-150.

Seymour, N. and W. Jackson. 1996. Habitat-related variation in movements and fledging success of American black duck broods in northeastern Nova Scotia. Can. J. Zool. 74:1158-1164.

USFWS. 1988. American black duck breeding habitat enhancement in the northeastern United States: a review and synthesis. USFWS. FWS/OBS-88/49.89 48 pp.