ResourcesIdentification of Important Habitats in Coastal New HampshireChapter 18. American Black DuckWaterfowl are important in coastal New Hampshire from recreational (hunting,
viewing) and ecological perspectives. One of the species on the GOMC list,
the American black duck (Anas rubripes), is of special interest,
due to a continued decline in population. The following model for black
ducks considers habitat use during four life stages: breeding, brood-rearing,
migration, and wintering. Black ducks appear to select wetland habitat
on the basis of vegetative structure, perhaps associated with food and
cover requirements, (Ringelman 1980). HABITAT REQUIREMENTS Habitat for breeding pairs include: (1) nesting cover and substrate (Reed
1970), (2) visual isolation from other pairs, and (3) high quality foraging
areas (USFWS 1988). Following hatching of the eggs, hens move their broods
to rearing wetlands, often considerable distances from the nest site (Ringelman
and Longcore 1982). Habitat requirements for brood-rearing include: (1)
cover from predators and weather, and (2) invertebrate-rich wetlands (USFWS
1988). Reproductive habitats (breeding pair, nesting, and brood-rearing)
must be managed as a unit to ensure successful production of black ducks
(USFWS 1988). Migration and winter habitat requirements include: (1) high
quality foraging areas and (2) cover from weather (Lewis and Garrison
1984). BREEDING PAIR HABITAT Hens may use a diversity of sites for nesting, covering the range from
uplands to lowland cover types. Therefore, we made the assumption that
where suitable breeding pair habitat occurred, hens could locate suitable
nest sites in the vicinity. A variety of wetlands provide habitat for
breeding black ducks. In inland Maine, wetland selection by breeding pairs
(pre-laying, laying, and incubation periods) in order of preference was
palustrine emergent, broad-leaved deciduous forested, and broad-leaved
deciduous scrub-shrub types. Unconsolidated organic bottom, needle-leaved
evergreen forested, and broad-leaved evergreen shrub wetlands were used
in a proportion less than their availability (Ringelman et al. 1982).
Ephemeral pools were important foraging sites for pairs breeding at inland
freshwater wetlands (Ringelman et al. 1982). Streams having sandy or stony
bottoms interspersed with invertebrate-rich detrital patches were used
in a proportion in excess of availability (Ringelman et al. 1982). Black
ducks nesting in coastal salt marsh in Nova Scotia foraged in the tidal
marsh (Reed and Moisan 1971). Researchers have documented a variety of wetlands that provide habitat
for breeding black ducks, but few studies (Seymour and Jackson 1996) have
objectively evaluated their relative suitabilities. Seymour and Jackson
(1996) documented black duck use of estuarine, lacustrine, riverine, and
palustrine wetlands during the breeding season over a 16 year period.
Breeding black duck pairs used the following, in decreasing order: inland
freshwater ponds and marshes, rivers, estuaries (not including tidal marsh),
lakes, and tidal marsh sites (Seymour and Jackson 1996). We used that
information to assign suitability scores to NWI wetland types (Table 6).
BROOD-REARING HABITAT Hens will move their broods considerable distances to rearing wetlands.
In inland Maine, hens and broods traveled as far as 3.3 km from the nest
to a 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 were often used by broods en
route to rearing wetlands (Ringelman and Longcore 1982). At inland freshwater wetlands in Maine, broods used emergent ponds in a proportion greater than their availability (based on water surface area) and lakes and evergreen scrub-shrub wetlands were used less than their availability. Dead scrub-shrub, unconsolidated bottom, and aquatic bed wetlands were not used by broods (Ringelman and Longcore 1982). Hens and their broods were associated with larger wetlands having alder, willow, and herbaceous vegetation (palustrine emergent and deciduous scrub-shrub classes). These had greater water surface area than wetlands not used by broods, based on discriminant analysis (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). Rearing wetlands all contained active beaver colonies. Palustrine emergent wetlands provided structure for high densities of invertebrates, protein rich foods required by developing young. Scrub-shrub and deciduous forested wetlands provide cover from predators and weather (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
alterniflorazone 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), over a 16 year period, observed an association of
brood size at fledging and wetland type. Brood size decreased in the following
sequence: inland freshwater ponds and marshes, estuaries (not including
tidal marsh), lakes, and tidal marsh sites. Greater cover from predators
and weather and potentially lower predator densities in palustrine wetlands
may have contributed to the highest brood size at fledging in these wetlands
(Ringelman and Longcore 1982). Greater predator densities in tidal marshes
may have contributed to the lowest brood size at fledging in these wetlands
(Seymour 1984). NWI wetland types were characterized as brood-rearing
habitat (Table 6) based on mean brood size at fledging. MIGRATION HABITAT Black ducks migrate into and through the study area from southern wintering
habitats around March through mid-April. Maximum numbers of black ducks
occur around Great Bay August through March (Short 1992). 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). We also characterized wetlands as migration habitat according to their NWI categories (Table 6). WINTER HABITAT Some black ducks winter in Great Bay and protected coastal waters, from
December through February. During the winter, food availability, disturbance,
and weather are factors that affect habitat use by black ducks (Lewis
and Garrison 1984). Inland wetlands are likely to be frozen, so only shellfish
and eelgrass beds would be routinely available. Animal matter made up
the greatest portion of the diet of coastal wintering black ducks in Maine
(Hartman 1963, Jorde and Owen 1990). Soft-shelled clams (Mya
arenaria) and the little macoma clam (Macoma balthica)
constituted 46% of the total food volume and occurred in four-fifths of
the gizzards collected from black ducks (n=138) in the Penobscot Bay estuary
during autumn and winter. High density soft-shelled clam beds attracted
large flocks of black ducks (Hartman 1963). Snails and amphipods constituted
8% and 7% respectively, of the total food volume (Hartman 1963). Animal
matter comprised 96% and plant material 4% of the aggregate dry weight
of esophageal samples collected from wintering black ducks in a marine
environment of coastal Maine (Jorde and Owen 1990). Periwinkles (Littorina
spp.), amphipods and blue mussels (Mytilus edulis) comprised
68% and soft-shelled clams 6% of the aggregate dry weight of food items
collected (Jorde and Owen 1990). Periwinkles and amphipods are typically
found in association with intertidal marine macro-algae. MAPPING OF HABITATS Habitats were mapped from digital themes including: bathymetry, wetlands
(NWI), eelgrass and algae distribution, and blue mussel and soft-shelled
clam beds. These data were overlaid and processed on a cell by cell basis
to create grid-cell coverages for each of the four life stages (Figures
of; Breeding Pair Habitat, Brood-Rearing
Habitat, Migration Habitat,
Winter Habitat, Combined Seasonal Habitat).
Scores were assigned on a 0 to 10 scale (10 being the most suitable habitat)
as follows. Table 6. Wetland Suitability as Black Duck Habitat*.
*Suitability Scored 0 (unsuitable) to 10 (optimum) Marine and estuarine wetlands having water depth > 1 foot are too
deep for foraging by black ducks; these were assigned a value of 0. Within
the correct depth range, eelgrass beds (Zostera marina) were
scored a suitability value of 7.5 (0 - 10 scale) as breeding pair, brood-rearing,
migration and winter habitats. Blue mussel and soft-shelled clam beds
were scored a suitability value of 10 for both migration and winter habitats.
Beds of the marine algae we mapped in this study (rockweed, Irish moss,
and tufted red weed) were scored a suitability value of 7.5 for both migration
and winter habitats. Palustrine, lacustrine, estuarine, and marine wetlands
were used in the characterization of breeding, brood-rearing, and migration
habitats (Table 6) but not winter habitat because of ice conditions limiting
their availability during winter months. COMBINING HABITAT SUITABILITY SCORES FOR ALL LIFE STAGES For successful reproduction, both breeding pair habitat and brood-rearing
habitat must be available, of sufficient quantity, and quality, and juxtaposed
on the landscape for successful reproduction (USFWS 1988). Therefore,
on a cell by cell basis we calculated a reproductive habitat suitability
score as the geometric mean of breeding pair habitat and brood-rearing
habitat scores. If either of the reproductive habitat components were
0 the overall reproductive suitability would be 0. On the other hand,
the relative mobility of mature black ducks allows them to fly to alternate
areas to use supplementary resources. Therefore, to identify the overall
habitat for black ducks, we overlaid the reproductive, migration, and
winter habitat coverages and selected the highest suitability score for
each cell. |
