Wood Duck Habitat Model
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Wood duck, Aix sponsa
Use of Study Area Resources:
Reproduction occurs throughout the study area (Adamus 1984, Breeding Bird Survey information); wintering is limited to southern Massachusetts and south through the Gulf Coast (Christmas Bird Count information). Wood ducks occur throughout the eastern United States and also the Pacific coastal region.
In the Northeast wood ducks use forested wetland complexes year round, including swamps, floodplains, bottomlands, beaver flowages, riparian corridors, oxbows, and scrub-shrub wetlands (Drugger and Fredrickson 1992, Hepp and Bellrose 1995). They favor shallow quiet inland waters in or near deciduous or mixed woodland (Palmer 1975). On migration (not mapped here) they can be found on fresh and brackish waters, and rarely on sheltered salt water areas (Palmer 1949).
Breeding and brood rearing cover: Suitable habitat includes nest trees in wetland complexes having floating or emergent vegetation with a ratio of 50-75% cover to 25-50% water (Sousa and Farmer 1983). Shrubby wetlands of willow, alder, buttonbush, and downed timber provide good cover and are used extensively, as are wetlands with shallow water and dense emergents such as bur-reed, arrow arum, duck potato, smartweeds, and American lotus (Hepp and Bellrose 1995). Preferred water depth is between 7 and 45 cm deep (Boone web page, Drugger and Frederickson 1992). Suitable trees for nesting are at least 40 cm dbh, with relatively large cavities 5 to 50 feet above the ground, and standing in or within a few hundred yards of water (Palmer 1949, Grice and Rodgers 1965 in Sousa and Farmer 1983, Palmer 1975). Conifers rarely provide suitable cavity structure (McGilvrey 1968 in Sousa and Farmer 1983). Wood ducks nest near human habitation if wetlands habitat is present (Bent 1923, Palmer 1975). Wildlife managers often place artificial nest boxes in suitable wetlands where regenerating forests are too immature to provide nest cavities (Clugston 1999).
Wood ducks raise one brood a year. Hatchlings are precocial, and leave the nest within a day of hatching. Ducklings leap from the nest with downy wings extended and have been known to jump as much as 89 m to the ground without injury, after which hens lead them to brood-rearing sites (Hepp and Bellrose 1995). Drugger and Fredrickson (1992) noted that broods may move up to 4 km to foraging sites, averaging 1.3 km. Travel is mostly along waterways, but also overland. Survival of broods is associated with the distance of ground travel; broods that moved less than 0.8 km had higher survival rates than broods that moved greater distances (Sousa and Farmer 1983).
Broods are led to shallowly flooded wetlands with a heavy understory of emergent vegetation or shrubs, small open water passages, and some woody debris (Drugger and Fredrickson 1992, Palmer 1975). Such areas offer concealment, forage, movement routes, and resting sites; similar sites are used by the adults during the flightless period of late summer molting (Palmer 1975).
Foods: "Wood duck foraging varies seasonally and between the sexes. Their winter diet is almost entirely plant based, of which 75% may be acorns. An increase in animal foods ... (to about 35%) occurs in both sexes in early spring. This percentage remains constant for the male wood duck through summer and fall while undergoing ...molts, but increases to about 80% for the female during egg laying. Female wood ducks increase the amount of invertebrates in the diet to meet daily protein needs during egg laying. After egg-laying, animal foods compose less of the females diet, while consumption of high energy seeds increases to meet the daily dietary requirements of incubation (Drugger and Fredrickson 1992). Foods commonly include acorns, seeds of beech, maple, elm, ash, sedges, bur-reed, pickerel weed, wild rice, sedges, grasses and small invertebrates, mostly insects (Palmer 1975, Clugston 1999, Drugger and Fredrickson 1992). Young feed especially on insects, aquatic invertebrates and small fishes (USGS 1999). Preferred water depth for foraging is less than 20 cm; deeper waters can be used for roosting and loafing (Drugger and Fredrickson 1992)
Wintering: Wintering wetland complexes ideally are centered on a permanent water body, and offer some persistent emergent vegetation (Sousa and Farmer 1983). These areas are similar to reproductive habitat, including bottomland hardwoods, beaver ponds, flooded forests, swamps, openings in marshes, and upper ends of tidal creeks (Palmer 1975, Drugger and Frederickson 1992), flowages, river oxbows, meanders and backwaters (USGS 1999). Wood ducks also winter in persistent herbaceous vegetation - cattails, soft rush, bulrush, bur-reed.
Area Requirements: Minimum habitat size for brood rearing is about 10 acres of wetland in contiguous units or in isolated parcels separated by no more than 100 feet of upland (McGilvrey 1968 in Sousa and Farmer 1983, USGS 1999). The distribution of naturally occurring nest cavities in forested wetlands ranged from 1 per 24 acres to 1 per 13 acres; in upland forest the density was about 1 per 5 acres, with an average of about 1 per 5.2 acres for all types (Sousa and Farmer 1983).
Limiting Factors: Adults may be preyed upon by great horned owls, raccoons, foxes or mink. Young may be taken by any of these, as well as by large predacious fish, bullfrogs, or snapping turtles. Eggs may be destroyed by raccoons, rat snakes, squirrels, mink, woodpeckers and starlings.
Cover suitability. Cover types suitable for wood duck foraging and resting (adults and broods) include marsh, near-shore open water, forested and shrub wetlands (see nominal scores in the table, below). Cover types for nesting include wetland and upland deciduous and mixed forest.
|Cover Types||Cover Suitability
(0 - 1 scale)
|Upland deciduous forest||0.8*|
|Upland coniferous forest|
|Upland mixed forest||0.5*|
|PEM, L2EM||Lake/pond, emergent vegetation||1.0**|
|PFOcon||Palustrine forest, conifer|
|PFOdec||Palustrine forest, deciduous||1.0|
|PSSdec||Palustrine scrub shrub, deciduous||1.0**|
|PSScon||Palustrine scrub shrub, conifer|
|PAB, L2AB||Lake/pond, aquatic vegetation||1.0**|
|L1UB, PUB||Lake/pond, unconsolidated bottom||1.0**,***|
|L2US||Lake, unconsolidated shore||0.5**|
|L2RS||Lake, rocky shore|
|R1UB||Riverine subtidal unconsolidated|
|E1AB||Estuarine subtidal vegetated|
|E1UB||Estuarine subtidal unconsolidated bottom|
|E2AB||Estuarine intertidal algae|
|E2EM||Estuarine intertidal emergent|
|E2RS, R1RS||Estuarine, tidal river rocky shore|
|E2SS||Estuarine intertidal shrub|
|E2US/R1US||Estuarine, Riverine intertidal unconsolidated shore||0.3**|
|M1AB||Marine subtidal vegetated|
|M1UB||Marine subtidal unconsolidated bottom|
|M2AB||Marine intertidal algae|
|M2RS||Marine intertidal rocky shore|
|M2US||Marine intertidal unconsolidated shore|
|NOTES||* if adjacent to wetlands
**if adjacent to deciduous or mixed forest
***exclude open water > 30 m from shore
Area Suitability. Wetland areas less than 10 acres in size were regarded as unsuitable for brood rearing, although they may be suitable for migration and breeding. Therefore, these smaller areas were given a reduced ( ½) habitat score overall. Patches of nesting cover smaller than 5.2 acres were regarded as unsuitable because of the reduced likelihood of having a naturally occurring suitable cavity.
Interspersion of nesting and foraging cover: Suitable reproductive habitats occur where foraging and nesting components are adjacent (within 180 m of each other), or in sites having both elements (e.g., wooded swamp). It was assumed that brood rearing habitat would be available within range (2+ kilometers) of any suitable reproductive/foraging complex.
HSI value = cover suitability x area suitability x interspersion suitability
Validation. The model was tested by comparing the proportion of Breeding Bird Survey occurrences having mapped habitat (17 of 20) to the proportion of a random set of points having mapped habitat (447 sites out of total of 798). The difference in relative proportions was significant (0.009). Modeled habitat was also found to correspond well with central Maine sites identified by MDIF&W biologist Allen Starr for placement of wood duck nest boxes (corresponding to 71 of 72 sites). Nest boxes had been placed in wetlands or open water having adequate brood rearing habitat.
Maine Department of Inland Fisheries and Wildlife marsh bird survey data (courtesy of T. Hodgman) also were used for testing. We compared the distribution of mapped habitat around a random set of 798 upland points to that for marsh bird survey stops at which wood ducks were observed in 1998 through 2000. Of the 97 sites with birds, 83 had mapped habitat, while only 174 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 wood ducks. Restricting the test to habitats scored above 0.5 gave even an higher probability of association, supporting our premise that more highly scored areas have a higher suitability for this species.
Adamus, P.R. 1984. Atlas of breeding birds in Maine 1978-1983. Maine Dept. of Inland Fisheries and Wildlife, Augusta, ME.
Bent, A.C. 1923. Life histories of North American wild fowl. U.S. Museum Bull. 126. Smithsonian Inst. Washington DC. pp. 158-171.
Breeding Bird Survey: http://www.mbr-pwrc.usgs.gov/bbs/
Boone, R. Maine Gap Analysis, Maine Cooperative Fish and Wildlife Research Unit; WWW page http://www.wle.umaine.edu/progs/unit/gap/
Christmas Bird Count information: http://www.mp1-pwrc.usgs.gov/birds/cbc.html
Clugston, D. 1999. Availability of nest cavity trees for wood ducks (Aix sponsa) at Sunkhaze Meadows National Wildlife Refuge, Maine. Northeastern Naturalist 6(2):133-138.
DeGraaf, R.M. and D.D. Rudis. 1983. New England Wildlife: Habitat, Natural History and Distribution. USDA Technical Report NE-108.
Drugger, K.M. and L.H. Frederickson. 1992. Life history and habitat needs of the wood duck. USFWS Fish and Wildlife Leaflet 13.1.6. Waterfowl Management Handbook. Washington, DC. 8p.
Hepp, G.R. and F.C. Bellrose. 1995. Wood Duck, Aix sponsa. In A. Poole and F. Gill, (eds.) The Birds of North America, No. 24. The Academy of Natural Sciences, Philadelphia, and The American Ornithologists' Union, Washington, DC.
Palmer, R. 1949. Maine Birds. Bull. Compar. Zool. 90-93.
Palmer, R. 1975. Handbook of North American Birds. Yale Univ. Press, New Haven CT. Pp. 252-277.
Sousa, P.J. and A.H. Farmer. 1983. Habitat Suitability Index Models: Wood Duck. U.S. Dept. of the Interior, Fish Wildl. Serv. FWS/OBS-82/10.43.
USGS 1999. Web page http://www.npwrc.usgs.gov/resource/1999/woodduck/woodduck.htm