Estimating habitat carrying capacity for American black ducks wintering in Southern New Jersey

Date
2009
Authors
Cramer, Dane
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Publisher
University of Delaware
Abstract
Midwinter waterfowl survey (MWS) data indicates a long-term decline in the number of wintering American black ducks (Anas rubripes), potentially due to the habitat limitations. My objectives were to (1) estimate the current carrying capacity of winter black duck habitat in southern New Jersey and (2) determine the type(s) of habitat most beneficial to wintering black ducks. I estimated the supply of energy (DUDS) during 3 sampling periods, October, January, and April, 2006-2008, using core sampling to estimate landscape biomass of seeds and invertebrates (n=1,020). I collected upper gastrointestinal tracts from hunter-killed birds and late season collections to remove the biomass of non-food items from standing landscape biomass estimates (n=64). I estimated the daily energetic requirement both allometrically and using time-energy budgets based on instantaneous scan samples (n=690). I modeled the population demand of energy (DUDD) using MWS results for the two winters of the study and annual abundance data to determine the proportion of black ducks in the study area during each sampling period. Estimates of DUDS ranged from 1,666,336 (± SE 577,906) to 2,167,281 (± SE 751,640), 1,698,567 (± SE 689,474) to 2,209,201 (± SE 896,748), and 889,968 (± SE 303,007) to 1,157,516 (± SE 394,098) in October, January, and April, respectively. DUDD ranged from 658,152 to 1,058,954, 1,994,400 to 3,208,950, and 458,712 to 738,059 in October, January, April, respectively. DUDD exceeded DUDS during the January sampling period, indicating that the availability of energy may be limiting the winter population of black ducks. My estimates of DUDS were limited by a lack of published true metabolizable energy values for important wintering black duck foods and equipment necessary to quantify nocturnal behavior to construct time-energy budgets. Further reductions to estimates of DUDS may be attributed to interspecific competition for resources, avoidance of developed areas resulting in an overestimate of available habitat, or the temporary suspension of food availability during winter freeze events. Future research should address limitations of this study by (1) determining TME values and a foraging threshold for important winter foods, (2) quantifying nocturnal behaviors for inclusion in time-energy budgets, (3) measuring the effects of interspecific competition, (4) determining how black ducks orient their winter home range in response to coastal development, and (5) assessing the frequency and duration of freeze events. Future research should address limitations of this study by (1) determining TME values and a foraging threshold for important winter foods, (2) quantifying nocturnal behaviors for inclusion in time-energy budgets, (3) measuring the effects of interspecific competition, (4) determining how black ducks orient their winter home range in response to coastal development, and (5) assessing the frequency and duration of freeze events. Mangers can increase DUDS by (1) reducing the individual daily energetic requirement by reducing flight behavior caused by disturbance or (2) increasing the acreage or consumable biomass of habitat. Establishing refuge areas of adequate size to accommodate winter requirements of black ducks reduces individual energetic costs brought on both directly through disturbance and indirectly through time spent in transit (i.e. flight). Mudflat habitat had the greatest availability of energy exploitable by black ducks. Habitat restoration potential exists with impounded waters, salt hay farms, and dense stands of Phragmites australis. Restoration efforts should focus on restoring the total function of saltmarsh habitat with mudflat habitat as a focal component
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