Bioenergetics, behavior, and sea level rise: current status and future implications for wintering dabbling ducks in Delaware
Date
2015
Authors
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Publisher
University of Delaware
Abstract
Waterfowl migrating and overwintering in the Atlantic Flyway depend on adequate availability of wetlands and associated habitats to survive and fuel subsequent breeding efforts. Long-term declines in some wintering waterfowl populations, such as American black ducks (Anas rubripes), have prompted researchers to investigate the bioenergetics of such species in an attempt to inform effective habitat management. Bioenergetics models typically seek to estimate energetic carrying capacity based on energy supply and demand. However, little effort has been made to explicitly and comprehensively assess the relative value of managed coastal impoundments vs. unmanaged tidal salt marshes to wintering waterfowl in the mid-Atlantic region in terms of both use and energetic potential. To address these questions, I sampled dabbling duck foods, observed behavior, and conducted point-transect counts on 7 managed impoundments and 3 unmanaged tidal salt marshes along the Delaware Bayshore. My objectives were to 1) estimate wintering dabbling duck population density and abundance between habitats and at the state-level, 2) characterize species-specific differences in behavior between habitats, and 3) estimate the current bioenergetic carrying capacity of wintering dabblers between habitats and extrapolate to the state-level, and 4) predict future trends in carrying capacity based on anticipated sea level rise (SLR) scenarios. I estimated population density in impoundments and salt marshes in Delaware over the winter period via point-transect counts (n = 2,128) and aerial survey counts. In general, I found that most dabblers tended to concentrate in higher densities on either freshwater or brackish impoundments compared with unmanaged salt marshes. Black ducks presented a notable exception, as both point counts and aerial surveys suggested black duck densities were higher on salt marshes than impoundments. I quantified the proportion of time spent in any given behavior for each dabbler species based on instantaneous scan samples (n = 6,400 scanning efforts per year) conducted alongside point-transect counts, November-March, 2011-2013. I observed dabbler behavior in four time periods (morning crepuscular, diurnal, evening crepuscular, nocturnal) in an attempt to capture behavioral variation over the 24-hr day. Using behavioral data, I constructed time-activity budgets for each habitat type, and compared behavioral proportions between habitats. I converted behavioral proportions into hourly energy expenditure (HEE) and daily energy expenditure (DEE) estimates based on activity-specific multipliers of resting metabolic rate. I estimated the range of dabbler DEE values over the winter period to be between 111.84-349.79 kcal/bird/day, the majority of which typically stemmed from flying and feeding behaviors. My DEE estimates were higher than previously reported values, and produced similar results to a mass-based allometric model. I estimated waterfowl food biomass in impoundments and various tidal salt marsh habitat types over the winter period by collecting soil core (n = 1,364), nekton (n = 426), and salt marsh snail (Melampus spp.; n = 87) samples in October, January, and April, 2011-2013. I converted biomass estimates to energy using true metabolizable energy (TME) values. I found that food energy density was highest in freshwater impoundments (range: 183,344-562,089 kcal/ha) for nearly all dabblers, and typically higher in brackish impoundments (range: 112,399-357,160 kcal/ha) than most salt marsh habitats, whereas mudflat (range: 50,745-137,473 kcal/ha) and subtidal (range: 51,402-136,326 kcal/ha) habitats typically contained the least energy. Extrapolating to the state-level, I estimated between 2.38 x 109-1.14 x 1010 kcal available in total within a 16 km buffer from the Delaware Bayshore, depending on species. Combining DEE and energy supply values, I estimated between 8.73 x 106- 7.06 x 107 duck use-days (DUD) available over the winter period. I used the Sea Level Affecting Marshes Model (SLAMM) to predict changes in habitat availability based on 4 SLR scenarios. I estimated that carrying capacity is likely to decrease in the future under all but the most conservative SLR scenario, due to the gradual replacement of high energy density natural habitat (i.e., low marsh, high marsh) with low energy density habitat (i.e., subtidal, mudflat). In the future, coastal impoundments will become increasingly important, provided they are properly maintained, as they will represent a growing proportion of the available DUD on the landscape. This study will assist managers in meeting population goals by highlighting key areas where habitat modification would be most effective. I urge further researchers to attempt to refine my carrying capacity estimates by incorporating appropriate foraging thresholds and more detailed models of competition into bioenergetics models, and by attempting to correct for biases in population estimation methodologies.