Foraging ecology of the endangered wood stork recorded in the stable isotope signature of feathers

Down feathers and regurgitant were collected from nestling wood storks (Myc teria americana) from two inland and two coastal breeding colonies in Georg ia. The stable isotopic ratios of carbon (C-13/C-12) and nitrogen (N-15/N-1 4) in these materials were analyzed to gain insights into the natal origins of juvenile storks and the foraging activities of adults. Down feathers di ffered in delta C-13 between inland and coastal colonies, having average is otopic values that reflected the sources of carbon fixed in biomass at the base of the food web. Feathers from the inland colonies differed between co lonies in delta N-15, while those from the coastal colonies did not. These patterns primarily reflected the foraging activities of parent storks, with individuals capturing differing percentages of prey of distinct trophic st atus at each colony. Collectively, the carbon and nitrogen isotopic signatu res of feather keratin were used to distinguish nestlings from each colony, except for instances where storks from different colonies foraged in commo n wetlands. The stable isotopic composition of food items in regurgitant wa s used to reconstruct the trophic structure of the ecosystems in which wood storks foraged. Predicted foraging activities based on the isotopic compos ition of keratin were generally consistent with the percentage of prey type s (freshwater vs, saltwater and lower trophic level vs. upper trophic level consumer) observed in regurgitant, except for the coastal colony at St. Si mons Island, where the delta C-13 of feathers strongly suggested that fresh water prey were a significant component of the diet. This inconsistency was resolved by aerial tracking of adults during foraging excursions using a f ixed-wing aircraft. Observed foraging activities supported interpretations based on the stable isotope content of feathers, suggesting that the latter provided a better record of overall foraging activity than regurgitant ana lysis alone. Observed foraging patterns were compared to the predictions of a statistical model that determined habitat utilization based on habitat a vailability using a geographic information system (GIS) database. Observed foraging activities and those predicted from feathers both suggested that s ome adult storks preferred to feed their young freshwater prey, even when s altwater resources were more accessible in the local environment, This conc lusion supports the contention that wood stork populations are sensitive to changes in the distribution of freshwater habitats along the southeastern coastal plain of the United States.