Presentation on theme: "Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal Plain Abby N. Powell, U.S. Geological Survey, Alaska Cooperative."— Presentation transcript:
Diets of two human-subsidized predators, common raven and glaucous gull, on Alaska’s Coastal Plain Abby N. Powell, U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, University of Alaska Fairbanks Emily L. Weiser, University of Otago, Dunedin, New Zealand Stacia A. Backensto, National Park Service, Fairbanks, AK Study site Introduction Common ravens (Corvus corax) and glaucous gulls (Larus hyperboreus) are primary avian predators in northern Alaska. Populations of both species in the coastal plain region are considered human-subsidized. Populations of both species have increased in areas associated with oil development due to availability of structures for nesting ravens and food sources for both species. Many species of conservation concern nest in the same region. The impact of these human-subsidized predators on these species is unknown. We used analyses of regurgitated pellets combined with stable isotope analyses to determine relative amounts of garbage and natural prey items in diets of adults and chicks. Results (continued) Methods We collected pellets and food remains from within a 10-m radius of each common raven nest in 2004 and 2005, andfrom one glaucous gull colony (8-9 breeding pairs) in and another colony (15 breeding pairs) in Pellets were dissected and food items were identified to the lowest possible taxonomic level. We opportunistically sampled mantle feathers from nearly- fledged ravens found dead at one nest in We captured live gull chicks and collected one mantle feather from each. Feathers were analyzed at the Alaska Stable Isotope Facility for carbon and nitrogen signatures. We corrected signatures using fractionation rates for carnivorous diets in birds along with an additional correction for δN for growing chicks. Acknowledgments Funding and other support were provided by the North Slope Borough Department of Wildlife Management, UAF Coastal Marine Institute, the U.S. Bureau of Land Management, UAF Angus Gavin Migratory Bird Research Grant, and ConocoPhillips Alaska, Inc. We thank the numerous technicians and volunteers who helped with this study. Results Conclusions Adult common ravens and glaucous gulls nesting close to the landfill used garbage as food, but it was more prevalent in gull diets. Both species prey on birds and mammals, including species of conservation concern. Young birds of both species likely consumed garbage, but gull chicks appeared to be fed a higher proportion of garbage than ravens. Other data suggest that breeding ravens forage close to their nests during chick rearing and are not using the landfill during this period, whereas gulls are frequent visitors. However, ravens remain in Prudhoe Bay year- round, while gulls leave the area after the breeding season. Management to reduce predation on species of special concern should include reduced access to garbage for gulls and limiting nest structure availability for ravens. Raven nests and gull colonies within 8 km of the Deadhorse, AK landfill, which services the oil fields (Kuparuk and Prudhoe Bay) in the region. Figure 1. Percent occurrence of each food group in pellets and food remains from Common Ravens at Glaucous Gulls at Prudhoe Bay, Alaska. “Other” includes fish, crustaceans, insects, and unidentified items. Figure 2. Stable carbon and nitrogen isotope signatures (mean ± s.d.) of food groups used in mixing models for estimating glaucous gull diets in northern Alaska; and stable isotope signatures of Common Raven and Glaucous Gull chick feathers. Percent occurrence of garbage in pellets ranged from 6-32% for ravens and 58-80% for gulls. Mammals were more prevalent (70-100%) in raven pellets than in gull pellets (13-40%). Birds were less prevalent than mammals as prey for both species (30-74% for ravens; 7-25% for gulls). Feather isotope values indicated that gull chicks were fed diets with higher proportions of garbage than raven chicks.