Download Diets of two human-subsidized predators, common raven and

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
Introduction
Methods
Results (continued)
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.
Pellets were dissected and food items were identified to the
lowest possible taxonomic level.
We opportunistically sampled mantle feathers from nearlyfledged ravens found dead at one nest in 2004. 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.
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.
16
Common Raven
Glaucous Gull
Potential Prey
14
12
Other
marine
Marine fish
Zooplankton
Freshwater
fish
10
δ15N
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.
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
2008-2009 and another colony (15 breeding pairs) in 2009.
8
6
Birds
Garbage
Small
mammals
4
Caribou
2
0
-32
-30
-28
-26
-24
-22
-20
-18
-16
δ13C
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.
Feather isotope values indicated that gull chicks were fed diets
with higher proportions of garbage than raven chicks.
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.
Results
Garbage
Birds
Mammals
Other
Study site
% occurrence in samples
100
 Young birds of both species likely consumed garbage, but
gull chicks appeared to be fed a higher proportion of
garbage than ravens.
80
60
 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 yearround, while gulls leave the area after the breeding season.
40
20
0
FS3
2004
FS3
2005
GC1
2004
GC1
2005
Common Raven
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.
LPC
2005
PM
2008
PM
2009
WC
2009
Glaucous Gull
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.
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).
 Management to reduce predation on species of special
concern should include reduced access to garbage for gulls
and limiting nest structure availability for ravens.
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.