Diets and foraging strategies of Glaucous-winged Gulls were
studied in areas with and without sea otters in the western Aleutian
Islands, Alaska. Gulls foraged on invertebrates (e.g., sea urchins,
limpets, chitons, mussels, and others) in the rocky intertidal
community and on fish at sea; this study was conducted on gulls
foraging intertidally.
Sea otters affected foraging strategies and diets of gulls by
reducing the size and density of intertidal prey available to them.
In the presence of low densities of sea otters (which had depredated
large sea urchins) gulls adjusted their foraging strategies by being
more selective while feeding on urchins. In the presence of high
densities of sea otters (which had depredated most intertidal prey)
gulls shifted their diets from intertidal invertebrates to fish and
the diversity of their diets was reduced.
Observations demonstrated that gulls foraged intertidally during
low tides and that most foraging occurred in the lowest intertidal
zones that were exposed. Consequently, gulls foraged in different
zones during spring and neap tides. When all zones were exposed gulls
selected the Alaria and Laminaria zones, which offered the highest net
rate of energy gain (En) . Gulls also selected particular prey species
and prey sizes. Selective foraging of gulls increased their En 126%
in areas without sea otters and 181% in areas with low densities of
sea otters.
Prey preference experiments demonstrated that preferences of
gulls for chitons and urchins were significantly correlated to En, but
assimilation rate, experience and search images were also influential.
Highly preferred prey species (chitons) were not strongly selected for
in the field because of their ability to adhere to the substrata.
Foraging behavior of gulls indicated that they hunted by En
expectation and left prey patches when a threshold En was reached.
Foraging behavior of gulls in the rocky intertidal community supported
optimal foraging theory for optimal diets, patch choice, and time
allocation to patches