Chick Hatching as a Trigger for Dietary Switching in the Western Gull

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Abstract
The diet of Western Gulls (Larus occidentalis) nesting on Alcatraz Island (San Francisco County, California) was investigated for the breeding seasons of 1983, 1984, and 1985. The major food types on Alcatraz were garbage (> 90% of which was chicken) and fish. Adults fed heavily on garbage early in the season, but switched to small fish later in the breeding season. Study of the diets of individual pairs revealed that the switch occurred when the eggs hatched. Experiments in which eggs were switched between nests to alter the timing of hatching also provided evidence that chick hatching triggers dietary switches. Hatching is known to alter many aspects of parental behavior in gulls, and prey choice appears to be another.

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