Feeding Rates of Warblers in Spring

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Abstract
Populations of migrant warblers were correlated with populations of lepidopterous larvae in spring in a heavily forested area of southern Illinois [USA]. The birds appeared to feed entirely on these larvae, mostly taken from oaks. Estimates of gross calorie intake indicated a positive energy balance for warblers in an area with larval biomass of 0.70 g/m3 and a negative balance in an area with biomass of 0.02 g/m3. Warblers chose either the smaller size classes (< 15 mm) of larvae or the smaller species (especially leafrollers) out of proportion to their availability, but the birds'' positive energy balance was dependent upon an irruption of geometrids. Lepidopterous larvae constituted 75-98% of the available foliage in invertebrates, and other groups were insufficient alone to support migrant populations. Larvae were .apprx. 80% water, and warblers ingested 1.2-1.7 times their own weight in larvae per day in order to attain gross caloric intake of 14.8-19.0 kcal per bird-day. Warblers in an area with insufficient food resources ingested 7.2 kcal/day and their behavior was consistent with a goal of energy conservation.