Feeding Rates of Warblers in Spring

Populations of migrant warblers were correlated with populations of lepidopterous larvae in spring in a heavily forested area of southern Illinois. 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 per m3, and a negative balance in an area with biomass of 0.02 g per m3. Warblers chose either the smaller size classes (under 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 invertebrates, and other groups were insufficient alone to support migrant populations. Larvae were about 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.