Camera traps at nest boxes reveal consistent importance of Lepidoptera in Eastern Bluebird ( Sialia sialis ) nestling diets

North American insectivorous bird populations have declined precipitously in recent decades. A more robust understanding of their nutritional needs, particularly during the breeding season, can help guide conservation and habitat restoration efforts. We monitored 38 Eastern Bluebird (Sialia sialis) broods in New Castle County, Delaware, USA, with camera traps over four breeding seasons to assess the composition of the nestling diet and examine how food provisioning varies over the season and between adult sexes. From the photos, we identified the prey adult bluebirds brought to nestlings to the lowest possible taxonomic level. Based on a review of 8,128 provisioning visits, caterpillars (i.e., larval Lepidoptera and Symphyta) were the predominant prey brought to the nest by both males and females in all years of the study and in both the first and second broods of each season. Most caterpillars are host-plant specialists, and native plants host a much greater number of caterpillar species than do non-native plants. The importance of caterpillars in the diet of Eastern Bluebirds and perhaps many other insectivorous birds highlights the urgent need to conserve native plants already present and make them a central component of habitat restoration.