Dietary and gut microbial patterns in a southern Indiana cerulean warbler (Setophaga cerulea) population and the relationship between breeding phenology and larval Lepidoptera

Arthropod populations are dramatically declining due to human activity and climate change. Furthermore, deforestation, agriculture, and extreme climatic events are driving a loss in moth populations worldwide. Larval moths are a staple food source for breeding forest birds and developing nestlings, making it critical to understand avian dietary patterns and prey choice dynamics. The Cerulean Warbler (Setophaga cerulea) is a declining Nearctic-Neotropical migrant that breeds in Eastern North America. These insectivorous birds glean arthropods from the canopies of white oak and hickory trees, but there is little knowledge about their prey preferences and diet composition. Previous studies have indicated that Lepidoptera larvae are fed to Cerulean Warbler nestlings more frequently than any other arthropod taxa. The objectives of this study were to describe the diet of adult Cerulean Warbler, quantify Lepidoptera larvae abundance in territories and consumption of Lepidoptera, determine prey composition differences between after-second year (ASY) and second year (SY) males and between breeding seasons, and the interaction between bird age and year. In this study I collected fecal samples from 53 adult male, one adult female, and two fledgling Cerulean Warbler. I collected caterpillar frass from Cerulean Warbler territories to compare Lepidoptera larvae availability to the frequency of Lepidoptera in the diet. Research was conducted in the Hardwood Ecosystem Experiment (HEE) in Yellowwood and Morgan-Monroe state forests in south-central Indiana during May – July, 2022 and 2023. Metabarcoding analysis was used to identify dietary items, and Lepidoptera larvae frass mass was determined by date in the breeding season. Cerulean Warbler consumed Lepidoptera and Araneae more frequently than any other prey order and selected prey items more often in the families Noctuidae, Tortricidae, and Philodromidae than other invertebrate families. Diet richness was greater in 2023 than in 2022 and there was a significant difference in prey composition between years. There were no differences in diet composition between age groups (ASY and SY) when samples were pooled for all years but there was a difference in prey composition between age groups in 2023. Lepidoptera family frequency in fecal samples fluctuated by date and Lepidoptera larvae varied in abundance throughout the breeding season suggesting that the adult diet changes with caterpillar availability. This is the most comprehensive study of the adult Cerulean Warbler diet and results demonstrate that Lepidoptera and Araneae are important prey orders for breeding populations. Long-term monitoring of the diet of imperiled bird species and the availability of arthropods in their habitats may provide further insight into patterns of bird declines and factors that influence dietary composition, particularly for bird species such as the Cerulean Warbler where we have limited knowledge of invertebrate consumption and prey preferences.