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An Assessment of Nestling Diet Composition in the Violet-Green Swallow (Tachycineta thalassina)

Abstract Aerial insectivores have undergone marked population declines in recent decades, including members of the Hirundinidae (swallows), which have long served as sentinels of environmental change. In contrast to other swallow species that breed in North America, we have a poor understanding of most aspects of the basic ecology and life history of the violetgreen swallow (Tachycineta thalassina), a widespread species found throughout the Pacific Northwest. In this study, we investigated the diet composition of violet-green swallow nestlings to document the consumption of food resources by offspring during adult feeding visits. We identified arthropods from 13 taxonomic orders in feeding boluses and found that representation of taxonomic groups was highly uneven and dominated by Diptera and Hemiptera. Although swallows did provision some large prey, the great majority (i.e., 92.6% of 1047) of food items were < 5 mm in length. Feeding boluses collected from the congeneric tree swallow (T. bicolor) at the same study area and during the same time period revealed similar patterns of size and taxonomic representation of diet composition of violet-green nestlings, raising questions as to how these species partition critical resources in areas of sympatry.