Abstract

Tree Swallows (Tachycineta bicolor) are migratory insectivorous birds that are declining in the NE portions of their range while increasing in the SE portion, including North Carolina. Although the reasons for this range shift are not fully understood, its influences can clearly be recognized in the distribution and breeding behaviors of the species, especially in the southern expansions of their range. Tree swallows, which historically only lay one clutch of eggs per breeding season, have increasingly been observed laying two clutches of eggs per breeding season across the southern parts of their range, with a behavior referred to as double-brooding. This does not appear to be occurring in western NC, where Tree Swallows depart the breeding grounds shortly after their young have fledged, which is typical across most of their range. However, double-brooding behavior has been observed at breeding sites in eastern parts of the state. We speculate that these shifts in range and brooding behavior may be due to changes in food resources during the breeding season- namely the availability of Nematoceran flies, a Suborder of Dipteran flies that Tree Swallows depend upon while feeding their young in the nest. We quantified insect diversity and biomass across two breeding seasons at Beaver Lake in North Asheville, NC, to examine whether insect availability (especially Nematocera) might allow for double brooding within this local breeding population. Using a malaise trap, we captured aerial insects taking fifteen 3-day samples during May and June of 2020 and 2021. We sorted the insects from each sample into 7 insect Orders and Suborders (Nematocera, Hymenoptera, Lepidoptera, Coleoptera, Aranea, Brachycera, and other orders), and examined how each order varied according to time of year, temperature, precipitation, and wind speeds. We found that the biomass of most orders of insects increased as temperatures increased throughout the summer, except for that of Nematocera. Rather, Nematoceran biomass fell sharply just prior to the end of the nesting season. Our results suggest that, even at this lower latitude relative to their northern breeding range, the Tree Swallow breeding season length in western NC may be restricted by insect availability. Further research on insect availability in the piedmont and coastal plains, where Tree Swallows have been observed double-brooding, would shed more light on the relationship between insect availability and double-brooding in this species.

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