Introduction

This is a swallow of open fields, meadows, marshes, beaver ponds, lakeshores, and other wetland margins, a species that uses trees only for nesting and occasional roosting. A hole nester, it depends on woodpeckers and other species that excavate and abandon cavities in dead trees, and to a lesser extent on nest boxes, which it accepts readily.

Such nest sites are limited, and competition for them is likely a driving force behind much of the breeding ecology and behavior of Tree Swallows, including early spring arrival, intense territorial defense against conspecifics, strong aggressive responses to potential nest site competitors, delayed plumage maturation in females, absence of mate guarding and frequent copulation, and sexually selected infanticide. Nest site availability also appears to limit the size of Tree Swallow populations in most areas.

Although mainly monogamous, extra-pair copulations are frequent in this species. In some populations, about 50% of the nests contain young not descendent from the resident male. A highly social bird, pairs can nest solitarily. Outside the breeding season, it often forms large flocks, up to several hundred thousand birds at some nighttime roosts.

Unlike other swallows, this species can subsist for extended periods on seeds and berries, especially the waxy fruits of Mryica sp., allowing some coastal populations to winter as far north as Long Island, NY.

Because Tree Swallows are hardy birds and easily attracted to nest boxes, they have proven popular subjects for field experiments testing evolutionary theory, environmental monitoring, and theories of population biology. This has resulted in an extensive literature on the species, selected portions of which appear here.