Tree Swallow

Along with the Barn Swallow, the Tree Swallow is the most familiar member of its family to residents of New Hampshire. It can be found statewide as long as there are nesting sites such as natural cavities and boxes. Typically these are in wetlands or along the edges of fields (the species avoids forest interiors), but it has even been found breeding at higher elevations in the White Mountains. Where there are plenty of nest holes, especially arrays of bird houses, Tree Swallows can nest in high densities. In such situations they are highly territorial, and boxes should not be placed closer than 30 feet apart. Placing boxes in pairs, however, allows other species like bluebirds to nest interspersed with the swallows and not being excluded by them.

Because they take readily to next boxes, Tree Swallows have been the subject of extensive research across North America, and one investigator has likened them to the “lab rats of ornithology.” Through careful monitoring of nest boxes, we have learned about subjects as diverse as the effects of climate change (e.g., nest initiation dates) and chemical contaminants, which factors influence reproductive success (e.g., food supply, weather), and mating systems (e.g., how often a single male may have two mates.

Unfortunately, like most other swallows, Tree Swallows are declining significantly, although this decline is currently focused on the northeastern United States and southeastern Canada. In these areas there are probably fewer than half as many Tree Swallows as there were as recently as the late 1990s, when the declines appear to have intensified. At the same time, the species is increasing and even spreading south into areas where it didn’t nest historically. Reasons for the declines are not well understood, and identifying causes is complicated by the diversity of population trends across the species’ range. As with other aerial insectivores, speculation focuses on the effects of pesticides, climate change, and effects during migration and winter. In the latter case the Tree Swallow differs from most other species in this group in that it winters almost entirely in the southern United States, which presumably exposes them to different threats than those faced by swallows wintering in South America. Even the data we do have don’t always agree, but the fact that Tree Swallows are one of the most heavily researched species on the continent ensures that people are still looking for explanations.