Chimney Swift

What would we call the Chimney Swift if early ornithologists had encountered it away from early colonial villages? It certainly didn’t nest in chimneys prior to European settlement, instead using hollow trees in mature forests. As chimneys became more common in eastern North America the swifts quickly adapted and probably even increased in population. Unfortunately, since the mid-1960s, when standardized bird monitoring started, their population has gone steadily down. Considering that this is still a common species in many areas, it’s hard to envision what our skies would be like if there were 3-4 times as many swifts in it on summer evenings.

Why might a species so comfortable coexisting with people be in such sharp decline? One possibility is that the very structures for which it is named are becoming less common and less suitable. Fewer new homes have traditional fireplaces, and even if they do their exterior chimneys may not be the masonry ones needed by swifts. Chimneys with metal linings can’t support swift nests, and those with caps prevent the birds from entering. Add to this the danger posed by cleaning chimneys during the breeding season and you have a whole gauntlet of threats facing our urban and suburban swifts. But at the same time, studies looking at chimney availability have usually found that there are plenty of suitable nest sites but not enough birds to use them, and attention has turned to the possible role of food supply.

Swifts feed entirely on flying insects, and declines in this prey base are often invoked as a threat to this and other aerial insectivores. One study in Canada investigated changes in swift diet by analyzing several feet of guano (representing 48 years of swift use) in a roosting chimney. Researchers documented a shift from beetles to true bugs around the time of the introduction of DDT, suggesting the pesticide altered insect communities in a significant way. What we still don’t know is whether a diet shift such as this can lead to reduced productivity, and studies are ongoing.

Chimneys are also important to this species as roost sites. After the breeding season, swifts can congregate by the thousands in old industrial chimneys, and the sight of them funneling into these at dusk is not to be missed. Smaller roosts occur year-round, and you can usually find them by watching swifts in your neighborhood just prior to sunset. They will gather in a swirling, chittering flock and start to circle near the roost. It may take a few nights to narrow down the exact chimney, but it’s well worth the effort. Artificial “swift towers” have been proposed as replacements for declining chimneys, both for nesting and roosting, but so far have not been used in the northern part of the species’ range. This is probably because they don’t retain heat as well as stone or brick and can’t keep swifts warm enough over cool late summer and fall nights.