Wild Side: The mysterious chimney swift

When was the last time you looked down into a chimney?

The elusive chimney swift excels at long-distance travel. —Tyler Jamieson Moulton

After the robin, the chickadee, and the blue jay, the chimney swift may have been the first bird I learned to identify. I was probably 5 or 6 years old, and once these birds were pointed out to me, I easily recognized their high-flying habits, dark, cigar-shaped body, narrow, curved wings, fast, erratic wing beats, and chittering call.

Only one species of swift normally occurs in our region, and it is very hard indeed to confuse with anything else. Yet in spite of six decades of acquaintance, the chimney swift is not a bird I know well. Its habits guarantee a measure of mystery.

Take nesting, for starters. As they name suggests, chimney swifts do indeed nest in chimneys, using their saliva to paste together a simple, shelf-life nest made of twigs. But when was the last time you looked down into a chimney? This is not a species that easily surrenders its breeding secrets.

Chimney swifts are presumed to have nested in caves and hollow trees before chimneys became available. In this preference for manmade nest sites, the chimney swift joins a very short list of birds, including barn swallows and barn owls. But the association of nests with chimneys appears close to absolute for the swift, and one wonders what new gig this species will discover a few decades from now, when suburbia is all-electric and chimneys are obsolete.

Away from the nest, chimney swifts are no easier to study. Their flight is rapid, erratic, and generally high off the ground (though they do occasionally swing low, like swallows, to hunt near ground level). You’d think that pickings would be slim for an insectivorous bird 200 feet off the ground, but this is apparently not the case.

Dietary studies have shown that chimney swifts consume a wide range of insects during their high-altitude foraging, prey items that are presumably lifted up by rising air currents. Behavioral studies have shown that foraging swifts are quite oblivious to the conditions on the ground beneath them. But it can be assumed that they have a fine sense for where warm air is rising.

Thus it is that in 60 years of observations, I’ve never really gotten a good look at a chimney swift! If I had, the view would not have impressed me: Fairly uniform grayish-brown in color, though lighter on the throat, and about five and a half inches long, this is a drab and modest species.

It has some snazzy anatomical twists, however. A swift’s feet are very small, and all the toes, each equipped with a curved claw, can swing forward. In that position, the toes amount to hooks from which the bird can hang, snagging any sort of rough surface (such as the inside of a chimney). The feet are useless for conventional perching, though, and this species, more than nearly any other bird, has committed to a life spent largely in the air.

Complementing the hooklike toes, each tail feather on a chimney swift ends with a spine, the extension of the central rib of the feather. When hanging by its toes, a swift can jab those spines into the substrate, completing a stable triangle of support. (Woodpeckers, though not closely related to swifts, have evolved similar tail feathers to help them perch upright while hammering on trees.)

But why bother perching if you can fly like that? Chimney swifts are the consummate aerialists, mixing bursts of speed powered by fast wingbeats with long, stiff-winged glides. They forage tirelessly, darting and zooming as they envelop insects in their broad mouths. They excel, as well, at long-distance travel, migrating between breeding sites across Eastern North American and wintering grounds in northwestern South America.

On the Vineyard, swifts arrive quite reliably in the first week of May. As with many of our breeding species, we don’t seem to get many transient swifts, and the breeding population fills in rapidly once the vanguard has arrived. The species leaves abruptly in late August and September, though the odd chimney swift, either transient or migrant blown back north by storms, can turn up into November.

Chimney swift numbers appear to be declining, here and indeed everywhere. The reasons are not wholly clear, but declines in insect populations are likely part of the picture. I hope the trend reverses; summer would not be the same without this mysterious bird darting over the Vineyard towns.