|Back to Home||Outdoors in the Land O'Lakes - May 10, 2012|
Swallows, swifts, nightjars and flycatchers: aerial insectivoresby Steve Blight
It’s early May, and almost all of “our” birds are back and are settling into their important summer routines of foraging for food, seeking mates and building nests to raise their young.
One group of birds is known as aerial insectivores – birds that feed primarily on flying insects, usually taken in flight. Known as a “guild” to bird scientists, aerial insectivores include swallows, swifts, nightjars and flycatchers. Nightjars are a small family of birds including Common Nighthawks and the Whip-poor-will, a bird familiar to many residents and visitors to the Land O’Lakes area.
Although there is plenty of overlap in diet, each bird species exploits somewhat different food resources in different ways. Swifts, nighthawks, swallows, and martins remain aloft to feed, whereas the flycatchers typically hunt from a perch, sallying out to catch flying insects, then returning to the perch.
Swifts and nighthawks feast on high-altitude insects, often above 20 metres. Meanwhile, the Purple Martin is a mid-level specialist that forages mostly above 10 metres, but also often enters the higher air space as well. Swallows are altitude generalists, cruising in all air spaces – high, medium, and low.
Flycatchers also divide up vertical air space. None of them forage much above about 20 metres, but forest-dwelling species (Olive-sided Flycatcher, Eastern Wood-Pewee, Least Flycatcher, Great Crested Flycatcher) feed mainly at moderate heights of about 5-15 metres. Eastern Kingbirds forage within this height range too, but also feed right down to ground level. Others, including Yellow-bellied, Alder, and Willow flycatchers and phoebes) are strictly low-altitude foragers and seldom feed at heights more than three metres.
Another feature linking the birds in this guild together is the rapid decline in their populations across much of North America. Declines in abundance and contractions in breeding range of many of these bird species were first detected in the 1990s. Since then, problems have become evident in many regions across North America and across many species within the guild. Initial concern over the extent and rate of aerial insectivore population declines in Canada originated largely from the second Ontario Breeding Bird Atlas. This work suggested major decreases in the occurrence of swallows, swifts and nightjars between 1981-85 and 2001-05. More recent information has identified particularly rapid decreases among Bank Swallows, Cliff Swallows, Barn Swallows, Chimney Swifts, and Eastern Kingbirds.
North American Breeding Bird Survey data indicate consistent 30-40 year declines for many species. Sadly, many long-time residents and regular visitors to our area would agree that they see (and hear, with respect to Whip-poor-wills) fewer of these birds than they used to.
Rates of decline vary among species. Declines are greater for long-distance than short-distance migrants, and are generally greatest and more widespread among swallows, swifts and nightjars. Flycatchers in general are also declining, although a few species are doing alright. Open-country species are declining more than those that inhabit forests and, interestingly, there is evidence to suggest that species that forage at moderate to high heights are declining most strongly. Those that forage exclusively at heights less than about three metres tend to be faring much better.
Although the one common trait across all Aerial Insectivores is their diet of aerial insects, it remains unknown whether their declines have a single common cause. Factors contributing to these wide-spread declines are likely to include some or all of the following:
Although these trends are worrisome for bird lovers, scientists from across North American are actively seeking to pinpoint the reasons for these declines. Once some of the questions have been answered, solutions can be developed and put in place to begin reversing these trends. Although having Whip-poor-wills singing through the night beside my bedroom window can be annoying, I’ll sleep better knowing they are there!