Winged Harbingers: the Decline of the ‘Common’ Barn Swallow

NIKI WILSON – On Science , Jasper Fitzhugh
September 29, 2011

If you walked down the airstrip in east Jasper National Park this summer, you likely crossed the flight path of barn swallows roosting in the cook shelter nearby. Maybe you witnessed the elegant dive of their light cinnamon and blue bodies as they hunted mosquitoes across the field with X-wing like precision.

The seasonal return of great migrators like the barn swallow is something we take for granted, as reliable as the change in season itself. Current population estimates suggest 4.9 million barn swallows spend their summers in Canada.

That seems like a lot, until you learn that in the 1990s there were more than 10 million of them. Scientists estimate there has been a 30-per-cent drop in numbers in the past 10 years, which is why this spring, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) recommended barn swallows be designated as a “threatened” species, meaning that if the trend continues, the species will disappear.

Barn Swallow on Nest. Photo Credit: Mark Bradley at Boreal Nature Photos.

Many Canadians are familiar with barn swallows. Although some still nest in the wild, many prefer to take up residence in barns, longhouses, bridges and culverts. Perhaps it is our regular interaction with these birds that makes their troubling drop in numbers so worrisome.

“It is the magnitude and swiftness of the decline that have scientists at COSEWIC concerned” says Dr. Marty Leonard, COSEWIC chair and biology professor at Dalhousie University.

According to COSEWIC, reasons for the decline are poorly understood, but are thought to be related to a number of factors that “are likely acting additively in unknown ways.” The first is a loss of nesting habitat due to the conversion of old-style wooden farm buildings to modern metal or closed structures. It is difficult to assess how much of a role this plays, as the timing of sharp population declines does not coincide with mass building modernization, and there are reports of suitable buildings standing empty. In addition, the reduction of open, grassland types of agricultural habitat appear to be limiting foraging habitat in some areas. However in Western Canada, the availability of foraging habitat may be increasing, suggesting that the loss does not, on its own, explain the decline in barn swallows.

Decline in insect populations is also a factor. Light pollution in and around urban centres, climate change, loss and degradation of wetlands, acid precipitation, large-scale use of pesticides, and the recent genetic development of insect-resistant row crops are among the many factors implicated in the decrease of insect populations. “DDT is still being used in some wintering habitat, and this may be a factor,” says Leonard. One study suggests that changes in flying insect populations in North America might be indicative of underlying ecosystem changes.

Summer cold snaps on the breeding grounds, a relatively new climate effect, may also be a cause of mortality. Warmer springs may mean birds are nesting earlier, using more energy and are more vulnerable to early spring temperature drops.

Most alarming is the fact that it is not just barn swallows. Many birds that migrate and feed on flying insects (aerial insectivores) are declining in number. The common nighthawk, whippoorwill, olive-sided flycatcher and chimney swift are just a handful of the many whose numbers are dropping. “We are seeing these declines in some aerial insectivores that don’t co-habitat with humans, so we know there is something happening outside that relationship.” says Leonard. Two more birds in this group, the black swift and bank swallow, will soon be assessed by COSEWIC.

In the meantime, ongoing research reveals a pattern of aerial insectivore decline that shows “a striking geographic gradient.” The decline appears to be most prevalent in the northeastern part of the continent. Studies also suggest that birds with longer migration routes experience decline more acutely.

The barn swallow is a long-distance migrant. Its current breeding range in North America includes parts of Alaska, all Canadian provinces and territories, the continental United States (except most of Florida), most of northern and central Mexico, and a few areas in Argentina. The bulk of the North American population winters in lowlands across South America. In Canada, barn swallow nests and eggs are protected on public land under the Migratory Birds Convention Act. However, most suitable habitat occurs on private land, and the barn swallow has no protection along its migratory route south.

Photo Credit: Mark Bradley of Boreal Nature Photos

The COSEWIC designation of “threatened” is the first step in having the barn swallow protected by law everywhere in Canada. The recommendation has been sent to the Minister of Environment, who has the power to formalize protection under the Canadian Species at Risk Act. Leonard says the legal designation of “threatened” requires the federal government to oversee the development of a recovery strategy.

Regardless of legal designations, extreme declines in barn swallows and other common bird species are sounding major alarm bells. The decline suggests things are going wrong at the scale of ecosystems, which will likely have implications across related food chains. Says Leonard: “I find it frightening that such common species are disappearing.”