Aerial foragers in decline

Citizen science monitoring programs, such as the Ontario Breeding Bird Atlas and the United States-based Breeding Bird Survey (BBS), provide important information on population trends and the conservation status of bird species. One trend that has emerged is the alarming decline in aerial foraging birds, a group that includes swallows, martins, swifts and nightjars. Data from the atlas and the BBS indicate that moderate to severe reductions in the populations of many species in this group have occurred across North America over the past 40 years.

These species feed on insects, and the increased use of pesticides in recent decades is believed to be partially responsible for the widespread decline of the birds.

Another likely cause of declining populations is loss of habitat. Earlier this year, the Committee on the Status of Endangered Wildlife in Canada (COSE WIC) recommended that the chimney swift and common nighthawk be listed as threatened species in Canada. In the last 10 to 14 years, these birds have declined by 30 percent and 49 percent respectively across the country. Ironically, both species have adapted well to urban environments and were once considered common in towns and cities. As forest habitat disappeared, chimney swifts switched from using hollow snags for nesting and roosting to using chimneys. With the loss of grassland habitat, the common nighthawk switched to nesting on gravel rooftops. Unfortunately, suitable breeding habitat is now becoming scarce for both species in urban areas as chimneys are modified or disappear altogether, and as tar replaces gravel roofing.

“Clearly a lot of these aerial insectivores are getting nailed,” says Marty Leonard, co-chair of the COSEWIC bird subcommittee and biology professor at Dalhousie University. “What is most disturbing is when you start seeing declines in common birds that were once widespread.”

Andrea Smith

Like The Atlas of Breeding Birds in Britain and Ireland (1976) that inspired it, Ontario’s atlas survey of breeding birds is a monumental exercise in citizen science, in which thousands of eager amateur (and professional) naturalists across the province report on the whereabouts of breeding birds, collecting vast amounts of data that simply would not be available otherwise.

“It’s incredibly valuable,” says Wendy Francis, Ontario Nature’s former director of conservation and science. Ontario Nature helped get the province’s first breeding bird atlas off the ground and is a founding sponsor of the second. “Nonprofit organizations can rarely do their own original scientific research. To have the capacity to do this kind of comprehensive study across the province, and to repeat it every 20 years and look at trends, is not something that’s available to most environmental organizations … We simply couldn’t afford to do it without the volunters.”

The granddaddy of the modern bird atlas movement, the first British atlas, was based on a survey conducted between 1968 and 1972 by about 10,000 contributing “twitchers,” or birdwatchers. Its system of grid-based atlassing quickly became the model used elsewhere. Denmark and France published their atlases around the same time, and much of the rest of Europe, New Zealand and some African countries quickly followed suit.

In the late 1970s, the notion literally winged its way to Ontario when George Francis, now a professor emeritus of environment and resource studies at the University of Waterloo, flew home after seeing Britain’s atlas in a British bookstore and tried to convince people the same could be done here. Eventually, says Cadman, the idea caught on. Now, most Canadian provinces and U.S. states have breeding bird atlases, and many researchers are compiling similar atlases for other animals and even plants.

“I’m all for citizens’ science,” says York University biology professor Bridget Stutchbury. “I think the volunteers on these projects have proven for decades how good they are at it and how passionate [they are] about it. There is no way in the world you could raise enough money to pay people to do what people are doing out of their own interest and concern for nature.”

Stutchbury is the author of the recently published Silence of the Songbirds, which describes declining songbird populations in North America and around the world. Stutchbury argues that globally dwindling bird numbers signal a dangerously unhealthy environment.

The new Ontario breeding bird atlas, however, tells a different story. It suggests that many of the same bird populations that the United States-based Breeding Bird Survey shows are in decline – such as wood thrushes and rose-breasted grosbeaks – appear unchanged here (at least as reflected in the changes in bird prevalence). The populations of other birds shown to be faltering across the continent, such as Wilson’s warblers, appear to have increased in Ontario.

A group effort

For dedicated bird watcher and atlas volunteer Dave Martin of the McIlwraith Field Naturalists, an Ontario Nature-affiliated member group, seeing a yellowbellied sapsucker topped his list of highlights while collecting data for the second Atlas of the Breeding Birds of Ontario. “They were low on the list of species that we expected to encounter, and we never dreamed they would be so widespread. What a delightful addition to our local bird life.”

Amassing an army of volunteers is critical to an undertaking like the atlas. Over a five-year period, an enormous amount of data on birds must be collected. Many of the devoted citizen scientists who signed on to help with the task were also Ontario Nature group members.

Atlas coordinators divided the province into 47 regions, which, in turn, were subdivided into 10-by-10-kilometre-square sections (except in northern Ontario, where sections ballooned to 100-by-100-kilometre squares). Volunteer groups trekked through anywhere from 20 to 90 sections.

In 2000, on the advice of a friend, Ahlan Johanson joined the Rainy River Valley Field Naturalists, also an Ontario Nature member group. As luck would have it, the atlas project started soon after. “My interest in birding was just peaking,” says Johanson. “The atlas provided an opportunity to search out birds I had always wanted to see.” Johanson saw his first pair of red-headed woodpeckers while in the field for the atlas.

Volunteers participated in owl prowls to observe nocturnal birds at their nesting sites, and honed their birding skills during winter by listening to bird calls and songs and studying bird photos.

By the time all the data was submitted, more than 150,000 volunteer hours had been logged in the field and some 1.2 million breeding bird records produced – an impressive tally that would not have been possible without the dedicated efforts of many.

Jim MacInnis

Stutchbury is sceptical of these results. She points out that the changes in distribution that show up as changes in the number of atlas squares in which birds are found provides a crude measure of shifts in actual populations. “It’s not really a criticism,” she says, “it’s just that there are limitations on the kinds of questions you can answer.”

Peter Blancher, a land bird scientist with CWS who analyzed breeding bird records collected for the atlas, agrees that comparing the results of the two atlases has not been easy. For starters, as many as 700 more volunteers clocked almost 25 percent more field hours in the new atlas compared to the first. Furthermore, birders spent different amounts of time within the same individual atlas squares between the two atlases. Blancher calculated the prevalence (the likelihood-of-being-found-in-a-square) figure for each species to overcome these pitfalls.

A change in the prevalence of birds between the first atlas and the second offers a glimpse of the direction in which bird populations might be moving, but it says nothing about the actual size of these populations. The new atlas attempts to address this by also including results from “point-count” bird censuses used to estimate how many birds of each species are living in Ontario.

Future point-count surveys will reveal increasingly accurate population shifts. But, for now, the atlas uses bird prevalence based on the atlas square data to reflect population changes – including the precipitous declines of three of Ontario’s endangered bird species. The likelihood of finding loggerhead shrikes, Henslow’s sparrows and northern bobwhites in an atlas square plummeted by 60 to 80 percent since the first atlas. Moreover, declines were detected for many neotropical migrants in the Carolinian region – the veery declined by 27 percent and the ovenbird by 22 percent. The prevalence of many waterbirds in the same region are also declining, including the American bittern (down 37 percent), common moorhen (down 38 percent) and spotted sandpiper (down 18 percent).

“The results would suggest that of all the regions in the province, the Carolinian is the one in which there should be greatest concern for bird populations,” says Cadman.

Conversely, the bird populations of the north seem to be thriving. Canada’s boreal forest is the breeding ground for about a third of all North American birds, and in Ontario, it makes up more than half of the province’s forested area.