Why are birds that feed on insects disappearing?
New findings point to answers that touch on a range of troubling environmental factors.
By Douglas Hunter
Late in our conversation, Jon McCracken, director of national programs at Bird Studies Canada (BSC) in Port Rowan, mentions whip-poor-wills. In turn, I observe that in some 15 years of exploring Georgian Bay, I have never heard or seen one. “Oh, 30 years ago, they were very common where you live,” he says.
For decades now, Ontario’s skies have been losing not only whip-poor-wills but many of their fellow aerial insectivores – that agile “guild,” or group, of birds that feasts on the wing, snapping up airborne insects. “Frankly, most of the avian insectivores are declining in northeastern North America,” says Mike Cadman, a songbird biologist with Canadian Wildlife Service (CWS), Ontario region, in Burlington. He quickly reels off a list of species: “Swallows, nightjars, a lot of the flycatchers and the swifts … We have no clue why that would be, and it seems fairly consistent across the group.”
The steady decline that has affected the guild since the 1960s, and which has been approaching freefall since the mid 1980s, has landed some of the birds on both the provincial and federal lists of species at risk. Flagged are the common nighthawk and olive flycatcher (threatened nationally, of special concern provincially), Acadian flycatcher (endangered nationally and provincially), chimney swift (endangered nationally, threatened provincially) and whip-poor-will (threatened nationally and provincially). But other, once numerous aerial insectivores have also declined as severely as those among the listed species. Provincially, the number of barn swallows has declined 64 percent, and chimney swifts 98 percent, between 1968 and 2008, according to North American Breeding Bird Survey data.
The causes are multi-faceted and have proven difficult to identify, and sometimes are little more than educated guesses. “It makes you think there might be something consistent and pervasive across the group,” says Cadman. “But one thing that is fairly noticeable is that you can come up with a reason for each species that is not the case across the group.”
For Gregor Beck, director of conservation and science at Long Point Basin Land Trust, one of the most worrisome aspects of the decline is that this broad range of species occupies very diverse ecological niches and habitats. “They have a huge range in feeding preferences. Some feed on small prey, others on medium or large insects. Some are daytime feeders whereas others are crepuscular or nocturnal. Some are open-country birds whereas others are woodland, shrubland, wetland or another habitat. This suggests that there are likely multiple issues at play.”
There may well be no single cause to account for the guild-wide crisis. Fortunately, we know much more than we did even a year ago about the nature of this troubling decline. In the past few months, important new findings have emerged, and ongoing studies promise major insights. This research is complementing – and at times challenging – a multitude of standard explanations and is pointing researchers in fresh directions. Indeed, new evidence suggests a connection to environmental factors much larger than the woes of any particular bug-eating bird. In the process of trying to solve the riddle of what has laid low one of these species, we may be on the verge of learning fundamental truths about the plight of this entire group of birds, as well as about broader environmental issues.
Through puzzling over the cause of the declines, researchers have not failed to recognize that, with names like barn swallow and chimney swift, they are clearly dealing with species that long ago adapted to humans. It follows that changes in the human environment would negatively affect the numbers of such birds. The decline in barn swallows, to take one species, has been blamed in part on new agricultural practices. Family farms have been giving way to more industrialized operations, and that means fewer wooden dairy barns dotting the rural landscape. Take away the barns, and you take away nesting opportunities.
The same goes for chimneys – and chimney swifts. A bird that centuries ago shifted its nesting preference away from hollow trees and caves presumably ran out of breeding shelter as people started installing chimney caps and liners and sealing the tops of old chimneys with wire mesh. Even the whip-poor-will’s decline is being linked to human-induced habitat change. The species is a creature of the Canadian Shield that frequents “edge” habitats between forests and clearings. As reforestation progresses throughout Ontario, those edges are disappearing. What’s been good for forests has been bad for the whip-poor-will.
Because data has been collected for only a few decades for many bird populations, we don’t know what their numbers were before they began adapting to human habitation. Perhaps populations of some species exploded after moving in with us, and are now dropping back to more “normal” levels. Data from Illinois, for example, suggests an increase in chimney swift numbers in the first half of the century that could be due to adaptation to human shelters. On the other hand, the increase in the nesting opportunities provided by chimneys could have been more than offset by the habitat loss caused by the clearing of hollow trees.
“A lot of these birds were helped by humanity and are now turning more towards the habitat that was there before,” says Cadman. “It raises questions of whether we should be worried. We should put the priority on learning more about these species.”
Beck, who surveyed swifts this year as part of his fieldwork in southern Ontario, believes people have had a direct effect, in some cases by altering both natural and human-made habitats. “There are definitely fewer and fewer nesting sites available,” he says, noting that the historic snags of old-growth forests are few and far between in the south, and open chimneys are growing scarce. “Some of the active sites I’ve observed are in abandoned buildings, which could be demolished anytime.” But he also suspects a linkage between habitat change, food supply and the numbers decline. “As natural habitats were lost and the species increasingly used human structures, changes in prey type may have been an inevitable and unfortunate companion.”
One expert, at least, has had a significant change in perspective recently. Joe Nocera, a research scientist at the Ministry of Natural Resources and adjunct professor at Trent University, became interested in aerial insectivores after chimney swifts were listed as threatened in Ontario has occurred at only one. While research into nesting boxes continues, their discouraging lack of use to date has led
Nocera to conclude that something beyond habitat is causing the plunge in swift numbers.
Fresh clues are emerging in a study whose findings were recently published in the journal Avian Conservation and Ecology. McCracken, one of the co-authors, is keenly aware of the guild’s problems through research on bank and barn swallows. With support from Ontario Power Generation, a team of biologists at Bird Studies Canada has been conducting a survey of bank swallow nesting sites along the shore of Lake Erie. (Other similar studies are also underway.)
McCracken’s study outlines two interesting spatial patterns for aerial insectivore populations. One shows that the largest drops have occurred in the east and north. “The more severe declines are in eastern Canada, from Ontario eastward, and they’re more pronounced in Quebec and the Maritimes,” he says. “We don’t know why, but it opens plausible research hypotheses.” For instance, the declines correlate with the environmental pattern of the impact of acid rain. Acid rain, McCracken notes, is associated with the loss of calcium in the environment, which could affect birds’ eggs and reproductive viability. There also may be a connection to the way contaminants such as mercury and lead are transported through the atmosphere and deposited far from their industrial source points.
The other spatial pattern McCracken found relates to migration. Birds with the longest southerly journeys, to South America, are suffering the most. “Fewer banded barn swallows may be coming back,” McCracken reports. Again, we don’t know why, in large part because we know little about the winter habitats of these birds. “The birds could be hit by a double whammy from their breeding grounds and wintering grounds,” he says. “I suspect that is the case.”
Another curiosity is that while broad population declines have occurred across the aerial insectivore guild, two distinct waves hit different species. “Nighthawks, chimney swifts and whip-poor-wills were declining in the mid-1960s,” McCracken says. “Most of the rest were doing okay. But then around the mid-1980s, something happened. That breakpoint is of real interest.
“There is something about this guild,” McCracken further reflects. “I suspect it has something to do with the food supply.”
If he is right about that, the answer may lie in a chimney in the heart of the Queen’s University campus.
The discovery started with Chris Grooms’s desire to do something for chimney swifts after they were listed as threatened federally in 2008. Grooms, who was then president of the Kingston Field Naturalists, is a technician with the Paleoecological Environmental Assessment and Research Laboratory (PEARL) at Queen’s University. The lab is especially well known for its groundbreaking work in the Canadian Arctic, where its lake sediment studies have shown that migratory birds are introducing industrial contaminants to nesting areas. They are ingesting the contaminants when feeding in the Arctic ocean and depositing them via their guano.
While speaking with one of the naturalist club’s older members, Grooms was surprised to learn that swifts once congregated in the massive chimney of the university’s Fleming Hall. It turned out that a banding study was even conducted there between the 1920s and the 1950s. Researchers banded about 2,000 birds daily and the flock at one point was estimated at 4,000. Grooms discovered that the chimney was sealed with wire mesh in the early 1990s, but by then the swifts were well into their steep decline.
The university agreed to have the mesh removed to again provide habitat for the birds. (Swifts built a nest the very first year.) Grooms and the naturalist club also decided to investigate what was in the massive chimney. When it was built between 1902 and 1904, Fleming Hall housed the plant that provided heating for most of the campus. Grooms was hoping the chimney might contain unrecovered bands from birds that died in the chimney. Instead, when club members opened the inspection door at the chimney base, about a metre square in area, they found a two-metre-deep column of organic matter.
What at first appeared to be more than a half-century of accumulated guano turned out to be something else entirely. Swifts cough up the hard bits of the insects they eat, much like owls do pellets of animal bones. While there was some guano, virtually all the material that had piled up in the chimney was insect remains. Grooms was planning to excavate and sift the material to find old bands when he realized its value as a research opportunity analogous to PEARL’s lake sediment cores. Soot and roof material at the base of the deposit appeared to mark 1933 as the year of a catastrophic fire in the building, and the top of the deposit had to date to 1992/93, when the mesh was installed. Ergo, about 50 years of sequential insect remains were deposited between those two dates; material below the fire layer dated back to 1928, when the heating plant was taken out of service.
In 2009, Grooms shaved off one-centimetre strips of a vertical section of the material to compile the “core sample.” The lab dated the sample strata and identified 1963 as the last year of atmospheric atomic testing. Among others, the investigation soon involved Nocera (who had done postdoctoral work at Queen’s); Leah Finity, a member of the Nocera Lab at Trent University; and Jules M. Blais, a biology professor at the University of Ottawa.
Preliminary findings were revealed at the annual conference of the Ecological Society of America in Pittsburgh last August. The team found that the crash in the swift population in the mid-1960s correlated with a dramatic change in diet. “True” bugs (insect species of the order Hemiptera) and beetles were replaced by flies, and nitrogen levels in guano deposits plunged. The swifts’ diet changed, a change the authors stated “could easily affect individual survival and brood rearing.”
In other words, kill off the bugs and beetles, and you kill off the aerial insectivores.
The findings suggested that swifts shifted their feeding behaviour because of a dramatic change in bug and beetle populations that may be related to the use of pesticides and other contaminants. They are now examining the samples for changes in contaminants such as metals, PCBs, DDT and hope this will be a way to gauge environmental change that hasn’t been done before.
“We can’t illustrate causation,” says Nocera, “but there is correlation between diet and the population drop. It’s the first historical evidence of what may be affecting other aerial insectivores.”
While the findings in the Fleming Hall chimney don’t knock all other causes out of contention – after swifts’ nitrogen levels recovered between 1977 and 1988, their depleted numbers continued to fall – they could go a long way to explaining why an entire guild of birds has been disappearing. Individual species, placed under stress by severe diet shifts and challenged by habitat loss, could have become more susceptible to a host of other factors, including pollutants.
Like other bird scientists, McCracken is intrigued by the new discoveries. He also thinks it’s too early to panic about aerial insectivores’ long-term viability. Take the bank swallow: while its numbers have been declining steeply in bird counts, last summer the Lake Erie survey team found “nearly 130,000 nests” between Rondeau and Turkey Point. Next summer, the survey may tackle the shore between Rondeau and Point Pelee. “On the good side,” he says, “all are common enough that we have some time to do the research to understand the problem before it’s too late.”
Douglas Hunter is a freelance writer based in Port McNicoll, Ontario. He has previously written for ON Nature on subjects ranging from cougars to fish farms.