Bird populations get lift and drag
Most bird enthusiasts would have some knowledge of broad bird population trends. We would say that Canada geese are thriving. Some might even say they’re doing too well. We also know that aerial insectivores, grassland birds, and many woodland warblers are struggling.
It’s interesting to drill down into the numbers to get a more detailed sense of how certain species are doing. Data from breeding bird surveys, the State of Canada’s Birds report and in particular research from a team led by Michael D. Cadman that was published in the journal Ontario Birds tell some interesting stories.
Chimney swifts, purple martins, whippoorwills, as well as cliff, bank, and tree swallows are all aerial insectivores, meaning they feed on insects while they’re on the wing. These six birds are among the 25 species that saw the steepest population declines in Ontario in the 30year period ending in 2013.
The challenges faced by these birds are many. They include changing insect populations, loss of nesting sites, migration hazards, and degradation of habitat, including across South American wintering areas.
Bobolink, upland sandpiper, vesper sparrow, ring-necked pheasant, and meadowlark are all grassland birds that shared the unfortunate distinction of being on that same top 25 list of declining Ontario bird populations. Habitat loss has driven this multi-decade trend.
Of the woodland warblers, the Tennessee warbler suffered an average annual decline of 3.7 per cent through the 30-year period. Other warbler populations, such as baybreasted, blackpoll and Canada, fell but were not among the top 25.
It wouldn’t surprise a birder to see some of the other entries on the list of declining birds. After all, loggerhead shrike, black tern and redheaded woodpecker are on the list of species at risk in Ontario.
Inevitably however, there are surprises on any list. Declines in house sparrows, herring gulls, brown-headed cowbirds, killdeers and spotted sandpipers — all between 3.5 and 4.5 per cent — were also charted.
At the other end of the spectrum, few birders would be surprised to learn of runaway growth of wild turkey, Canada goose, cormorant, turkey vulture, ring-billed gull, and mallard populations. Canada goose numbers have skyrocketed by 18 per cent a year over three decades.
I was chatting about goose populations earlier this month with Ken Abraham, a professor at Trent University and waterfowl scientist with Ontario’s Ministry of Natural Resources and Forestry.
He explained that because of their ability to adapt to changes in the landscape, most goose species have been doing well. He highlighted in particular snow goose populations across the province.
It might seem counterintuitive to have bald eagle numbers, another species at risk in Ontario, growing significantly, but this big bird is in the top 25. Consider, however, that following the DDT debacle, their numbers bottomed out. This is why the osprey is also on the list with average annual population growth of 4.8 per cent.
I am surprised there has been sustained growth in a couple of warbler species. The northern parula and the pine, blue-winged, and palm warblers are all on that list.
Another surprise for me is the yellow-bellied flycatcher. With an annual three-decade population growth averaging 3.5 per cent, why am I not seeing a lot more of this bird? Am I misidentifying other flycatchers? They are flying south now through to mid-September so I’ll have to be more attentive.