For declining boreal birds, hope is in the details
When it comes to environmental science, answers to major questions often depend on seemingly minor details. When seeking solutions to a problem, small, but crucial questions must be answered before research can move ahead.
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A young male rusty blackbird in its fresh "rusty" fall plumage. Unlike most songbirds, it's difficult to tell adult and juvenile rusty blackbirds apart.
(photo: Pam Sinclair)
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This is certainly the case with the rusty blackbird, Euphagus carolinus, whose populations are declining rapidly over much of its range. Scientists, like the Yukon's Pam Sinclair of the Canadian Wildlife Service, have discovered that the past four decades have been especially tough on this once-abundant North American songbird; the decline has been a precipitous 90 per cent.
"We don't know exactly what's going on. Is it that the birds are not surviving the winter? Are the birds not reproducing well? We don't know these sorts of details and until you know the mechanism of the decline, you can't really do anything about it," says Sinclair, who has been leading a rusty blackbird research project here since 2005.
The rusty blackbird winters in the shrinking hardwood forests of the Mississippi Valley, east to the coast. The disappearance of secure feeding and roosting sites in those southern forests is "undoubtedly part of the story" of the decline of the birds, says the ornithologist. "But it doesn't seem to be the whole story because the rate of decline in recent years is more dramatic than the loss of winter habitat."
Learning just how this multifaceted crisis is coming about has been made more difficult by a deceptively small challenge: It is difficult to tell adult and juvenile rusty blackbirds apart.
That's where Sinclair and her Yukon colleagues come in. The birds breed in the boreal forest, and autumn finds many of them congregating at the readily accessible Whitehorse landfill, stoking up on calories for the long journey south. They also pass through the Yukon's two bird observatories, at Teslin Lake and Albert Creek, which contribute to the study.
"In order to figure out why these birds are declining, you need to figure out if the young birds in their first year are not surviving, or whether the birds are not reproducing well in the first place. Where is the deficit in the population coming from?" she says.
In most songbird species telling the adults from the young is a fairly straight-forward proposition. That's especially true in the fall when the young are marked differently than their elders. Their plumage is different, their moult pattern is different, and often the young do not regrow all of the flight feathers on the wings, leaving an obvious contrast between old and new feathers. But wouldn't you know it? Those most threatened of North America's blackbirds present a special case. The differences between young and adult rusty blackbirds are more subtle, and the most reliable difference is only available to observers in autumn.
That's when Sinclair applies a technique called "skulling," a sort of bird phrenology. "It's a bit of a funny thing," she says. The skulls of the younger birds, those who have just hatched during the boreal summer, are incompletely formed. Their skulls "are in two thin layers and as they grow they develop little pillars of bone between those two layers, and these appear as little white dots." The skin on the head is so thin, that by wetting and gently moving it scientists can locate those little white dots, the ends of the pillars.
By the time the rusty blackbirds have reached their wintering ground, and become the focus of study by southern researchers, adults and young all have the pillars. "So we've been taking these known-age birds and looking for other ways we can distinguish the ages. We've been looking for details of their colour patterns on their faces and the rest of the body, but most of the focus is on the face."
The Yukon scientists have discovered a correlation that can be used by their southern colleagues. Young birds, the ones without the fully formed bone pillars, all have very fine white eye rings, unlike the adults.
The threats to rusty blackbirds only became a concern recently, says Sinclair. They used to be very abundant over most of their range and the first scientific paper about the species decline came out only 10 years ago. That study basically consisted of perusing old books on birds for different regions of North America. Some books dated back into the 1800s. In the texts researchers could read where the birds were considered abundant, common, uncommon or rare. The Birds of Manitoba, from 1891, for instance, described the rusty blackbird as "an enormously abundant migrant." Later bird guides tell a different story.
Rusty blackbirds are still fairly numerous in the northern boreal forests, says Sinclair, but that is changing. There's quite a bit of development, these days, at least in the southern boreal, including forestry, hydroelectric projects, peat harvesting, agriculture, and oil sands. Such activities can disturb wetlands and kill off the water-born creatures birds feed on. Climate change may also be hastening the decline. When the permafrost layer melts, the wetlands above it can dry up or be choked by brush.
Most disturbing of all, is that the rusty blackbird is only one of many boreal bird species in decline: Solitary sandpipers (which move into abandoned rusty black bird nests), lesser yellowlegs, horned grebes, black poll warblers, white-winged and surf scoters, lesser scaup... Hopefully much of what is being learned about these blackbirds can be applied to efforts to help their boreal neighbours as well.
- For more information, the US Smithsonian National Zoological Park maintains a fascinating rusty blackbird page.
