The triple threat of severe habitat loss, climate change and pollution has caused a precipitous drop in these sublimely coloured birds across North America.
By Tim Tiner
The muskeg-nesting rusty blackbird may be the most sharply declining land bird in North America. On its wintering grounds in the swamps and bottomland woods of the southern and eastern United States, this species’ numbers have decreased by more than 85 percent since the mid-1960s. Much of its prime habitat in the deep south is gone but, until recently, the bright-eyed recluse was little studied and its population plunge unheralded.
Unlike most blackbirds, which have multiplied with the spread of agriculture, the inconspicuous, retiring rusty species is a habitat specialist and intolerant of change. In the United States, it was long lumped with its relatives as a farm pest and, in the 1970s and 1980s, killed in control programs that targeted mixed-flock roosts of up to millions of birds. Rusty blackbirds, however, forage mostly on wetland insects, rarely crops. In January, the loophole that had allowed their destruction was finally closed.
In 2006, the Committee on the Status of Endangered Wildlife in Canada designated this beleaguered blackbird a species of special concern. A draft management plan, due from the Canadian Wildlife Service in March 2012, may well be the first government conservation framework for the bird in North America.
Canada hosts 70 percent of the global breeding population of rusty blackbirds in a range that spans the country. Nearly three-quarters live in the boreal forest. In Ontario, small concentrations are scattered as far south as Algonquin Provincial Park, but the vast majority are in the Hudson Bay Lowlands. They nest in dense water’s-edge thickets of small evergreens and shrubs along slow-moving streams winding through muskeg and on the fringes of peat bogs, beaver ponds, sedge meadows, swamps and other wetlands. The slate-grey females build bulky nests of twigs, grass and lichens, plastered with aquatic vegetation; solitary sandpipers often use these nests in later years.
The rusty blackbirds that breed the farthest south seem to be in the most trouble. On the Canadian Shield, the birds’ occupancy shrank by a third in the 20 years leading up to the 2007 publication of The Atlas of the Breeding Birds of Ontario 2001–2005. In Algonquin park, their occurrence fell by almost half during that time. Range retraction is also documented in Quebec, the Maritimes and New England.
Biologists say the distribution of the entire population may be shifting north. Data collected for the most recent breeding bird atlas indicates that the probability of observation in the Hudson Bay Lowlands actually 37 percent. Recent studies also suggest that rusty blackbird populations may be secure in the Mackenzie Valley and Alaska. Continent-wide, however, the annual North American Breeding Bird Survey indicates that this species has declined by more than 98 percent since the mid-1960s – although the survey covers less than a third of the nesting range. The vast majority of the breeding grounds remains inaccessible.
Joe Nocera, a species-at-risk research scientist with Ontario’s Ministry of Natural Resources, last summer abandoned a pilot project in Algonquin Provincial Park monitoring rusty blackbird nests, and concluded that replicating the project farther north would be impractical. “They are really hard to find,” he says, noting that rusties nest in less accessible places than other blackbirds, and their creaky songs, which both the glossy black males and their mates sing, are not very loud. Nocera’s team found nests in only five percent of the sites surveyed. What’s more, the nesting concentrations are quite remote from towns in the Hudson Bay Lowlands, so studying them requires the use of helicopters. “The logistics of working up north would have been too expensive,” explains Nocera.
Given the bird’s elusiveness, the best available measure of the species is the annual Christmas Bird Count, and that shows an 86 percent drop in numbers over the last 38 years. Winter flocks of hundreds or thousands were once common but are now rare. The loss of southern wetlands, flood plain forests and cyprus lagoons to agriculture and urban development is suspected of being the biggest cause of the decline. About 80 percent of this habitat, at the epicentre of the species’ winter range in the Mississippi Valley, has disappeared over the past 150 years, much of it between 1950 and 1980.
Russell Greenberg, head of the Smithsonian Migratory Bird Center, says other factors, such as the way management agencies control water levels, could also be important in the decline. Greenberg co-chairs the International Rusty Blackbird Working Group, an informal body of researchers formed in 2005 to share information and direct research. The group has organized three annual midwinter rusty blackbird blitzes since 2009, involving volunteers searching for flocks across the southeastern United States. “We hope to offer [the information] to state agencies to show them the hot spots so that they can set up more focused monitoring programs,” he says.
Another potential cause of the rusty blackbird’s decline emerged in recent research led by Acadia University’s Samuel Edmonds: in birds nesting in Maine, mercury was found at levels high enough to likely affect reproductive success. Both mercury and acid rain from industrial emissions fall heavily in the northeastern United States and the Maritimes, notes Edmonds. “We suspect that the acidification of wetlands and freshwater habitat has allowed for greater retention of mercury in this region and greater bio-availability of mercury to wildlife.” The rusty blackbird’s diet of aquatic insects makes it particularly susceptible, while its tolerance of mercury is lower than that of many other water birds.
Wetlands loss isn’t just a problem in the southern United States, of course. Canadian wetlands are declining, especially in the northeast. In the boreal zone, an estimated five percent of the rusty blackbird’s habitat has disappeared, and another four percent could be lost over the next half century with the creation of more hydroelectric reservoirs – which can also increase mercury concentration in the water – and a massive expansion of oil sands extraction.
Still other issues may be affecting this species. Evidence is mounting that climate change is already melting permafrost, drying northern peatlands and ushering red-winged blackbirds and grackles farther north; these birds aggressively displace rusty blackbirds. The warming is suspected to be changing water chemistry, disrupting aquatic food chains and possibly skewing critically important timing of insect development for bird breeding. Greenberg notes that rusty blackbirds in the boreal forest feed their young large numbers of newly transformed dragonflies, caught as they emerge from the water.
“We just don’t have the monitoring program to tell you if these things are affecting rusty blackbirds,” laments Greenberg. He notes, however, that awareness of the bird’s plight has come a long way in the past five years. With other boreal nesters – such as lesser yellowlegs and solitary sandpipers – also in decline, much depends on that awareness going much farther.
Tim Tiner is the author of several nature guidebooks and a long-time contributor to ON Nature.