A just-published new study has chipped away at the mystery surrounding what is limiting populations of Rusty Blackbird, one of North America’s fastest-declining songbirds.
According to the International Rusty Blackbird Working Group, blackbird numbers have plummeted 85-95 percent since the mid-1900s, but the reasons are not well understood.
The blackbird’s preferred breeding habitat is in short or stunted conifers in bogs and fens and along beaver flowages deep in the wetlands of the boreal forest, mostly in Canada, but the bird also lays in regenerating clearcuts in upland settings, where its nests may be subject to predation by red squirrels.
Eager to disentangle factors responsible for the blackbird’s decline, a team led by Shannon Buckley Luepold, of the State University of New York College of Environmental Science and Forestry, Syracuse, spent the breeding seasons of 2011 and 2012 in the Moosehead Lake region of north-central Maine and the Umbagog Lake-Androscoggin River region of northern New Hampshire.
The researchers located 72 Rusty Blackbird nests and collected extensive data on habitats where the birds nested, including vegetation density, spruce and fir cone production, and red squirrel abundance. The team also installed motion-triggered infrared cameras at 29 nests.
“What we found,” says Buckley Luepold, “was that red squirrels were indeed the most frequent predators of Rusty Blackbird nests, at least in Maine.”
The researchers documented eight predation events on camera: two in 2011 and six in 2012. Red squirrels accounted for at least four of five events in 2012 in Maine. All of the observed red squirrel predation occurred in the summer of 2012, following an abundant spruce-fir cone crop in 2011.
The results suggest that nest-predation rates may be driven more by ecological processes, such as masting (years of higher-than-normal cone production), than by timber harvesting.
“That said,” explains Buckley Luepold, “we also found that dense vegetation was the most important predictor of nest survival, so harvest practices that reduce density of young trees, such as pre-commercial thinning, could potentially be detrimental.”
In addition to red squirrels, the researchers’ IR cameras caught a hawk (identified as either a Sharp-shinned or a Cooper’s), a Blue Jay, and a white-tailed deer preying on Rusty Blackbird nests.
Buckley Luepold’s research paper, “Habitat Selection, Nest Survival, and Nest Predators of Rusty Blackbirds in Northern New England, USA,” will be published in The Condor: Ornithological Applications, the peer-reviewed international journal of the Cooper Ornithological Society.
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