Cones, Squirrels, and Rusty Blackbird Nests: Declining Songbirds Caught in a Complex Web
The bird that’s experienced the steepest population declines in
North America in recent decades is also one that few people have heard
of: the Rusty Blackbird. Rusty Blackbird (Euphagus carolinus)
populations have decreased by about 95% in the last fifty years, but the
reasons are not well understood; it doesn’t help that their preferred
breeding habitat, stunted conifers deep in the wetlands of the boreal
forest, makes finding and studying them difficult. New research in The Condor: Ornithological Applications
aims to disentangle some of the interacting factors that may be
responsible for the decline. Shannon Buckley Luepold of the SUNY College
of Environmental Science and Forestry and her colleagues spent two
years collecting data on Rusty Blackbirds nests and their surrounding
habitat in Maine and New Hampshire, and uncovered a web of connections
between timber harvesting practices, spruce and fir cones, red
squirrels, and Rusty Blackbird nesting success.
In the breeding seasons of 2011 and 2012, Buckley Luepold and her
co-authors were able to locate 72 Rusty Blackbird nests and install
motion-triggered, infrared cameras to watch for predators at 29 of them.
Enduring swarms of biting black flies, numerous flat tires, and even a
night spent stranded in the woods with a broken ATV, they also collected
a variety of data on habitats where the birds nested, including
vegetation density, spruce and fir cone production, and red squirrel
abundance. After abundant cone production in 2011, squirrel numbers
increased significantly in 2012, which is when nest survival was lower
and when all of the observed red squirrel predation on eggs and
nestlings occurred. A big year for cone production leads to a big year
for the squirrels that eat them. This is bad news for Rusty Blackbirds,
as their eggs are also on the squirrels’ menu.
“What we found was that red squirrels were indeed the most frequent
predators of Rusty Blackbird nests, at least in Maine,” explains Buckley
Luepold. “However, all of the red squirrel predation we observed
occurred in the summer of 2012, following an abundant spruce–fir cone
crop in 2011. Our results suggest that nest predation rates in Rusty
Blackbirds may be driven more by ecological processes such as masting
(years of higher-than-normal cone production), rather than timber
harvesting, which we found not to be a strong predictor of nest
survival. That said, 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,” says Buckley Luepold. In addition to red
squirrels, cameras also caught a hawk, a Blue Jay, and even a
white-tailed deer preying on Rusty Blackbird nests.
“Buckley Luepold et al.’s study elegantly chips away at the mystery
surrounding what is limiting populations of Rusty Blackbirds, North
America’s fastest declining songbird,” says Luke Powell of the
Smithsonian Migratory Bird Center, who first suggested the possibility
of a link between cone production, squirrels, and blackbird nest
success. “Thanks to their work, we now know that red squirrels are the
most frequent predators of Rusty Blackbird nests, and that understanding
the complex relationship between red squirrels, cone cycles, and the
pre-commercial thinning of softwoods near wetlands may be the key to
maximizing next success.”
Nesting Rusty Blackbirds face a variety of threats. Image credit: S. Buckley Luepold
This camera trap image shows a red squirrel raiding a Rusty Blackbird nest and eating an egg. Image credit: S. Buckley Luepold
