Flocks of robins aren't earlybirds
I've had a number of startled readers drop me a note to report seeing flocks of 20-or-so robins converging on backyard trees, gobbling up leftover fruits and berries. We had a bunch of them in our back yard in Cockeysville on New Years Day, feasting on some sort of red berries in a low tree at the edge of the woods. Sun reporter and columnist Fred Rasmussen spied a robin in a tree in Ruxton:
"Isn't it a little early for them to be back in Maryland? It really shocked me. Seeing them in late February or early March is more normal," he said.
Well, apparently that's when we expect to see them. They're the traditional harbinger of spring, after all. But it's not unusual to have a flock of foraging robins in Maryland in mid-winter, according to David Cursom, director of bird conservation at Audubon Maryland DC.
"There is a population that overwinters in the coastal plain of Maryland," he said. "The largest groups are over on the Eastern Shore, a regular roost of robins. I believe around 14,000 have been counted in Easton."
Robins, it turns out, are strongly migratory birds, but they breed all over North America. So, there are populations that breed well to our north, for whom Maryland's coastal plain is "South." Those that breed here likely migrate in October to the southeastern states, and along the Gulf of Mexico. They return in March and April.
"The groups people are seeing now are part of the wintering population that are moving around. As the weather fluctuates between cold and mild, the robins move accordingly to find food," Cursom said.
Although some of my correspondents have expressed worry about the birds as they search for meager pickings in bitter cold weather, Cursom said "there is no reason to be concerned. There is a certain amount of winter mortality among all birds related particularly to when there is a lot of snow on the ground. But that's natural ... (the birds) come here and if they don't find enough food they just move on and find it somewhere else."
It's not clear whether the robins' current migratory and over-wintering behavior is any different, in a warming climate, from what they have always done, Cursom said, "but I think it would be a very worthy line of research."
"It shows the value of keeping written records, and doing regular counts of birds so you can record the times they are absent, as well as present," he said. He noted the Audubon Society's annual Christmas Bird Count, which has now accumulated more than a century of data on bird populations in North America.
As interesting as the birds' foraging behavior is their roosting behavior, where thousands of birds of a feather flock together overnight during the winter, then fly off at sunrise to forage for food. Scientists have not yet figured out why they do it.
"There are some interesting theories," Cursom said. One is "thermo-regulation," the idea that the birds crowd together to take advantage of each other's body heat to help them through a cold winter night.
Another theory is that the birds somehow share information during their roosts. "Maybe birds can size up their roosting partners' body condition" and judge from that whether it would be fruitful to follow them to their foraging grounds the next morning, he said.
In addition to the big robin roost in Easton, there has been, for many years, an enormous roost of crows in northwest Baltimore, near the Seton Industrial Park. Evening commuters on northern and western sections of the Beltway can see crows by the hundreds as they fly toward the roost at sunset.
Comments
Tell me where there aren't crows in this area.
Posted by: bryanintimonium | January 28, 2008 10:41 AM
We see very few crows around our place in Cockeysville. They used to be nuisances, breaking into garbage bags and leaving a mess. But they seemed to disappear around the time that West Nile virus began to be a problem. But now we have huge numbers of buzzards.
Posted by: frank roylance | January 29, 2008 1:44 PM