Surviving the Night in a Communal Roost

Posted:  Wednesday, January 3, 2018 - 11:30am

A friend from Edgecomb sent us back in December a few photos, along with his depictions, a description of throngs of gulls that he was noticing flying from east to west at dusk every evening. From his vantage point, he could see the high-flying birds moving from the Boothbay area, across Barters Island and the Sheepscot River and then across Westport Island, disappearing from view.

Over the years we’ve noticed similar movements at dusk, of gulls moving in different places, including a regular movement of gulls over our state capital, Augusta, toward the west.

For a few months now and continuing through the winter, another bird species that many people will see flocking in the evening are American crows. Readers who travel the Interstate 95 corridor north of Augusta probably have noticed the often massive concentration of crows near the Waterville exits in the evenings.

All of these observations relate to the same phenomena: communal roosting.

While waiting for our son to emerge from a basketball practice at the Augusta armory a few weeks ago, we enjoyed watching and listening to more than 500 crows coming in to one of these communal roosts in the pine trees behind the building. The racket they made even after darkness had fallen was impressive. And as impressive as such a congregation is, the size of it was small potatoes compared to some that have been documented in birds. In the southeastern U.S., nighttime communal roosts of blackbirds (red-winged blackbirds, grackles, cowbirds) have been found that numbered into the millions. And this is nothing compared to Africa’s red-billed quelea, a relative of the well-known house sparrow, whose roosts have been estimated in the tens of millions.

The question of why some birds roost together in these large groups is still something that is not completely resolved.

The “information center” hypothesis, described in the 1970s by two scientists (including the same Amotz Zahavi that we talked about in an earlier column), suggests that these communal roosts serve as places where individual birds can learn about the location of food sources. The idea is that those individuals that don’t know much about where the best places are to find food in an area could follow older, more experienced birds that do as they leave the roost in the morning.

Others have suggested that perhaps the reason older, more experienced birds might participate in such roosts, given that they are essentially drawing more birds to their food source, is that they gain something from having the younger birds around—perhaps more eyes to watch for predators at the feeding site or to find food that they could also exploit (as in gulls at a dump). Or it could be that in the roost itself, the older and more dominant birds can take the safest or warmest spots in the roost, being essentially shielded from predators by the less dominant birds and/or warmed by the birds surrounding it.

Maybe all of these ideas are true in different circumstances for different species?

With the particularly frigid temperatures we are currently enduring, one has to wonder whether just being together in a roost might not be the critical factor in whether any individual bird stays warm enough to survive the long winter night!

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology. Dr. Wells is one of the nation's leading bird experts and conservation biologists and author of “Birder’s Conservation Handbook. His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization working statewide to protect the nature of Maine. Both are widely published natural history writers and are the authors of the book, “Maine’s Favorite Birds” and the newly published “Birds of Aruba, Bonaire, and Curaçao” from Cornell Press.