Essays

Moseying: Exploring the Natural World

Why do birds form large winter roosts and flocks?
September 17, 2008

Five hundred starlings dove out of the line of trees next to the road, swooping low in front of my truck. Pounding rain, too much for wipers set on high, only partially revealed the tight-knit flock of birds. I could not put on my brakes out of fear of hydroplaning. Thud! Thud! Thud! Thud!

I pulled off of the road to clean a blood mixed with feces off of the windshield. I grabbed a wet rag out of the back of the truck and as I scrubbed, back hunched over against the windblown rain, I glanced back to the row of trees. The birds had flown because the farmer had started up a big tractor and were already returning as the big machine lumbered towards a barn.

Starlings are one of a number of species that form big night-time roosts when nesting season is over. Great-tailed grackles do the same, much to the dismay of homeowners who have to clean thick crusts of droppings off of vehicles and driveways. Red-winged blackbirds, cowbirds, and Brewer’s blackbirds will crowd into dense cattail swamps in ranch ponds and water retention reservoirs, too. Sometimes the winter roosts of these assorted blackbirds will grow into flocks of several million, causing health problems for cities.

When the roosts reach such incredible size, city officials are often asked to “fix the problem.” A non-lethal method, such as the erratic explosions of a noise-making cannon, is first tried, but it often does not work. High-powered jets of water are tried next, but often the city officials have to resort to lethal means. A chemical has been developed that takes the oil out of the feathers of birds, which makes them lose their insulating ability. When sprayed on a huge roost of birds when the temperature is near freezing or below, significant mortality finally occurs.

We sometimes receive reports of other species of birds in large flocks. With the first cool fronts of fall, folks often report seeing a hundred or more hawks sitting in a field. Usually these are migrating Swainson’s Hawks, heading south to Venezuela. In late August and early September Mississippi kites gather in trees in town, before they also head to South America for the winter. In late September, flocks of yellowheaded blackbirds pass through. By mid-October flocks of lark buntings will appear in ranch pastures in the area. Most flocks number less than two hundred, but sometimes several thousand will be together. “The ground rose up and flew away,” is an apt description of their synchronous flight in a tight flock that undulates as one organism as the flock traverses the landscape.

Every Llanero (resident of the Llano Estacado) should visit one of the big salinas (salt playas) where sandhill cranes roost at night during the winter. There is no grander sound or sight when 10,000 cranes come streaming in at sunset to stand in shallow water safe from predators. At some of the water supply reservoirs in West Texas and Eastern New Mexico, huge “rafts” of geese and white pelicans will gather in the winter. A flock of shining white Snow Geese that covers ten acres of water is an incredible and wondrous vision.

The Midland Naturalists (Midnats) have also seen large flocks of “seagulls” at regional water supply reservoirs in the winter. The most dedicated “birders,” like Rosemarie Stortz, will sit for hours examining each bird, looking for the rare and unusual. Serious birders will also spend hours examining similar rafts of ducks far out on such lakes, and then spend hours sifting through hundreds of sandpipers gathered on muddy shores.

In the winter, robins often gather in large flocks in different regions of West Texas. On Christmas Audubon bird counts I have counted 8000 robins in one pecan orchard just north of Fort Davis, and near Iraan one year robins continued to come to an oak grove for over an hour as the evening darkened. Bluebirds will often gather in large flocks in the winter as well. “The hillside turned blue when the first rays of sun hit the junipers,” has been proclaimed by Midnats on Christmas counts from the Guadalupe and Davis Mountains to Robert Lee and Big Spring. In the west, mountain bluebirds are the magicians, but eastern bluebirds perform in the juniper country to our east.

Large visible flocks of birds are spectacular. In the urban forest and in the riparian forests of the draws and canyons mixed species flocks of small birds filter through the trees. A dozen or more species of birds will forage through the landscape, slowly drifting along. Kinglets and warblers will be on the outside edges of the trees. Creepers and nuthatches will work the bark of the trunks of the trees. Juncos and a number of species of sparrows will tumble along the ground, flying a few feet and landing to peck for a few minutes, then whirling up to go a little further. If present, blue jays and gnatcatchers will join in the fun. These flocks form for protection from the bird-hunting Sharp-shinned and Cooper’s Hawks or the species of falcon known as the Merlin, but they do not roost together at night.

Ornithologists have studied species that form the large nighttime roost, trying to understand the reasons for the roost flocking behavior. The roosts are often used year after year, often for decades. As the birds settle in for the night (and before they disperse in the morning) such roosts are cacophonous riots of noise. The birds come from all directions and often perform impressive massed maneuvers over the site. In some studies, hundreds of birds were marked with large colored bands on their legs. The roost seems to serve as regulatory system. Certain groups roost in one particular area, and usually leave to forage in the same direction. Many of these roosts will have four “flight waves,” with most of the birds leaving during the second and third waves. One hypothesis for that the birds of the first and earliest wave are the groups that returned to the winter roost up to two weeks after the roost was established for the winter. Another major function of the roosts seems to be a mechanism for scheduling migration, as some of the birds will continue to migrate even further south. The roost is an “epideictic rite,” an orchestrated “ritual” that governs the dispersal of the birds (both locally and regionally.)