The bird that gives away her children

Written by Jack Sanders
Thursday, 08 July 2010 11:00

The brown bird sits on a branch, watching for another bird carrying grass or twigs, the telltale signs of construction. She’s a Brown-headed Cowbird and she’s looking for a nursery.

Cowbirds are “brood parasites,” laying their eggs in active nests of other bird species in the expectation that the nest-builders will raise the cowbird young. In the process, the eggs or chicks of the nesting species are usually destroyed or starved as the bigger cowbird baby dominates the brood.

The cowbird was once only a Plains dweller, following the herds of buffalo and eating the insects they stirred up. Because herds were always moving, cowbirds moved, too, with no time to build a nest and sit on it. So they learned to use the services of other birds, laying their eggs — as many as 65 a season! — in the nests of other species.

The mother cowbird visits the nest when the parent is away, and can lay an egg and depart in between 10 and 30 seconds. It takes 10 to 13 days for the cowbird egg to hatch, a bit shorter incubation than most songbirds, which can give the baby cowbird an additional advantage over its usually smaller “brethren.”

Being plains birds, cowbirds avoid forests, but when European immigrants began felling the woodlands, the cowbirds moved eastward. Even a modern power line running through a woods opens a pathway to more potential nests to parasitize in the forest edges.

Most birds raise cowbird eggs while their own eggs or chicks wind up pushed from the nest. This is threatening some species, particularly woodland warblers, which the cowbirds couldn’t reach in the past.

A few years ago, Patrice Gillespie of north Wilton described what can happen after a cowbird plants her egg in another’s nest.

A large juvenile cowbird was “twittering and fluttering its wings with feigned helplessness as a tiny Chipping Sparrow dutifully brought it food,” Patrice wrote. “Apparently the chipper had cared for the gawky cowbird since it hatched from a strange egg surreptitiously laid in the small sparrow’s nest.  The rather homely young cowbird towered over its dapper caregiver.”

The killing off of buffalo and other wild herd animals, plus the felling of the Eastern forests, led to changing habits and ranges for the cowbird. Cattle and other domesticated animals substituted for the wild roaming animals, and the millions of acres of Eastern fields and pastures attracted the cowbird to our area, where they have adapted to farm and suburban life.

In some places, like Michigan, cowbirds are trapped because they are such a threat to rare warblers.

Some species have caught on to the cowbird trickery, and destroy the foreign egg. If a Yellow Warbler finds a cowbird egg in its nest, it may build a new nest atop the old — with the egg still in it. Yellow Warblers have been known to build as many as two nests atop an original nest, if the first two were parasitized by cowbirds.

On the Plains, some species have learned to recognize a cowbird egg and remove it from the nest. Perhaps our eastern birds will become as egg-wary.

Times have been changing in some of the Northeast. Many farmers’ fields and pastures of the 19th Century have reverted to forest and the cowbird may be a bit less common than it was.  However, cowbirds do well in suburbia, which is open enough to meet their feeding needs and woody enough to meet their nest-hunting needs.

Coming up

Hummingbirds: Feathered Gems, Gina Nichol, former Audubon staff member, illustrates the amazing adaptations of these birds and describes their fascinating life histories, Saturday, July 17, 3:30-4:30 pm, $5, Audubon Greenwich, 613 Riversville Road, Greenwich.audubon.org.

Sharon Audubon Festival, nature walks, bird programs, music, food, displays, Saturday and Sunday, Aug.14 & 15, 9: to 5, 860-364-0520, sharon.audubon.org.

Birdwatching cruises of the Norwalk Islands, aboard the new 45-foot C.J. Toth ferry, three hours, Saturdays, July 10, 24, Aug. 7, 21; Wednesdays, July 14, 21;  Thursdays, Aug. 5 and 19, at 7:30 a.m., $22 adults, $12 kids,  Norwalk Seaport Association, Seaport Dock,  corner of Washington and Water Streets, South Norwalk, reservations, 203-838-9444.

Copyright 2010 by Jack Sanders. Send sightings or comments to: jackfsanders [at sign] yahoo.com, or to Bird Notes, Box 1019, Ridgefield, CT 06877; or call 203-438-1183, extension BIRD (2473), and leave a message with your report, spelling your first and last names and telling us your town. If you need help identifying a bird, try your local nature center. If you find an injured bird, call wildlife rehabilitator Darlene Wimbrow of Redding, 203-438-0618, Wildlife in Crisis of Weston, 203-544-9913, or Wild Wings of Greenwich, 203-637-9822. The columnist’s website is www. sandersbooks. com.