SCOTT SHALAWAY Brown-headed cowbirds are stealthy nesters

Last year chipping sparrows returned to my yard on April first. On the 17th I found a nest with five eggs. Robins and song sparrows also sometimes lay eggs in April. So it should come as no surprise that brown-headed cowbirds return in time for the earliest nests. They are stealth nesters -- they lay their eggs in other birds' nests.
I often get reports of cowbird activity from puzzled watchers. A female cardinal, for example, feeds a young begging bird. Yet the observer is certain the young bird is not a cardinal. It's gray with light streaks on its breast and lacks a crest. What is it, and why was the cardinal feeding it? These are the obvious questions such observations raise.
Such mystery birds are undoubtedly young brown-headed cowbirds. The cardinal feeds the young cowbird because she's been duped into raising it in her own nest.
Brown-headed cowbirds are common blackbirds found on farmland, forest edges, and at feeders. Males have shiny black bodies with distinctly brown heads. Females are uniformly grayish brown.
Nest parasites: The cardinal had raised the cowbird because she incubated its egg. Cowbirds are & quot;nest parasites. & quot; They do not build a nest of their own. Instead, they lay their eggs in the nests of other birds. These & quot;host & quot; birds incubate the cowbird eggs and raise the chicks.
Peculiar behavior: Presumably, this peculiar behavior originated on the prairies where cowbirds lived in association with the huge herds of bison that once roamed the Great Plains. The cowbirds ate insects kicked up by the bison as they grazed their way across the prairie. The bison, however, did not stay in one place long enough for the cowbirds to build their own nest and raise a brood, so the cowbirds adopted the parasitic lifestyle.
This strategy enables cowbirds to avoid most of the work associated with reproduction. Building a nest, defending a territory, incubating eggs, and feeding young are time-consuming and energy-demanding chores. Cowbirds avoid these responsibilities by parasitizing other birds' nests. Beginning in April and continuing well into summer, female cowbirds invest hours each morning watching the coming and goings of other birds to locate their nests. .
Where they lay: Warblers, vireos, sparrows, and buntings seem to get more than their fare share of cowbird eggs. Other birds won't tolerate the strange eggs. Catbirds and robins, for example, recognize cowbird eggs and remove them from their nests. Yellow warblers and phoebes abandon parasitized nests or sometimes build a new nest atop the old one.
Each morning a female cowbird lays one egg in a nest it has found. Before laying the egg, she sometimes removes one of the host eggs. A female lays one egg per day for five or six days. After a few days of rest, she repeats the process. Over the course of a nesting season a single female cowbird may lay as many as 40 eggs.
Sometimes a host nest contains more cowbird eggs than host eggs. These eggs probably come from several females.
After the eggs are laid, the cowbird has the advantage. Cowbird eggs hatch a day or so before the host eggs, and cowbird chicks grow faster than host chicks. Because of this head start, cowbird chicks are bigger than host chicks and get most of the food the adults bring to the nest. The foster parents feed the chicks that beg most vigorously, so often their own chicks starve.
Most people never notice cowbird parasitism until they see a small adult feeding a considerably larger fledgling. A young cowbird begging food from an adult warbler or song sparrow suggests a bully demanding candy from a child.
Destructive effort: Cowbirds have such a destructive effect on many songbirds that some people may be tempted to destroy them or their eggs. But cowbirds are native songbirds protected by state and federal law, so special permits are required to control cowbird populations. And that's one way biologists protect endangered song birds such as Kirtland's warblers in Michigan and black-capped vireos in Texas.
On the other hand, it's difficult not to admire the cowbird's ingenious and most efficient reproductive strategy.
UCatch Scott's radio show on the internet every Saturday afternoon from 2 to 4 at www.1360wptt.com.