When I walked to my office this morning, I noticed a female brown-headed cowbird perched high atop the tallest tree in the yard. For 10 minutes she surveyed the surrounding area.
With binoculars, I could tell she was scanning in every direction. She was looking for her next victim.
Cowbirds, members of the blackbird family, are brood parasites; they lay their eggs in the nests of other birds.
It’s an ingenious reproductive strategy. Why bother building a nest, incubating eggs and raising young when you can get other birds to do it for you?
Every year I get reports of cowbird behavior from puzzled readers. A female song sparrow, for example, feeds a young begging bird.
Yet the observer is certain the young bird is not a song sparrow. It’s gray with light streaks on its breast and is much bigger than the parent.
What is it, and why was the song sparrow feeding it?
Such mystery birds are undoubtedly young brown-headed cowbirds. The song sparrow feeds the young cowbird because she’s raised it in her own nest. Brown-headed cowbirds are common on farmland, forest edges and at feeders.
Males have shiny black bodies with distinctly brown heads. Females are uniformly grayish brown.
The song sparrow had raised the cowbird because she incubated its egg. Cowbirds do not build a nest of their own. Instead, they lay their eggs in the nests of other birds.
These ‘host’ birds incubate the cowbird eggs and raise the chicks –hence the term brood parasite.
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. Warblers, vireos, sparrows, and buntings seem to get more than their fare share of cowbird eggs.
Some birds don’t tolerate 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 on top of 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 35 to 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. It can suggest a bully demanding candy from a child.
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 state and federal agencies protect endangered species 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 deceptive reproductive strategy.