Many biologists have a love/hate relationship with brown-headed cowbirds.
We love their ecological and behavioral ingenuity, but we hate the impact they have on a variety of native nesting songbirds.
Cowbirds are stealthy nesters — they lay their eggs in other birds’ nests. Biologists call such species brood parasites.
It’s why I often get questions about small adult birds feeding much larger chicks.
Understandably, readers want an explanation. The mysterious larger chicks are undoubtedly young brown-headed cowbirds.
The smaller adult feeds the young cowbird because she’s been fooled 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.
Cowbirds dupe a variety of small songbirds such as warblers, vireos and native sparrows by laying their eggs in other birds’ nests.
That frees cowbirds from building a nest of their own. The “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 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 potential host nests.
Warblers, vireos, sparrows and buntings seem to get more than their fair share of cowbird eggs.
Some birds don’t tolerate unfamiliar eggs. Catbirds and robins, for example, recognize cowbird eggs and remove them from their nests.
Phoebes abandon parasitized nests, and yellow warblers sometimes build a new nest on top of nests containing cowbird eggs.
Early each morning, female cowbirds lay one egg in a nest it has found. Before laying the egg, she sometimes removes one of the host eggs.
Females lay one egg per day for five or six days. After a few days of rest, they repeat the process.
Over the course of a nesting season, a single hen cowbird can lay as many as 40 eggs.
Sometimes a host nest contains more cowbird eggs than host eggs. After the eggs are laid, the cowbird has the advantage.
Cowbird eggs hatch a day or so faster than host eggs, and cowbird chicks grow faster than host chicks.
Because of this head start, cowbird chicks get bigger faster 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. The parasitic reproductive lifestyle is brilliant.
Cowbirds invest no time or energy nesting, but each female can still add several dozen young to the population.
For all these reasons, I can’t help admiring cowbirds.
On the other hand, I loathe cowbirds because they have such a destructive effect on many songbirds’ nesting efforts.
Like all native birds, cowbirds are protected by state and federal law, so killing them or removing their eggs from other species’ nests is not an option without special permits required to control cowbirds.
And that’s exactly how governmental biologists protect endangered songbirds such as Kirtland’s warblers in Michigan and black-capped vireos in Texas.
When cowbirds are removed from these species’ nesting habitats, their nesting success soars.
Love ’em or hate ’em, it’s easy to admire the cowbird’s ingeniously efficient reproductive strategy.
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