The bird we love to hate: Cowbirds make others do their parenting for them

A chuckling gurgle and a squeaky whistle announce the presence of one of nature's oddest and most maligned birds, the brown-headed cowbird.

They're a native species but have an alien-like habit: They perch on treetops looking for signs that other species are building nests, planning to take advantage of all that activity.

Cowbirds never build nests or care for their own chicks. Instead, they outsource the job, with a female cowbird laying her eggs, one by one through the nesting season, in the nests of other birds, compelling others to raise her offspring. A female cowbird may produce up to 40 eggs in a season, using nests of 220 or so other species. This practice is called brood parasitism and is unique on our continent to cowbirds.

You may have seen the outcome of one of these odd matchups: A tiny chipping sparrow might be spotted hopping over the ground, gathering up seeds and insects to stuff down the throat of a much larger, noisy young cowbird. Many other species are vulnerable, too, from Northern cardinals to Eastern phoebes to American goldfinches

This system works well for cowbirds, but is stressful for parent birds that must raise an interloper. A cowbird egg often hatches a bit sooner than others in the nest, so the cowbird chick gets fed earlier and grows faster. In some cases, the natural chicks don't thrive and only the cowbird chick survives to fledge from the nest. In many other cases, all chicks fledge, but the parents work nearly to exhaustion to satisfy the large interloper.

We humans like to revile cowbirds for this practice, but humans have been the cause of their brood parasitism spreading widely in the bird community. Cowbirds traditionally were prairie birds, following bison herds for the seeds and insects they kicked up, only parasitizing other Plains birds. But humans moved in and cut forests, essentially creating more plains, allowing cowbirds to move into more areas to parasitize more species.

Some birds have developed ways to foil cowbirds. For example, yellow warblers will start over, building a new nest on top of their first one, if they find a cowbird egg in it. Other birds toss out an egg if it doesn't look like their own, but this often has a cost: Female cowbirds keep an eye on their surrogate nests, so if a catbird, say, dumps out a cowbird egg, the female cowbird may swoop in and break or remove all the eggs, thereby compelling the catbird to start a new brood. And that new nest will likely include a cowbird egg.

However, most birds don't seem to recognize a foreign egg, so end up raising its chick as their own.

Why do cowbirds do this? The standard explanation is that the behavior developed as a response to their nomadic lifestyle on the Plains, which left no time to remain in one place for weeks at a stretch. Ornithologists aren't quite sure if this theory covers all the facts, but it's about all we have at this point.

How do birds raised entirely by foster parents know their true selves, i.e., how does a cowbird know it's a cowbird and not its warbler or sparrow foster parent? Soon after they become independent of their foster parents, young cowbirds begin to gather where they hear adult cowbirds making a "chatter" call. The young birds then learn from the adult birds how to be cowbirds.

Cowbirds don't build nests, they don't incubate eggs and they don't have to feed a nestful of begging chicks. But they don't get off scot-free, either, since females spend the summer making the rounds, checking on each nest holding her eggs. And males perch on treetops, making calls to gather fledglings in open spaces. It goes to show that evolution can be a very strange business sometimes.

St. Paul resident Val Cunningham, who writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.