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Re-Wilding Route 1: Cowbirds, con artists of the bird world

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Posted on: June 6, 2024


European folklore is liberally sprinkled with tales of babies stolen from their crib by fairies and replaced with either a sickly fairy child or just a doll made from leaves and berries. Seldom do these stories end well for either the human parents or the changeling child.

In backyards across most of America, an avian changeling story also plays out every nesting season. Small songbirds – mostly warblers, but also thrushes, sparrows, finches and similar-sized species – come back to their nests to find a new egg they didn’t lay nestled in with their clutch. This new egg is usually bigger and often a different color, but even if the warbler parents notice anything is amiss, their incubation instinct still kicks in, and they typically care for the odd egg as their own.

Like the changeling fairy tales, this does not end well for these parents or their initial offspring.

A large chick hatches from this egg, often earlier than the original eggs. And that young chick is a badly behaving bird bully, outcompeting its smaller nestmates and gobbling up all the food when the parent birds arrive with dinner.

This masquerade can even take a deadly turn, with the larger chick smothering or kicking the smaller nestlings out when the parent birds aren’t looking. Even when it’s the only chick left in the nest, it keeps the parents running ragged, feeding a youngster that ends up twice or more their size before it leaves the nest.

Where did this changeling egg come from??

The brown-headed cowbird (Molothrus after).

Brown-headed cowbirds, despite their unorthodox habit of farming out child care, can be attractive and cheery birds in the urban landscape. During the winter, the brown-headed males, with their glossy black bodies, and drab grayish-brown females are regular visitors to backyard bird feeders, happily eating corn, millet and milo alongside their regular sparrow, junco and finch compatriots. They also throng together with other ‘blackbirds’ – starlings, red-winged blackbirds, grackles and others – in huge mixed flocks that glean corn and other grains in agricultural fields and livestock pens. Male cowbirds serenade their mates with liquid, tinkling notes.

Wood thrush nest (blue eggs) parasitized by a brown-headed cowbird [photo credit Brian Henderson under CC BY-NC 2.0]
While they are a common, even abundant species for us now, brown-headed cowbirds are a relatively recent addition to the Maryland avian fauna. Another colloquial name for the cowbird is bison bird – a nod to the fact that before European colonization, cowbirds followed roving bison herds across the great plains, feasting on grasshoppers and other insects stirred up by the constantly moving mammals, along with flies and ticks they found on bison necks and backs.

Because the herds were always on the move, female cowbirds found it impossible to build a nest, lay their eggs and raise young while still keeping up with the bison. So, they took a page out of the playbook of their tropical cowbird cousins, all of whom commandeer the parental efforts of other birds. Brown-headed cowbirds laid their eggs in nests of other birds in thickets and tree margins bordering rivers and lakes along the herdÕs route. Scientists call this behavior ‘brood parasitism,’ and it is found in other bird species as well, including common cuckoos in Europe.

Our English word cuckold, used to refer to a man whose wife has cheated on him, derives from the old word for cuckoo. The genus name for the brown-headed cowbird, Molothrus, pulls no punches either, pairing the Greek molos- (battle or fight) with -throskos (to sire or impregnate).

When colonists started cutting down the eastern forests and replacing them with agricultural fields, adding cows and horses to the mix, cowbirds moved eastward to take advantage of these newfound herds and fresh habitat. Here they discovered a whole new community of forest birds to exploit as nannies for cowbird youngsters.

Female cowbirds are consummate spies, watching the comings and goings of other bird species as they build their own nests. The female cowbird waits until there’s an egg or two already in the nest before zipping in when the nest owner is momentarily away. While some songbirds take an hour or more to lay an egg, cowbirds lay theirs in about 10 seconds, and then they’re gone. Sometimes the cowbird kicks out the existing eggs for good measure. They may even destroy nests with young birds in them to encourage the songbird parents to rebuild a new nest with new eggs.

A few of the foster parents have learned ways to fight back. Sometimes they kick the odd egg out of the nest. Sometimes they build another nest on top of the parasitized one. Sometimes they abandon the nest altogether, starting again in what they hope will be a more secure location.

Despite never having seen another cowbird in their lives, young cowbirds instinctively find other teenaged cowbirds to hang with as soon as their adoptive parents stop the gravy train. Brood parasitism behavior is ingrained, not learned, so these teenagers are able to take up parasitic behaviors as soon as they are of mating age.

Songbirds dwelling deep in forests have long been immune to cowbird parasitism- but no longer. As we humans crisscross dense forest with open roads, we’re inviting cowbirds to occupy even these formerly safe songbird strongholds.

Luckily, however, most songbird species are pretty resilient. In some cases with rare or endangered species – such as Kirtland’s Warbler in Wisconsin – efforts were made to control the cowbird population until the warblers recovered sufficiently to no longer need the protection. Over time, our Maryland songbirds are likely to reach evolutionary detente with cowbirds just as their Great Plains counterparts have.

Have questions for Rick about the world of nature in and around the Maryland suburbs, or suggestions for future columns? Drop him a note at rb*******@gm***.com.



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