Cowbirds, nature's ultimate parasite

In the birding world, it’s commonly known as parasitizing.

Merriam-Webster defines it: ‘to infest or live on or with as a parasite.’

Ticks and mosquitoes come to mind, but do you think of a bird as a parasite?

The brown-headed cowbird is actually known as a brood-parasite, meaning that the female cowbird lays its eggs in the nests of other species.

No nest building whatsoever for the brown-headed cowbird. This strategy was very effective when this bird was limited to the short-grass plains and followed herds of buffalo eating the insects they would stir up by their movements.

“Brown-headed cowbirds were originally a bird of the grasslands and Great Plains closely tied to roaming bison herds,” said Liz Bradley, Wildlife Biologist for Montana Fish, Wildlife and Parks in Region 2. “But human disturbance has since created more conditions where cowbirds can live, and with their unique breeding strategy, they can sometimes outcompete other local songbirds. There are 53 species of birds in Montana that have been known to be parasitized by brown-headed cowbirds and most birds don't realize they're being fooled and will actively feed the cowbird chicks as if they were their own.” 

Also known as the cow-pen bird or buffalo bird, the female and juveniles resemble sparrows, but are larger, completely dull gray-brown, and have un-streaked backs.

“Cowbird chicks can even sometimes grow larger than the host itself which creates an odd image of a small bird feeding a larger one,” Bradley said.

Birds are often celebrated as exceptional parents, incubating their eggs for weeks in all weather conditions while diligently traveling back and forth from the nest countless times bringing food to their offspring.

But the brown-headed cowbird shuns this responsibility and the female expends her energy producing up to 36 eggs each summer, according to Cornell Lab of Ornithology.

Once she finds a nest, she will remove one or more of the existing eggs by pushing it out, and then lays her egg, or eggs, in the nest.

The cowbird is not particular in the nest she selects as they have been known to parasitize more than 140 species of birds, with chipping sparrows, red-winged black birds and yellow warblers the common hosts in this area.

Most birds simply accept the imposter eggs and raise them as their own even though the incubation period is shorter for cowbirds.

Hatching earlier and being bigger means cowbirds often thrive at the

expense of the host’s own babies. Because of this, their parasitism can threaten the livelihood of some birds, especially those with smaller populations.

This behavior has made them one of the most reviled birds in North America to the point some good samaritans have removed their eggs from their hosts nests, which is ill advised for a few reasons.

First off, they are protected under the Migratory Bird Treaty Act making it illegal to tamper with their eggs without a permit.

Secondly, most birds do not recognize the cowbird eggs are different from their own, but they monitor the mass and could abandon the nest entirely if they suspect ‘fowl’ play.

This nesting behavior may seem cruel or clever, but it is the only way they have to reproduce as they cannot build nests of their own and rely on other species to incubate and raise their offspring.