Ask SIB: Brown-headed Cowbirds – Brood Parasite

In a recent email, Joleen Ardaiola said:

At my feeders there’s a momma pine warbler feeding a baby cowbird. The cowbird is huge compared to the pine warbler.

Jackie Brooks replied:

You would think that if birds can recognize infertile eggs and remove them, don’t mate with just any species, that they would recognize #1 cowbird egg and remove it, and #2 realize that this baby is not their species and stop feeding it.

This brings up two questions. First, why do Brown-headed Cowbirds lay their eggs in the nest of other birds? Second, why don’t those other birds recognize the imposter and continue to feed a baby bird that may be twice the size of the parent like in Joleen’s observation?

Historically, the Brown-headed Cowbird lived primarily in the Great Plains where they followed the bison herds. The bison would shake loose the seeds and insects the cowbird eats. Since these bovine herds moved constantly, a cowbird sitting on a nest would lose its access to the abundant food source. The solution-drop their eggs in another bird’s nest and move with the herd. The cowbirds evolved this lifestyle over many millenniums. They prefer meadow forest edge habitat, something uncommon in the ancient history of our country.

All was well and good and balanced until Europeans entered this country, cutting down forest, creating grassy openings, and introducing stationary bovine-the cow. Humans changed the environment to something that the Brown-headed Cowbird loved. It did not take long in evolutionary terms for the cowbird to adapt. The Brown-headed Cowbird arrived in Georgia in 1957! It has spread all across the country and on into Mexico.

Why do birds raise the cowbird young? For starters, one can see from the numbers above, many birds do not accept cowbird eggs or young. Birds like the American Robin or Gray Catbird spot the odd egg and may chuck it out of the nest. Other birds will build a second nest right on top of the first one burying the intruder’s eggs and all their own at an ecological cost. Birds like Yellow Warbler (yes, it is still one of the cowbird favorite hosts), Eastern Kingbird, Blue Jay, Brown Thrasher, Cedar Waxwing, and Baltimore Oriole will not accept a cowbird egg or young more than 88% of the time. These are the rejectors. A potential victim can keep from hosting a cowbird by four methods. First, avoid the cowbird, keeping a close watch on the nest to prevent the cowbird from entering. Second, recognize the egg as something different and destroy it. Third, abandon the nest completely including its own young. Fourth, recognize and refuse to feed the chick, though this is very rare.

Other birds, the acceptors, will readily raise cowbird young. As seen in the photograph or Joleen’s comment, often the cowbird baby is huge compared to the parent. Evolution can be considered a war similar to the war we face with spammers. A bird over time may recognizes the intruder and rejects that intruder. The cowbird will devise a way to extract an evolutionary toll on the bird that rejects the egg. This could be adding new eggs or destroying a nest where its egg has been rejected. Since the cowbirds arrived very recently in our part of the country, many birds have not evolved enough to notice the difference.

Birds operate on instinct. One of the reasons you enjoy eggs in the morning is a chicken looks at her nest and sees that it is empty and lays eggs. Once the prescribed number of eggs fill the nest, she stops laying. Before that happens, the farmer removes the eggs, and the chicken brain says, lay more eggs.

An accepter species may lay some eggs before the cowbird sneaks in and removes one or more eggs and lays her own egg (a 20-40 second operation). The acceptor sees the requisite number of eggs and starts brooding. The cowbird egg will hatch in about 10 days, faster than most species. This gives the cowbird chick a leg up on getting fed. It may even push the remaining eggs out of the nest. A mother bird sees an open mouth and a begging baby, and its instincts say stuff food down this hole. The baby cowbird provides a huge hole and loud noise the acceptor cannot resist for the 25 to 39 days the cowbird baby demands care. Then one day, the young cowbird’s brain clicks. It leaves the foster parent and instantly recognizing other cowbirds, joins the flock. The host sees there is no longer a mouth to feed, and MAY start another nest, one hopefully a cowbird will not find.

The female cowbird lays on average 41 eggs per year, a huge number. This all sounds like the life of Reily for the cowbird and says we should be overrun by cowbirds. Not so, records show that only about 3% of the babies survive to adulthood.

Another question people often ask is, “Should I intervene?” That is your preference. If you remove the cowbird egg, the female cowbird may return and either lay another egg or trash the nest or the host will be able to raise its full brood. From our perspective, brood parasitism sounds cruel, but nature has factored in predation and parasitism into the survival of species.

Sources:

Phillipsen, Ivan. “Brood Parasites are Devious Birds” The Science of Birds podcast

Lowther, P. E. (2020). Brown-headed Cowbird (Molothrus ater), version 1.0. In Birds of the World (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.bnhcow.01

Submitted by Bob Mercer. Photos by Bob Mercer.