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Why You Should Never Remove Cowbird Eggs From Host Nests

Begging cowbird chick

It’s that time of year again when the general public gets a close look at the hidden lives of birds. While some species’ nests are inaccessible to humans, almost everyone has story about finding a nest on their property, finding fallen chicks, or witnessing a parent bird feeding fledglings. Birds are all around us, and during this special season we get a chance to spy on their incredible family lives. Often, this can lead inexperienced birders down the route of trying to identify their new neighbors based on the shapes of their nests or the appearance of their eggs. In this process, it is more common than one would expect for a nest to be found with one or more different looking eggs inside. Brown and speckled, and usually larger than the others, the cowbird’s eggs are common finds. Upon learning about the cowbird, however, many people feel inclined to remove these mismatched eggs. This is not a good idea.

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What is a Brood Parasite?

Cowbirds are brood parasites. This means that their reproductive strategy is to lay their eggs in the nests of other birds, leaving the foster parents to either recognize the unfamiliar egg and reject it or to raise the cowbird as their own, usually to the detriment of their actual chicks. The young cowbird chick is often much larger than its foster siblings who usually do not survive. As this hatchling grows, it will often outsize its foster parents as well, leaving them scrambling to feed such a large and needy baby. While this may seem unfair, it is the only way of reproducing that the cowbird knows. These birds do not engage in nest building and without this parasitic behavior, they would be unable to breed.

Changeling Trouble

Upon reading about the havoc that cowbird eggs can wreak for the unsuspecting foster parents, many well-meaning nature-lovers are disgusted by this seemingly villainous bird. The next step for such people seems obvious: destroy the interloper. This is a very bad idea for a number of reasons.

Firstly, it is illegal. In the United States, at least, native birds and their nests and eggs are protected by the Migratory Bird Treaty Act. While cowbirds may be culled in regions where they threaten endangered, generally these birds are a normal native member of their ecosystem’s wildlife. They have as much right to live and breed as the foster family species do. It just so happens that their reproductive strategy is unsavory when viewed through the lens of the human moral compass.

Interfering with a cowbird egg may also guarantee the failure of the nest from which it is removed. This can happen for a few reasons. For one thing, female cowbirds usually remove or destroy one egg within the nest so that the parent birds do not find an unexpected number. Seeing the wrong number of eggs or sensing that a nest has been tampered with may cause the host parents to abandon the nest altogether, dooming their remaining eggs as well as the culled cowbird egg. While a cowbird does not guarantee that the host parents’ own eggs will die, an abandoned nest does.

Brown headed cowbird
Photo by Joshua J. Cotten on Unsplash

Shockingly, the nest may also be destroyed by the mother cowbird, who, despite foisting her eggs upon another bird, often checks up on their progress. If she finds that her egg has been rejected from the nest, she may destroy the nest altogether. Research suggests that she may do this to force the host parents to start over so that she can try again. Alternatively, this may be a strategy to prevent savvy parents from breeding and producing a whole generation of birds which can sense interlopers, thus destroying the breeding strategy of the cowbird.

This brings us to my last point. In order to grow stronger, birds must adapt. If birds which regularly fail to recognize cowbird eggs are able to breed normally with no consequences, then they will only pass on their genes and influence their species’ evolution away from becoming more and more able to handle brood parasites. What if you remove a cowbird egg now and then in a few years every single nest is vulnerable to cowbird parasitism because of that one nest’s survival? While it may feel good to “protect” nesting birds, the best way to enjoy nature is with respect and humility. I promise, she knows what she’s doing.

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