Bird watchers often wonder about the identity of odd eggs in bird nests. In a nest with three pale blue eggs with a few spots, there may be one larger tan egg covered with spots. In the US, these odd eggs are usually deposited by brown-headed cowbirds, Molothrus ater, and pose risks for both the host's and the cowbird's eggs.
Problems of Being a Parasite
Brown-headed cowbirds are nest parasites, having lost the ability to construct nests or feed their own offspring and rely totally on other species to raise their offspring. By laying eggs in another species' nest, the mother cowbird consigns her baby to compete with the host species' babies for food and space and to eat whatever the host parents bring for their own offspring. Some problems for the cowbirds are:
- Cowbirds have to find nests while they are being constructed so they can lay their eggs before incubation begins. A female usually lays her egg after two or more host eggs have been laid.
- The female cowbird often removes one host egg from the nest, replacing it with one of her eggs the same or next day. Still, some birds recognize their own eggs and remove or destroy a cowbird's egg, cover the entire nest over and start again, or abandon the nest to build a new one.
- The cowbird baby has to obtain enough food to survive. It does so by starting life just a little larger than the host's babies – usually hatching a day earlier than the host babies, being aggressive – often climbing on top of or pushing the host's babies out of the nest, and begging loudly for food.
- Cowbird babies need food rich in protein. Cowbirds usually do poorly in cardinal, Cardinalis cardinalis, mourning dove Zenaida macroura), and house finch (Carpodacus mexicanus) nests because the seed diets they provide have low levels of protein.
Problems of Hosting a Parasite
Parent birds confronted with cowbird eggs and chicks are caught on the horns of a dilemma:
- By removing or destroying the cowbird egg, they might damage one or more of their own eggs.
A bird with a large bill, such as a woodpecker, jay, or grackle simply picks up the egg and flies away with it. A warbler, finch, or sparrow with a small bill can neither handle such a large package nor puncture a parasite's egg without its bill slipping off the surface and possibly cracking one of its own eggs. Thus, some bird species that reject cowbird eggs simply start over from scratch and build a new nest. The yellow warbler (Dendroica petechia) covers its entire nest, including the eggs, with a new one - attempting to keep a preferred nest site without having to raise a cowbird chick. Some pairs construct four or five nests before obtaining a cowbird-free nest.
- By feeding a cowbird chick, host chicks often languish, starve, or even are killed by their foster sibling.
Cowbird chicks hatch out a day or two before the host chicks. By being fed a day or more before the host chicks hatch, the baby cowbird has grown considerably before its nestmates appear. This head start ensures the cowbird can squawk louder and present a wider-open mouth than its foster siblings each time the parents arrive with food. One “flaw” in bird behavior is that the louder a chick squawks, the more food it gets. It makes little sense to feed a sickly chick (the chick might not survive), and squawking is a signal to the parents that the bird is both hungry and in good health. Sometimes all host nestlings starve to death while the cowbird chick receives most of the food.
- When young cowbird chicks come in contact with other objects in the nest, they attempt to push them up over the edge of the nest cup. This behavior reduces competition from host chicks and ensures more food for the cowbird chick.
What Hope for the Parents?
All is not lost if a cowbird dumps her egg in a bird's nest. Many species normally have large clutches (sets of eggs) and can feed up to a dozen offspring. In this case, a few of the host babies often survive.
In some instances, having a cowbird chick reduces the incidence of nest predation and total nest failure is unlikely. If a few babies survive, that is better than losing the whole clutch. This is often the case with our Eastern and mountain bluebirds, Sialia sialis and S. corrucoides.
Nest predators tend to take the largest, most active chicks (cowbirds) from the nest first. If a nest predator is interrupted while feeding, as is often the case, host chicks would remain.
Is Dependency on Parasitism Advantageous?
Cowbirds originally inhabited the prairies of the west and Midwest. Opening the forests of the east increased available habitat and exposed many new species to cowbird parasitism. Female cowbirds lay about 40 eggs a year for two years. Presently, in most areas, an average of about 2.4 of a cowbird's 80 eggs survive and many populations are doubling in size in about eight years.
The species most seriously affected by cowbird activity are those that only recently came in contact with cowbirds because of human activity. These species have few or no cowbird defenses and their numbers have recently plummeted. Some will be driven to extinction, others will survive.
Being dependent on other species for reproduction is not necessarily an evolutionarily stable strategy. Those bird species driven to extinction by cowbird and human activities will no longer provide nesting opportunities for cowbirds. As many birds have already done, some surviving species will eventually evolve anti-cowbird behaviors that reduce the effectiveness of cowbird reproduction. Although it will not happen soon, the tide will eventually turn, and cowbird numbers will drop again.
The photo of the cowbird egg in a house finch nest was used by permission of the freelance photographer, Suzanne Guida.
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