Birds as Parasites

In the spring and early summer months, it is enjoyable to watch birds nesting and raising their young.  If we’re lucky enough, we get to watch the juveniles grow from eggs into copies of their parents; however, some witness the occasional oddity: a bird feeding a chick that is clearly not its own.  This is a result of nest or brood parasitism.

Male Brown-headed Cowbird (photo: Wikipedia)

 Brood parasitism occurs when a parasitic species, such as the Brown-headed Cowbird (Molothrus ater), lays its eggs in another species’ nest.  In this host-parasite relationship, the parasite receives all the benefit, and the host incurs all the costs.  Since the parasite doesn’t have to worry about building and defending its own nest, it can put all of its energy toward producing eggs.  The female of the Brown-headed Cowbird, which is considered North America’s most common brood parasite, can lay up to three dozen eggs in a single summer, none of which are raised by the Cowbirds themselves.

For the host, the cost of being parasitized can be great.  The host’s own nestlings may grow more slowly due to a more competitive parasite nestling, or the host may lose its entire clutch due to nest abandonment or the destruction of the host’s eggs or hatchlings by the parasite.

Nest with a Brown-headed Cowbird egg (photo: Wikipedia)

 One has to wonder how the host parents could not notice the presence of a parasitic egg or hatchling, but some do.  Some host species are able to recognize eggs that are not their own and destroy them, but for most the distinction is difficult to make, since some nest parasites’ eggs mimic those of the host species, or the parasite’s egg is too difficult to destroy due to its size or harder shell.

If the parasite’s egg is not destroyed, it usually hatches before the host’s own eggs.  Shorter incubation times insure that the parasite grows more quickly to gain advantage over the other nestlings.  Even at this stage, it is possible for some species to recognize a chick that is not their own and discriminate against it; however, the vocalizations of the parasitic nestling can actually stimulate higher rates of feeding from the host parents, enabling it to out-compete the host nestlings.

Brood parasites can be dangerous to bird species that are already in decline. It is for this reason that entrance size of nest boxes is so important.  For cavity-nesting species like Eastern Bluebirds, keeping the opening to the box at a specific size decreases the odds that a parasite like the Brown-headed Cowbird will be able to fit into the box and lay its egg in the nest.

To download nest box plans that suit the need of various species Click Here.  If you’re not handy or don’t have enough time to make your own nest box; visit the World Bird Sanctuary’s Wildlife Hospital to purchase an  inexpensive nest box donated by some of our talented supporters.

Submitted by JoHanna Burton, World Bird Sanctuary Naturalist