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
