Brown Eggs Replacing Finch
Name: Jean C.
My name is Jean Case and I teach second grade in a small
town in South Dakota. We have a nest in a hanging flower pot outside of
our classroom. It is a purple finch nest with small white eggs. A few
days ago, we noticed that there was a new egg in the nest, but this egg
is larger and has brown specs on it. Then, the next day we noticed that
one of the finch eggs had disappeared. Today, we noticed another of the
large brown speckled eggs. Can you please tell us what kind of bird this
could be and why it layed eggs in the finch nest.
Sincerely, Jean and second graders
This is the description of brown-headed cowbird eggs from the Birder's
Handbook: EGGS: White to grayish-white, marked with browns. 0.8" (21mm).
(Copyright 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.).
Does it match? Cowbirds are nest parasites that lay eggs in the nests of
other birds, sometimes ejecting a host egg at the same time. Apparantly they
evolved this behavior in centuries of following buffalo herds on the
prairies, which meant the cowbirds, like the buffalo, were constantly on the
move, making conventional nesting difficult.
This is very typical behavior of a parasitic species of bird. The
most commonly known one is the Cowbird which lays large light brown to
speckled eggs in the nests of other species. They have been known to kick
out the other eggs to make room for their own. When the Cowbird chick
hatches it is generally larger and more vocal than the chicks which belong in
the nest. Because most parenting instincts of birds depend on the
vocalization and open mouth image of the chicks, the Cowbird chick gets most
of the food. When the Cowbird chick is strong enough, it will kick out any
competitors. You can remove the cowbird eggs yourself so the remaining
hatchlings will be care for properly. Don't worry about leaving your scent;
it is an old wives' story. However, if the nest is disturbed too often the
parents will abandon it. I have successfully observed hatchlings and parent
care when I've limited nest visits to ten day intervals.
Your unfortunate speckled finches are being parisitized by a cowbird.
Cowbirds lay eggs in the nests of other birds because the other birds will
do all the work of raising the cowbird chicks for them. To make matters
worse for the host birds, the cowbird parents will, as you've seen, destroy
the host birds's eggs, and so will the cowbird chick once it hatches.
So, you and your class may get the "privelege" of watching cowbirds
reproduce at the expense of the purple finches. Nature is not always pretty.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012