The last sentence of your first paragraph posed the rhetorical question It
also seems that eventually this could sway the balance of birds in a
negative way. The following excerpt from a larger article (
http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12202-32591--,00.html#cowbirds
) illustrates how the presence (or, successfully, the absence) of cowbirds
can impact a bird population (read, especially, the 3rd and 4th paragraphs):
[excerpt]
Brown-headed Cowbirds
The brown headed cowbird (Molothrus ater), once called the buffalo bird,
was common in the open plains. Cowbirds followed the vast herds of American
bison and then cattle, eating the insects that swarmed around the hoofs of
the grazing herds. Unable to move with the wandering herds while maintaining
a nest, these birds developed an unusual behavior; they began to lay their
eggs in the nests of other birds. The cowbird chicks, which hatch earlier
than most songbirds, are more aggressive and will out-compete their nest
mates for food. This added competition reduces the number of non cowbird
young that fledge.
As land in Michigan was opened up during logging and agricultural
development, cowbirds moved into the new areas, and the Kirtland's warbler
was an extremely vulnerable host. The egg laying activity of the cowbirds
began to impact the Kirtland's warbler population.
Studies have revealed that when one cowbird egg is laid in a warbler nest,
only one to three warbler chicks may survive. If two cowbird eggs are laid
and hatched in a warbler's nest, none of the warbler chicks survive. Heavy
cowbird parasitism is believed to have been a major factor in the decline of
the Kirtland's warbler population. In 1972, the U.S. Fish and Wildlife
Service, in cooperation with the USDA Forest Service, Michigan Department of
Natural Resources, and the Michigan Audubon Society, began controlling
cowbirds with large live traps that are placed in Kirtland's nesting areas
during spring and early summer. The traps, which are baited with millet,
water, and several live cowbirds, are checked daily and any trapped cowbirds
are euthanized. Non target species are released unharmed. Since 1972, an
average of 4,000 cowbirds per year have been removed from Kirtland's warbler
breeding areas.
Kirtland's warbler reproductive success has improved dramatically since
cowbird trapping began. The nest parasitism rate has declined from the 1966
71 average of 69% to less than 5%. Average clutch size has increased from
2.3 eggs per nest to more than four. The average number of young warblers
fledged per nest increased from less than one to almost three birds during
the same period. The 2002 annual census counted over 1000 singing males for
the second year in a row.
From: Farrel Graves bgra...@usfamily.net
To: mou mou-...@cbs.umn.edu
Subject: [mou] Bird Behavior-Sad
Date: Tue, 11 Jul 2006 08:04:56 -0500
Like most things in life hearing or even reading about a problem doesn't
drive home a point like seeing it first hand. From reading I knew cow birds
lay eggs in other birds nests. One day I spotted a chipping sparrow (several
are at our feeders) hopping through the grass with this really big bird
hopping right behind it. The bigger bird almost appeared big enough to harm
the sparrow if it landed on it. And it stayed on the sparrows heels. Then I
realized the sparrow was searching for food to feed the big bird. It was a
cow bird. I witnessed it begging for food for a couple of weeks. It looked
so odd. The sad part is the chipping sparrow had none of it's own with it.
After reading more I realized the bigger bird out muscled the sparrow babies
and likely rooted them out of the nest. It's really a shame that some birds
can't realize the difference. It also seems (since the cow birds are not
only big but also hatch earlier than the birds in the host nest) that
eventually this could sway the balance of birds in a negative way.
I have a picture if anyone is interested. It looks like the sparrow is
putting it's head into the mouth of a lion...
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