This raises the question of how a cowbird knows its deposited egg has been
removed. I thought Cowbirds developed this parasitic behavior due to never
staying in one place on enough to nest (following the cattle). Perhaps if the
urban cowbirds don't have cattle to follow around, they have time to see if
their egg deposits are still safe.
And while we're discussing Cowbirds...I'm still puzzled by the anger some
birders have for them. It seems to me this behaviour is what nature developed.
Any similarity to dead-beat dads should not be held against the birds. Are we
anthropomorphising too much here? We don't consider a lion cruel for taking
down a gazelle for lunch, do we?
John Broman
Lovettsville, VA
On 13/Jun/2007 23:44 Sandy Hevener wrote ..
And the other side of the cowbird egg story:
(From March 10, 2007 Science News)
*
Mafia Cowbirds: do they muscle birds that don't play ball? * By S. Milius
Cowbirds in Illinois that sneak their eggs into other birds' nests
retaliate violently if their scam gets foiled, researchers say.
The brown-headed cowbirds of North America outsource nest building and
chick raising. female cowbirds dart into other birds' nests, quickly lay
eggs, and rush away. The nest owners are left to care for big demanding
cowbird chicks.
Why don't the dupes throw out the odd eggs? When scientists removed
cowbird eggs from warbler nests, more warbler eggs later got smashed or
carried off than did eggs in nests with cowbird eggs in place. It was
cowbird retaliation, conclude Jeffrey P. Hoover of the Illinois Natural
History Survey in Champaign and Scott K. Robinson of the Florida Museum
of Natural History in Gainesville.
That's the first evidence of gangsterlike behavior in cowbirds, say Hoover.
A decade of monitoring prothonotary warblers in nest boxes in
souther-Illinois swamps gave Hoover the idea for the new experiment. The
nest boxes sit on poles coated with axle grease to thwart raccoons,
snakes, and most other raiders. Egg-laying cowbirds still strike, and
Hoover had for years left the cowbird eggs alone. In 2002, he and other
researchers removed cowbird eggs. Nest vandalism suddenly increased.
No one saw the vandals, but Hoover and Robinson turned to an idea put
forward in 1979 by Israeli biologist Amotz Zahavi. He'd suggested that
by tending the weird-looking eggs and chicks, the foster parents protect
their own progeny. In a rare test of the idea, cuckoos retaliated
against magpies in Spain that rejected cuckoo eggs, scientists reported
in 1995.
In the new experiment, the researchers recorded egg damage in only 6
percent of the warbler nests where cowbird eggs remained unmolested. In
contrast, 56 percent of nests were vandalized after the researchers
removed the cowbird eggs. When the scientists removed the cowbird eggs
but added new fronts to the nestt boxes with holes too small for
cowbirds, there was no damage
So, the nest thrashes are cowbirds, Hoover and Robinson conclude in a
paper now online for an upcoming Proceedings of the National Academy of
Sciences.
When the cowbird eggs stayed in the next, some warbler chicks starved
because the pushy cowbird nestlings took so much of the food. Yet with
the retaliation attacks, the nests where cowbird eggs had been removed
produced, on average, only 40 percent as many warblers as the
cowbird-fostering nest did, say Hoover.
"This is a surprising result," says Stephen Rothstein of the University
of California, Santa Barbara.
Rothstein hasn't tested whether cowbirds retaliate, but he says, "My
bet, before this paper, would have been defnitely no." He's now
reconsidering but says, "I'd like to see more direct evidence to date,
"
such as video.
So would Naomi Langmore of the Australian National University in
Canberra. Still, she describes the evidence as compelling."
"Best evidence of date, " Says Rebecca Kilner of the University of
Cambridge in England.
Comment: Hummmmm. Wonder if we should throw cowbird eggs from the nest?
Posted by:
Sandy Hevener
General Delivery
Blue Grass, VA 24413
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