For the same reason that the European Cuckoo also knows when the host bird,
notably Wrens, detect an egg of another species and removes it. How do they
know? They watch nests from a distance, according to wildlife biologists. The
Cuckoos also select some species to parasitize more than others, too. Warblers
are less likely to eject eggs, Wrens are far more likely.
IE Ries
Scott Baron <brnpelican@xxxxxxxxx> wrote:
Birders get upset about Cowbirds here in the East,
where cowbirds are relatively recent colonists from
farther West. The story goes that when forests were
cleared, these westerners expanded their range to the
Atlantic. Our eastern forest birds didn't evolve with
Cowbirds, so are less likely to remove the Cowbird egg
or abandon the nest. I've never heard any
anthropomorphizing about these birds. Rather, humans
have caused a range expansion in one species (Cowbird)
that has negatively affected several others.
John's question about why Cowbirds know that the
deposited egg has been removed is a good one. Anyone
have an answer out there?
Scott Baron
Fairfax, Va.
--- John Broman wrote:
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:play ball? * By S. Milius
(From March 10, 2007 Science News)
*
Mafia Cowbirds: do they muscle birds that don't
Cowbirds in Illinois that sneak their eggs intoother birds' nests
retaliate violently if their scam gets foiled,researchers say.
outsource nest building and
The brown-headed cowbirds of North America
chick raising. female cowbirds dart into otherbirds' nests, quickly lay
eggs, and rush away. The nest owners are left tocare for big demanding
cowbird chicks.scientists removed
Why don't the dupes throw out the odd eggs? When
cowbird eggs from warbler nests, more warbler eggslater got smashed or
carried off than did eggs in nests with cowbirdeggs in place. It was
cowbird retaliation, conclude Jeffrey P. Hoover ofthe Illinois Natural
History Survey in Champaign and Scott K. Robinsonof the Florida Museum
of Natural History in Gainesville.in cowbirds, say Hoover.
That's the first evidence of gangsterlike behavior
nest boxes in
A decade of monitoring prothonotary warblers in
souther-Illinois swamps gave Hoover the idea forthe new experiment. The
nest boxes sit on poles coated with axle grease tothwart raccoons,
snakes, and most other raiders. Egg-layingcowbirds still strike, and
Hoover had for years left the cowbird eggs alone.In 2002, he and other
researchers removed cowbird eggs. Nest vandalismsuddenly increased.
turned to an idea put
No one saw the vandals, but Hoover and Robinson
forward in 1979 by Israeli biologist Amotz Zahavi.He'd suggested that
by tending the weird-looking eggs and chicks, thefoster parents protect
their own progeny. In a rare test of the idea,cuckoos retaliated
against magpies in Spain that rejected cuckooeggs, scientists reported
in 1995.egg damage in only 6
In the new experiment, the researchers recorded
percent of the warbler nests where cowbird eggsremained unmolested. In
contrast, 56 percent of nests were vandalizedafter the researchers
removed the cowbird eggs. When the scientistsremoved the cowbird eggs
but added new fronts to the nestt boxes with holestoo small for
cowbirds, there was no damageRobinson conclude in a
So, the nest thrashes are cowbirds, Hoover and
paper now online for an upcoming Proceedings ofthe National Academy of
Sciences.warbler chicks starved
When the cowbird eggs stayed in the next, some
because the pushy cowbird nestlings took so muchof the food. Yet with
the retaliation attacks, the nests where cowbirdeggs had been removed
produced, on average, only 40 percent as manywarblers as the
cowbird-fostering nest did, say Hoover.Rothstein of the University
"This is a surprising result," says Stephen
of California, Santa Barbara.retaliate, but he says, "My
Rothstein hasn't tested whether cowbirds
bet, before this paper, would have been defnitelyno." He's now
reconsidering but says, "I'd like to see moredirect evidence to date,
"University in
such as video.
So would Naomi Langmore of the Australian National
Canberra. Still, she describes the evidence ascompelling."
the University of
"Best evidence of date, " Says Rebecca Kilner of
Cambridge in England.cowbird eggs from the nest?
Comment: Hummmmm. Wonder if we should throw
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