My understanding is that this is only one study done in Ill. or Ind.
There needs to be a lot more research done on this. It has always
been general knowledge with us birders that cowbirds lay eggs and go
on. Never thinking about the egg any more. This study gives us an
interesting new twist to consider. It was the male that tore up the
nests, was my understanding. Perhaps they hang around, and the
females go on.
I was always fascinated with the very large flocks of cowbirds in the
fall of each year. How do they find one another? How do the
Juveniles of that year find the adults? and do they ever have
"family" groups as such? It just looks like one large cowbird
convention to me. And it has to be call and song that gets them
together again, but the young birds had not heard that "voice" in the
nest. So it is in the egg! It is a remarkable thing considering.
And I must admit that I hate seeing the cowbird eggs in our bb box
nests, but they fledge before the bbs and there is at least one
bluebird that survives to also fledge. I would prefer it not happen
but it is not EVERY box, and all species need their methods of
breeding. It is the only parasitic bird on our continent, we can
handle that.
However, it also seems to me that there are more and more cowbirds as
we cut back all the woodlands to build housing and malls, leaving
only "edges" for our other birds and making it all that much easier
for the cowbirds to find a nest to drop an egg in. And I have read
that a female can lay 70 eggs in a season! So if I remove one of
them I won't feel too badly.
Barbara
On Jun 14, 2007, at 2:29 PM, 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:
(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
You are subscribed to VA-BIRD. To post to this mailing list,
simply send
email to va-bird@xxxxxxxxxxxxx. To unsubscribe, send email to
va-bird-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field.
You are subscribed to VA-BIRD. To post to this mailing list, simply
send email to va-bird@xxxxxxxxxxxxx. To unsubscribe, send email to
va-bird-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field.