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Re: nest predation



>On Fri, 29 May 1998, Pat Grant (Library: Serials Catalog wrote:
>> If the unwilling
>> foster parents had thrown out the alien egg, or otherwise failed to
>> take care of the young parasite, the visiting absentee parents would
>> trash the nest and kill any other young in it.
>
>Fascinating!  Therefore the cowbird is selecting for good parenthood in
>another species, i.e., offspring with genes for parenting of cowbird young
>will survive at a higher rate than those whose parents evict the
>parasites.


Unfortunately, there's another dimension to cowbird behavior.  Cowbirds are
open-ground birds, and you typically find them in grassy fields and open
thickets.  But the birds they parasitise - certain warblers, vireos, maybe
thrushes - nest in trees.  As a result, cowbirds only parasitise birds that
(a) won't throw an alien egg from the nest, and (b) happen to nest at the
edge of a forest.

The thing is, there's much more forest edge now than there was two
centuries ago, and cowbird populations are exploding as those of
parasitised birds - black-capped vireos, Kirtland's and golden-cheeked
warblers, and several others - drop.  Some areas have begun cowbird control
programs, which raise complaints from many groups but do show a measureable
affect on the populations of putative parasite species.  My wife and I had
to stop filling our birdfeeders in Austin when we discovered cowbirds
taking up about 85% of the bird biomass at the feeders - they sound cool,
but I don't want to help them.


chris





-=--=--=--=--=--=--=--=--=--=
Christopher Brochu, Ph.D.

Postdoctoral Research Scientist
Department of Geology
Field Museum of Natural History
Lake Shore Drive at Roosevelt Road
Chicago, IL  60605  USA

phone:  312-922-9410, ext. 469
fax:  312-922-9566

cbrochu@fmppr.fmnh.org