Greetings!
At the risk of starting a flame-war, here is my
2-cents worth.
Brown-headed Cowbirds are not native to this area.
They are native to the short-grass plains of the
mid-western states. They have moved both east and
west with people and their cattle. They are now
distributed across most of the US.
Cowbird reproduction strategy is to to place their
eggs in nests of other small passerine birds and let
the other species raise their young for them. The
adult female Cowbird sometimes removes one egg and
replaces it with one of her own. The Cowbird chick
typically hatches out before its parasitized
nest-mates, gets larger faster, and out-competes them
for food. I do not think the baby cowbird has the
same egg-pushing behaviour as the Cuckoos to push the
other un-hatched eggs out of the nest before they
hatch. Cowbird eggs are also usually not the same
color and size of their nest-mates, again unlike the
Cuckoos. The young Cowbird simply out-screams its
nest-mates for food and attention, and they starve.
The birds native to the short-grass prairie have
evolved to deal with Cowbird parasitism by recognizing
and abandoning nests that have been parasitized.
Yellow warblers, for instance, will build a new nest
right on top of one that has been parasitized. I have
seen pictures of Yellow warbler nest stacked 12 high,
one atop the next. Birds here have not evolved with
this form of parsitism, so do not recognize it, and
therefore will raise any chick in their nest.
This type of parasitism is believed to be larger
responsible for declines in several species of birds
(Kirtland's Warbler), and certainly contributes to
noted declines on many others. "The parasitic cowbird
almost exterminated the Kirtland's warbler in northern
Michigan until drastic cowbird eradication procedures
in the breeding range of Kirtland's warbler were
adopted." (Ernst Mayr,
http://www.pnas.org/cgi/content/full/94/6/2091)
I also believe there are extensive trapping and
eradication programs going on in several areas of the
country. Texas comes to mind.

Personally, having seen WAY to many Cowbirds both here
and in Montana, I think they should be added to our
list of "undesirable, destructive, alien species"
(Starlings, House Sparrows, Cats) that are harming our
native bird species. And I think aggressive steps
need to be taken to limit their populations. People
created this situation by our interference and our
lifestyle, so we should be responsible for controlling
some of the effects. And if we can not agree to do
this on a large, regional scale, I feel no compunction
about control efforts done in my own back yard. If my
neighbors feel as strongly about it, but in the
opposite direction, they are also free to encourage
these nasty aliens in their yards.
But I do not see how you can be anyone who truly
appreciates birds (and watching them and studying
them) and NOT want to try to do something about this
problem.

Thank you for listening to my rant.

Please do NOT respond via Tweeters if you wish to
Flame me out, unless you actually have something
useful to say to the whole group. You can blast me
directly for any of my views ;)

Guy McWethy

=====
Guy McWethy
Renton, WA
mailto: lguy_mcw at yahoo.com

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