Subject: the dreaded cowbird thread
Date: Jul 9 13:15:27 1998
From: Dennis Paulson - dpaulson at mail.ups.edu


Hi tweeters,

Someone in the grounds crew here just accidentally killed a female junco on
its nest, with a weed whacker, no less; an ignominious way to go.

But this sad story is the background for an interesting observation. She
also brought us the nest, and it has 5 eggs in it--2 juncos and 3 cowbirds.
Several pairs of juncos nest on campus, but I don't often see cowbirds. Yet
this nest, on the ground at the base of a Douglas-fir in an arboretum-like
grove of same, was visited presumably by 3 different female cowbirds, as
they normally lay no more than one egg per nest. With this level of
parasitism not only the young juncos were doomed but possibly one or more
of the young cowbirds. I would suspect the other pair or two of juncos on
campus are likewise parasitised.

Brood parasitism is undoubtedly one of the major sources of mortality on
passerine birds in this area. When you're entertaining cowbirds at your
feeders during the summer, that's worth a thought. The feeder is probably
more important for them than it is for the other birds coming to it, most
of which have a higher proportion of insects in their diets, so by
discontinuing feeding in summer you may actually be helping the songbirds
in your neighborhood.

Dennis Paulson, Director phone 253-756-3798
Slater Museum of Natural History fax 253-756-3352
University of Puget Sound e-mail dpaulson at ups.edu
Tacoma, WA 98416
http://www.ups.edu/biology/museum/museum.html