And random cowbird comments.

I can top Dennis's unfortunate junco nest by one or two eggs, I think.

In the course of a study of nesting song sparrows in Discovery park, I was
shown a nest that had six eggs in it, of which *four* appeared to be
cowbird eggs. I don't linger over found nests, so as not to draw
predators' attention to them, so I could have gotten the exact number of
cowbird eggs wrong in my quick inspection (cowbird and song sparrow eggs
aren't *that* different in appearance), but today I banded the one hatched
song sparrow chick, and the nest contained

1 song sparrow chick, 6 days old
2 cowbird chicks, same age and about the same size
1 undeveloped cowbird egg
1 partly developed song sparrow egg

One egg had disappeared during incubation. So I think my initial
assessment of 4 cowbird and two song sparrow eggs was probably right.

I fully expect the one large sparrow chick to fledge, despite its two
loud nestmates.

It's also typical in multiply parasitized nests for some or most of the
cowbird eggs to fail to develop - this nest is unusual in that two
hatched.

As to how many cowbirds contributed, I suspect it was more than one, but
doubt it was more than two. I have often found two cowbird eggs in a nest
that differed from each other in their markings, suggesting more than one
female. However, I've also found two nearly identical cowbird eggs in the
same song sparrow nest, and have found cowbird eggs laid in old abandoned
nest many times. That suggests to me that sometimes female cowbirds are
stuck with the decision of where to get rid of an egg before they've found
a "fresh" unparasitized nest to lay in. Under those circumstances, it's
probably in their favor to lay two eggs in one nest, and maybe even more.

The song sparrow nesting season is just about over in Disc. Pk. This
particular nest was the latest one I know of (among about 30 sparrow
territories) and that may explain the oddity of four cowbird eggs. The
sparrows do have about a month of the breeding season before the cowbirds
arrive back in town, and enjoy higher nesting success then. I suspect the
same thing is true for juncos.

Chris Hill
Everett, WA
cehill at u.washington.edu