I believe Robins are ejectors, they are able to discriminate between their
own eggs and the cowbird eggs and are large enough to eject them if they
find them in the nest. Robins are also very aggressive at nest defense,
which proves a very good deterrent from keeping cowbirds from slipping in
and dropping an egg. There is very little if any parasitism of nests of
other highly aggressive species such as kingbirds, mockingbirds, and Western
Wood Pewees.

Jeff Kozma
Yakima