Robins remove a pale cowbird egg when it shows up in their blue clutches, yet wood thrushes, who also lay solid blue eggs, just accept the anomaly. The birds that put up with cowbird eggs haven't had time to develop defenses, he argues. He points out how poorly tuned and full of disadvantageous quirks both the parasitism and the defenses are. That's just the sort of sloppiness one would expect from a work in progress, he says. Sievert Rohwer of the University of Washington in Seattle adds to the explanation.
Nothing evolves overnight.
However, he proposes that other forces are also at work. Tiny birds may never evolve ejection behavior because they risk cracking their own eggs. Even orioles damage one of their own eggs, on average, for each five parasite eggs they eject. Egg-tolerating species in the ancient heart of cowbird territory tend to be smaller than egg rejecters, he reports. Complicating the picture, Sealy reported in 1996, is the fact that the smallest egg ejector, the 15-gram Eastern warbling vireo, routinely punctures cowbird eggs and heaves the pieces out of the nest, whereas warbling vireos west of the Great Plains generally do not.