I too have seen reasonably high numbers of foot deformities in both
blackbirds and pigeon-types - I have seldom seen the same thing in
crows, however, although they're certainly abundant around here.

This leads me to wonder if it's a matter of selective pressure. In
most places there's very little predation mortality on pigeons, and
what there is probably doesn't differentially affect pigeons with
normal feet versus those with foot deformities, does it? It also would
have minor impact on their foraging success, I would think. Crows, on
the other hand, would probably find it difficult to forage effectively
without two functioning feet for grasping food items, and so deformed
feet in crow populations are weeded out rapidly.

How's that for a hypothesis? Now, how one would test it, I don't know.

Cheers,

Josh Hayes, Quantitative Sciences, U. of Washington
josh at pogo.cqs.washington.edu