Deformed Yellow-rumped Warbler

On Thursday I banded a dozen Yellow-rumped Warblers, including this bird with deformed feet.  I can only speculate about the cause of the missing toes.  Is this the result of a developmental deformity?  Is it a bird that recovered from avian pox?  Had it suffered frostbite (the bird was an adult, so it was alive last winter)?  Had it barely escaped from a predator? In my banding career, I have never seen anything quite like this.  One wonders how this bird perched.  Most deformed birds do not survive.