No one has yet mentioned a common cause of missing toes in urban birds.
Bird feet easily get tangled in string, which can then wind around the
toes, cut off circulation, and kill the toe (and sometiems the foot). I
know that this can occur in doves of Honolulu, because on my very first
day I was ever in Honolulu, I noticed that one of the doves on my hotel
balcony was hobbling around with its feet entangled in black thread. It
was almost unable to walk. By luring it close with bread I was able to
grab it and cut the string free. The string was very tightly wrapped
around the toes of both feet and had cut deeply into the soft tissue, and
I am sure this dove would have lost toes if I hadn't caught it. (Don't
worry, I let it go right afterwards, and yes, I had federal & state
permits to do this.) I have seen other pigeons in the continental U.S.
who had similarly gotten their feet tangled in a bit of trash.

Another cause of toe loss in Hawaiian birds is lesions from avian pox. In
1993 I studied birds on the Big Island, and I captured many 'i'iwi and
'apapane (native Hawaiian honeycreepers) that had pox lesions on their
feet, legs, and bills, and I captured other birds who were healthy but had
missing toes (presumably having recovered from pox. We tested their blood
for pox exposure.) I don't know if the doves at lower elevations also
have endemic avian pox, but it wouldn't surprise me if they did..

Endocrine disruptors, suggested by another subscriber, are unlikely to
cause toe loss. Endocrine disruptors tend to target tissues that are
sensitive to reproductive hormones, such as brain (& behavior), testis,
ovary, liver, etc.

cheers,
Kathleen Hunt
U.W. Zoology