cterus_ should be the presence of a more massive set of muscles
concerned with protraction and depression than is found in non-gaping
groups. Beecher found the situation to be exactly as expected in that
genus and in other genera which also gape. Meadowlarks (_Sturnella_)
and caciques (_Archiplanus_) gape and pry in soil and wood respectively
(Beecher, 1951a:422 and 426).

The lengthened beak would be a problem when the White-winged Dove
attempted to pick up objects such as seeds, which do in fact constitute
the largest percentage of its diet in spite of its nectar-feeding
habit. A similar situation exists in the genus _Icterus_, which is
primarily adapted for gaping even though it shows a preference for
insects when they are abundant (Beecher, 1950:53). The lengthened beak
could be compensated for by (A) migration of the anterior end of the
jugal bar toward the rostral tip of the bill and away from the
fronto-nasal hinge with a simultaneous enlargement of the adductor
muscles of the lower mandible, or (B) enlargement of the one muscle
that functions simultaneously as an efficient retractor of the upper
mandible and adductor of the lower mandible, namely _M.
pseudotemporalis profundus_. _Mm. pterygoideus dorsalis et lateralis_
perform the same function, but because of their position on the lower
mandible they, apparently, are stronger retractors of the upper
mandible than they are adductors of the lower.

It will be recalled that the jugal bar bears the same, or nearly the
same, relationship to the cranium in the white-wing as it does in the
Mourning Dove and that the heads, excluding the beaks of both species,
are of nearly the same proportions. Also, _Mm. adductor mandibulae
externus_ and _pseudotemporalis superficialis_, the chief adductor
muscles of the lower mandible, were not noticeably enlarged in the
white-wing. It is also important to note that other combinations of
migration of bone and/or enlargement of muscles could successfully
solve the problem of providing sufficient leverage for the proper
functioning of the lengthened mandibles, but it is my thesis that the
second alternative sufficed for seed-eating habits and that that is the
adaptation that was established; it is, in fact, the only one present
in the White-winged Dove.

It is unlikely that this enlarged muscle and beak are the remains of
another series of jaw muscles that have converged toward the condition
in Mourning Doves. Columbids are almost