scles of closely related species with similar habits
to be similar. The beak of the White-winged Dove is longer in
proportion to the length and height of the skull (exclusive of the
beak) than is the beak of the Mourning Dove. The lengthened beak is
probably an adaptation for nectar-feeding, which has been documented by
McGregor, Alcorn and Olin (1962:263-264) while investigating
pollinating agents of the Saguaro Cactus (_Cereus giganteus_), and by
Gilman (1911:53) who observed the birds thrusting their bills into the
flowers of the plant. Gilman indicated, however, that he could not be
sure if the birds were seeking insects, pollen, or nectar. In any event
the lengthened bill probably facilitates getting food by birds that
probe parts of flowers. Hensley (1954:202) noted that both Mourning and
White-winged doves were "exceptionally fond of this source of
nourishment." But he also points out an "interesting correlation"
between the presence of the white-wings in the desert and the flowering
of the saguaro. During his studies the appearance of the first
white-wing preceded the opening of the first saguaro flower by two
days. The flowering and fruiting season lasted until August, the month
of termination of the white-wing breeding season.

Since Hensley makes the correlation solely with the white-wings, I
assume that there is no other obvious correlation between plants and
birds among the remainder of the avifauna of the desert. Probably the
Mourning Dove has failed to adapt to nectar-feeding as yet, and the
White-winged Dove is the primary exploiter of this food niche. It
should be noted, also, that the head of the Mourning Dove is smaller
than the white-wing's, and perhaps there is no need for an elongated
beak for reaching deeply into the flowers.

The lengthened bill should produce no difficulties in protraction of
the upper mandible and depression of the lower for the reason that in
the dove there is no known resistance offered to these movements. The
genus _Icterus_ furnishes an example wherein resistance is met in the
process of opening the mandibles; individuals of this genus thrust
their closed bill into certain fruits and forcibly open their mandibles
against the resistance of the pulp by strong protraction and
depression, thus permitting the juices of the fruit to lake and
ultimately to be consumed (Beecher, 1950:53). Beecher refers to the
technique used in fruit-eating as "gaping." The result of gaping in
_I