In the species dissected, the only variable muscle that I consider significant in revealing relationships is _M. pseudotemporalis profundus_. It is markedly enlarged in the White-winged Dove in relation to the homologous muscle in the Mourning Dove. The muscle is enlarged in such a manner that a lateral expansion of its mass is apparent in superficial or dorsal view (compare figures 15 and 16). This, of course, indicates a muscle with powerful contraction, which has been unable to enlarge its circumference symmetrically because the eye is immediately dorsal to the muscle. Therefore it has expanded laterally. Ventral expansion is blocked by the presence of other muscles, and medially there is no surface for the insertion of additional fibers on the orbital process of the quadrate. The jaw musculature has been known for some time to be highly adaptive (Beecher, 1951a and b, 1953; Bowman, 1961; Burt, 1930; Engels, 1940 and Goodman and Fisher, 1962) and it would not be unreasonable, I think, to expect the jaw muscles 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.