Phylogeny of the Waxwings and Allied Birds

by M. Dale Arvey

Structural affinities of Dulus and Bombycilla are more pronounced than are those of Dulus and Ptilogonys, for example. Many of the structural features of Dulus parallel those of Phainoptila, and it seems likely that the Dulinae were separated early in the history of the family, perhaps as an isolated offshoot of the early migratory Bombycillinae.

CONCLUSIONS

Nomenclature, as used by a taxonomist, should of course indicate affinities as well as apply a name, and the rank of the family should be applied to a structural unit based on common anatomical characters that are more fundamental than, in my opinion, are those used by Ridgway (1904) in proposing family status for the silky flycatchers and the palm-chats. The characters in the diagnosis ([page 478]) of the family Bombycillidae are common features regarded as warranting a single family unit for the waxwings, silky flycatchers, and palm-chats. The differences in morphology used by previous workers to characterize each of these groups: (1) the silky flycatchers; (2) waxwings and; (3) palm-chats are regarded as more properly characters of only subfamily rank.

The existing coloration of the species of the Bombycillidae appears to have been acquired relatively late, geologically speaking. The three subfamilies responded to ecological stimuli in three different ways, and the resulting color patterns are unlike in the three groups. Dulinae to this day have a color pattern that is most like the ancestral color pattern, and this is recapitulated in the juvenal plumage of the Bombycillinae before they attain their adult plumage.

Consideration of the geographic distribution of the species of the family indicates that the center of origin of the family Bombycillidae was south of the present range of the waxwings (subfamily Bombycillinae). Waxwings probably are the descendants of a migratory population that diverged from the primitive population at an early time in the history of the family. Owing to their adaptations to survive in the north, waxwings no longer return south in the autumn. Palm-chats (subfamily Dulinae) are descendants of an isolated population of the family stock that developed communal living habits as one specialization. Silky Flycatchers (subfamily Ptilogonatinae) became modified to catch insects, and have specializations that roughly parallel those of the Tyrannid flycatchers.

Osteologically, the various species of the Bombycillidae are remarkably similar. Small variations do exist, but these are primarily differences in relative size. The modifications of the beak enable palm-chats to feed on parts of plants, and the beak of Phainoptila shows some similarity in this respect. Rounded wings, which cause a bird to fly by means of short, relatively weak strokes, are correlated with a comparatively long humerus, whereas long and pointed wings, which enable a bird to fly with more powerful strokes of the wing, are correlated with a relatively short humerus. There is a positive correlation between a short humerus and a long external condyle, and between a long humerus and the absence or smallness of the external condyle.

In the Bombycillidae short bones of the leg are adaptive, and long bones of the leg are the generalized condition. Although all passerine birds were differentiated relatively late in geologic time, long hind limbs still could have been present in the immediate ancestors of passerine birds. As adaptive radiation took place in the class Aves, some birds, the Bombycillidae included, became more and more adapted for an arboreal, and eventually an aerial habitat, with consequent loss of saltatorial and running ability.

Birds, like mammals, have a short femur, the most proximal element in the leg, if the species is adapted to run fast. If the species is not adapted to run fast, birds, unlike mammals, have the tibiotarsus longer than any of the other elements; in mammals that are not adapted to run fast, the femur and tibia are approximately the same length. In non-running birds as compared with running birds, the leg element distal to the tibiotarsus, and the one proximal to it, are considerably shortened. In waxwings, all three elements of the hind limb are shortened, indicating that the reduction in length has been, evolutionarily speaking, a rapid process, in order to reduce the limbs to a convenient size as soon as possible.