Phylogeny of the Waxwings and Allied Birds

by M. Dale Arvey

It is here postulated that the center of origin for the ancestral stock of the Bombycillidae was in a region of North America, which at the time concerned was temperate or possibly even semi-tropical in climate. Probably Northern Mexico was the place and probably the climate was temperate. It is reasonably certain, because of the distribution of the species of the family, that they originated in the Americas. In the absence of paleontological data (Bombycilla alone is reported, in essentially its modern form, from the late Pleistocene—Wetmore, 1940a), the place and time of origin cannot certainly be determined.

The distribution of the family is such that the more primitive groups are in the south. These are the Ptilogonatinae in Central America and Mexico, and the isolated Dulinae in Haiti and the Dominican Republic. This distribution would support the view that the origin was in the south. However, the Holarctic Bombycillinae are so typically birds of northern latitudes that, were it not for such close relatives south of their range, it would appear logical to infer a northerly origin with a subsequent shifting of populations both southward and northward. The phyletic age of the family is probably great, however, as evidenced by the spotty distribution of the birds.

In the evolution of this family, population pressure possibly played the initial role in forcing members of the primitive, southern stock to seek habitable areas on the periphery of the range. Some birds also, being possessed of the "adventuresome spirit", aided the northerly movement, thus effecting an extension of the breeding ranges to the north. So far as is now known, this family did not seek living space in South America. By extending its range, a species might find more abundant food and nesting sites. This process of extending the range probably would be costly to the species concerned, because only those individuals best able to adapt themselves to the new environmental conditions would be able to survive long enough to reproduce their kind.

The return flight to the south could, in time, be dispensed with, except in the coldest weather or when the local berry- and fruit-crop failed. Birds such as waxwings are, of course, able to subsist on dried fruits and berries in the critical winter season when strictly insectivorous birds, not so catholic in their food habits, must return south. It appears that waxwings are descendants of migratory birds that have adjusted themselves to a life in the north; and they are judged not to have evolved from year-round residents of the north.

Even a short migratory journey in spring by part of a population of birds, while the other part remained in the original range, would quickly isolate one breeding population from the other, resulting in the formation of different genetic strains that lead to subspecies, species, and finally to genera and families. Any variation away from the ancestral, "sedentary" stock would become established more quickly because of such isolation at the breeding period. By the same token, the parental stock can, and no doubt does, become modified to suit its environment more perfectly, thus accelerating the tempo of this type of divergent evolution.

The original "split" of the Bombycillines is thought then to have been the result of migration on the part of some of the ancestral stock, with subsequent loss of regular migration because the need to return south was lost. Early in development, and before the migrational tendency was entirely lost, an isolated population, which later became sedentary, as it was an island population, diverged to give rise to the Dulinae. The Dulinae are a homogeneous group since on the islands now inhabited by the birds, they have not been isolated sufficiently long to produce even well-marked subspecies.

Fig. 49. Hypothetical family tree of the Bombycillidae.

The present day Phainoptila is most nearly like the ancestral group, and the remainder of the Ptilogonatinae have diverged to fit conditions similar to those to which the Tyrannid flycatchers, which parallel them, are also fitted.

In comparatively recent geological time, two basic lines developed from the Bombycilline stock, the future B. garrula and B. cedrorum. Possibly garrula originally was isolated in Europe and Asia, and later came into contact with B. cedrorum, following the time at which the two species were genetically well differentiated. It appears certain that B. japonica was an offshoot of the Bombycilline stock at an early time, since it has characteristics that seem relatively unspecialized. It possibly was isolated in the Orient.