ASPECTS OF EVOLUTION IN THE PARROT GENUS AGAPORNIS

Abstract

Summary. Agapornis, an African genus of parrots allied to Loriculus of Asia, has usually been classified in nine species.Their geographical, altitudinal and ecological ranges are described and their temperature relations are worked out. There is a general agreement with Bergmann's rule.Only two of the Agapornids appear to be in any respect ecologically specialized: to A. swinderniana, the only one confined to tropical evergreen forest, the seeds of figs may be an essential food; and A. pullaria is more or less completely dependent on the nests of arboreal insects for nesting‐sites. The other seven Agapornids are birds of dry country with a wide range of food and are indiscriminate hole‐nesters.All nine birds are practically allopatric. The four closely allied birds in East Africa, which produce fertile hybrids very freely in captivity, seem nowhere actually to meet in nature. Certain vegetation types, especially Brachystegia‐Isoberlinia woodland, appear to be an effective barrier, for reasons not clear.Comparisons are made of colour, size and proportions, degree of sexual dimorphism, juvenile plumages, hybridization and breeding biology. Special points of interest are:— The existence of well‐developed sexual dimorphism and juvenile plumage in some Agapornids and their total absence in others. The development of contrasting eye‐rings; in two forms from feathers, in four others from naked white skin; that is, using two different physiological bases. The persistence of certain features in the tail pattern throughout the genus, in contrast to the variability of other parts of the plumage. The existence of two pairs of allopatric forms, each consisting of one pink‐headed and one containing more melanin; but the distribution of the melanin being very different in the two “melanic” forms. The fact that some forms of Agapornis carry their nesting‐material in their contour feathers (a habit otherwise known only in Loriculus) and others do not. Although certain of the tectonic and climatic changes in Pleistocene Africa would, by sundering populations, have facilitated evolution of the existing forms, the isolating mechanism so provided seems inadequate.Most of the Agapornids fall into two groups, the older comprising the West African, Abyssinian and Madagascar forms. Main trends of evolution in the genus have been to eliminate (a) the habit of carrying material in the contour feathers, and (b) sexual dimorphism, partly by “grading up” the female plumage of the fore‐parts and partly by eliminating the black under wing‐coverts of the male.A proper picture of relationships within the genus demands the use of the newer taxonomic categories, cline, semispecies and superspecies, in addition to the species and subspecies.The genus provides a good field for genetical analysis. At least five sharply characterized forms, including subspecies, semispecies and full species, readily produce fertile hybrids, and one of the five has the habit of carrying material in the contour feathers.