THE SPECIES OF APUS.

Abstract

Summary. The correct determination of the species of Apus is difficult. Their extreme adaptation to aerial life allows for few trivial or incidental differences. Some species look much alike and some conspecific races look very unlike. In several instances two conspecific races look more different from each other than one of them does from another species. Forms of the same species tend to be darker in areas of higher rainfall and smaller at lower latitudes, the differences in some cases being so striking that true affinities were not previously recognized. The instances of strong resemblance between races of different species are due partly to all the species of Apus possessing a similar basic colour pattern and partly to convergent adaptation. The relative length of the first and second primaries provides a useful taxonomic character, the second primary being longer in the forms with strongly forked tails. The winter ranges of the European species are reviewed. The following changes in specific status from the list by Peters (1940) are advocated. The more tentative proposals are indicated by (t).