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It is well known that closely related, sympatric species have evolved species-specific features which serve to minimize the possibility of "wrong" choices being made during pair formation. The amount of evolution of such species-specific features is roughly proportional to the deleterious effects of the "wrong" choices made in species recognition. Of course, if upon initial contact, the forms interbreed too freely panmixia will occur and both will eventually lose whatever genetic identity they may have had. On the other hand if, by the time of contact, the forms have incidentally developed differences sufficient to serve automatically as isolating mechanisms from the outset, then the further evolution of such characters will not occur as a result of "mistakes" being made. Thus, it would seem that the post-contact evolution of species-specific features which serve as isolating mechanisms depends upon rather particular conditions involving contacts between forms which find themselves neither impartially interfertile nor completely isolated at the start (see Sibley, 1957, for a thorough discussion of these phenomena).
The species recognition features evolved in birds are mainly visual and/or vocal in nature. Either may predominate, depending on the nature of the selection pressures involved and upon the nature of the genetic variability available upon which the selection can exert its influence. Visual recognition will tend to be emphasized by selection in those species in which visual features are most advantageous, and the same may be said for vocal features. The relative advantage or disadvantage is probably determined largely by the ease in which either may be perceived in the physical environment in which pair formation typically takes place (Dilger, 1956). Ducks of the genus Anas, many trochilids, paradiseids, phasianids, etc., probably rely largely on visual species recognition (Sibley, 1957) ; and thrushes of the genus Catharus have been shown to rely most heavily on vocalizations for their species recognition (Dilger, 1956).