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ON the basis of recent anatomical, behavioral, and serological investigations, I feel that the genus Hylocichla of the A.O.U. checklists should be considered congeneric with Catharus except for the Turdus-like Wood Thrush, which has been left in Hylocichla pending further investigation (Dilger, 1956). The North American species of Catharus (the Veery, fuscescens; Hermit Thrush, guttatus; Olive-backed Thrush, ustulatus; and Graycheeked Thrush, rninirnus) and the Wood Thrush, Hylocichla rnustelina have a roughly allopatric distribution across the continent (Figure I). There is much broad overlapping of ranges, and in mortfane areas of the eastern United States as many as four species (If. rnustelina, C. fuscescens, C. guttatus, and C. ustulatus) may be sympatric. The four Catharus species are very closely related; judging by their behavior, anatomy, and serology; and all five are similar in appearance. All are forest or forest-edge inhabitants. They either do not hybridize or do so very rarely as no hybrids have been reported. While these forms were differentiating in isolation, they must have developed differences which reduced the probability of interbreeding upon subsequent contact. Whatever these differences were they probably became greater through selection upon subsequent species contacts until they achieved the level of perfection which we observe today. It is not hybridization that is necessarily selected against, but, rather, the formation of mixed pairs. These will be selected against since they are biologically disadvantageous. If the production of offspring is impossible, both birds will have "wasted" their time, and, in effect, two potentially productive pairs will have been wasted; the mixed pair and the birds they would have mated with if a correct

Hostile Behavior and Reproductive Isolating Mechanisms in the Avian Genera Catharus and Hylocichla