Random Mating in the Northern Flicker Hybrid Zone: Implications for the Evolution of Bright and Contrasting Plumage Patterns in Birds

The Red-shafted and Yellow-shafted Flickers hybridize in a narrow zone on the western Great Plains of North America. The two subspecies are markedly different in six plumage traits. Plumage phenotypes were scored for the male and female of 125 mated pairs from the hybrid zone. Correlation analyses revealed no tendency towards pairing of similar phenotypes. A binomial test was performed to determine whether flickers that have parental phenotypes and are presumably immigrants rather than hybrids tend to choose mates similar to themselves. This test again indicated random mating in the hybrid zone. These data complement earlier reports in providing an additional test of alternative hybrid zone theories. Random mating along with evidence of high reproductive success in hybrids suggests that reinforcement of premating reproductive isolation is not occurring and that speciation will not result from the dynamics of the hybrid zone. At least some of the plumage traits seem to be important in the integrated courtship and territorial behaviors of flickers. Since these plumage differences have no significance in mate choice, it is inferred that they evolved in an intrasexual selection context rather than in intersexual or species-recognition contexts. Selection gradients appear to be important in restricting hybridization to a narrow zone. The gradients could result from ecological selection or from sexual selection.