Two subspecies of Northern Flicker (Colaptes auratus), Yellow-shafted Flicker (C. a. auratus) and Red-shafted Flicker (C. a. cafer), form a hybrid zone that stretches the length of North America and is apparently stable. The most widely accepted hypothesis for the zone, the “bounded-hybrid superiority” hypothesis, predicts that the fitness of hybrids should be equal to or higher than that of parental types within the hybrid zone. We compared the reproductive performance at various stages of the reproductive cycle among Northern Flickers of different phenotypes at a location in the hybrid zone in which individuals are predominantly reddish. Individuals with red and yellow coloring showed similar levels of aggression toward taxidermy models of pure Red-shafted and Yellow-shafted flickers, and there was no association between phenotype (color class) and the likelihood of winning agonistic contests. In addition, the color of the male and female parents was not related to nest initiation date, clutch size, hatching success, risk of predation, or number of fledglings. These results do not support the tension-zone model, which predicts lower fitness of hybrids, but neither do they strongly support the concept of hybrid superiority, because all phenotypes had equivalent reproductive success. Annual changes in selection pressure, such as those caused by weather, could mask fitness differences between red and yellow individuals measured at a single geographic location and result in a hybrid zone that is variable in location and width over time.
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Vol. 126 • No. 2