Asymmetries in male aggression across an avian hybrid zone

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Abstract
Recent studies suggest that competitive asymmetries are causing the hybrid zones between hermit and Townsend's warblers to move, such that Townsend's warblers are replacing hermit warblers. Here we examine the contribution of male aggression to this competitive asymmetry by measuring aggressive responses to mounts. We presented male mounts of the two parental species to Townsend's and hermit warblers outside the zone and to hybrids within the zone. Outside the zone, Townsend's males are more aggressive to both conspecific and heterospecific mounts than are hermit males. This asymmetry should move the zone in the direction inferred from previous studies. Hybrids fall between parentals in their aggressiveness, which should accelerate the movement of the zone. Remarkably, we found no relationship between phenotype and aggression in individual males at a locality within the hybrid zone. The forces of selection and dispersal that maintain narrow hybrid zones should generate such a correlation if aggressive differences between the parental species are genetically controlled. We resolve this conflict by proposing a behavioral model of competitive sorting within localities. If birds are sorted across the hybrid zone according to competitive abilities, and competitive interactions within neighborhoods are more or less complete, then the correlation between phenotype and aggression within any given neighborhood will be eliminated. We tested this model by examining the relationship between phenotype and aggression across the zone. Warblers in hybrid neighborhoods on the Townsend's side of the zone are more aggressive than warblers in hybrid neighborhoods on the hermit side, indicating that competitive sorting is occurring.

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