OWEN-ASHLEY, N.T.*; BUTLER, L.K.; ROHWER, S.; WINGFIELD, J.C.: The Role of Testosterone in a Moving Avian Hybrid Zone

Three narrow hybrid zones exist between hermit (Dendroica occidentalis) and Townsend’s (D. townsendi) warblers in Oregon and Washington. Asymmetries in character transition curves suggest that these zones are moving, with Townsend�s warblers replacing hermit warblers. Previous work has shown that male Townsend’s warblers are superior to male hermit warblers in measures of aggression and territory maintenance. Hybrid males are intermediate in aggression and territory maintenance, suggesting that the hybrid zone movement is due in part to heritable differences in traits related to behavioral dominance and territory holding ability. We tested the hypothesis that these behavioral differences are mediated by differences in testosterone (T) secretion in response to a territorial intruder. We exposed parentals outside the hybrid zone and hybrids inside the zone to simulated territorial intrusions using taxidermic male mounts and local song playback, and we netted and bled territory owners for plasma T samples after the presentations. Townsend�s and hybrid males had higher T responses than hermits, but there was no difference between Townsend�s and hybrid males. Thus, we provide a proximate explanation for the replacement of hermit warblers by Townsend�s warblers, but differences in plasma T levels do not explain intermediate levels of hybrid aggression. To explain the unexpectedly high T levels in hybrids, we investigated the relationship between phenotype, aggression, and T levels among hybrids. Our study suggests that interspecific differences in hormone secretion may play a role in broad evolutionary processes such as hybridization and species replacement.