Behavioral asymmetries in a moving hybrid zone

Three narrow hybrid zones exist between hermit and Townsend's warblers in t he Pacific Northwest. Character transition curves suggest that these zones are moving and that Townsend's warblers have a selective advantage over her mit warblers. This study compares year-to-year return rates, male persisten ce on territories, pairing success, pairing patterns, and territory quality of hermit and Townsend's warblers and their hybrids in the Washington Casc ades hybrid zone. There was no difference in the year-to-year return rate b etween the parental species. Townsend's males were more successful in maint aining territories and attracting mates than hermit males. Among mated pair s there were few hermit males compared to females, independently supporting the inferiority of hermit males in competition for territories or female m ate choice for To Townsend's-like males. I found no difference in the quali ty of hermit and Townsend's territories; however, in poor habitats Townsend 's males were more successful at attracting mates, suggesting female prefer ence for Townsend's-like males. In high-quality habitats, there was no diff erence in pairing success between the parental species. The fitness of hybr ids relative to parentals affects the width and movement of the zone. Hybri ds were intermediate in their ability to maintain territories and to attrac t mates, which should increase the width of the zone and accelerate its mov ement. Fewer hybrid males returned to from one year to the next, and among mated pairs there were few hybrid females relative to males, suggesting hyb rid inferiority, which should narrow the hybrid zone and slow its movement.