Behavioral asymmetries in a moving hybrid zone

Abstract

Three narrow hybrid zones exist between hermit and Townsend's warblers in the Pacific Northwest. Character transition curves suggest that these zones are moving and that Townsend's warblers have a selective advantage over hermit warblers. This study compares year-to-year return rates, male persistence on territories, pairing success, pairing patterns, and territory quality of hermit and Townsend's warblers and their hybrids in the Washington Cascades hybrid zone. There was no difference in the year-to-year return rate between the parental species. Townsend's males were more successful in maintaining territories and attracting mates than hermit males. Among mated pairs there were few hermit males compared to females, independently supporting the inferiority of hermit males in competition for territories or female mate choice for Townsend's-like males. I found no difference in the quality of hermit and Townsend's territories; however, in poor habitats Townsend's males were more successful at attracting mates, suggesting female preference for Townsend's-like males. In high-quality habitats, there was no difference in pairing success between the parental species. The fitness of hybrids relative to parentals affects the width and movement of the zone. Hybrids were intermediate in their ability to maintain territories and to attract mates, which should increase the width of the zone and accelerate its movement. Fewer hybrid males returned to from one year to the next, and among mated pairs there were few hybrid females relative to males, suggesting hybrid inferiority, which should narrow the hybrid zone and slow its movement.