Three hybrid zones between Hermit and Townsend's Warblers in Washington and Oregon

Abstract

We describe two hybrid zones between Hermit Warblers (Dendroica occidentalis) and Townsend's Warblers (D. townsendi) in Washington, and introduce a third zone in Oregon. To quantify phenotypes, we scored eight standardized characters in each age class and summed them to form a hybrid index score. Because no linear geographic feature delimits the hybrid zones between these warblers, we sampled the forests of Washington in a grid-like fashion to define the shape of the zones. For both Washington zones, the shape of the character-transition curves for the hybrid index suggests asymmetries in gene flow, with Townsend's-like phenotypes penetrating farther into the Hermit side of the zone than vice versa. Covariances between character states are high at the phenotypic center of the zone. Following Barton and Gale (1993), we use these covariances and the width of the zone to estimate σ, the root mean square dispersal distance. Both Washington zones are narrow relative to σ, indicating that selection is preventing these zones from increasing in width. Hybrid superiority within an ecotone, recency of contact, and habitat modification seem insufficient as explanations for the abrupt transition that takes place in these zones. Asymmetries in the character-transition curves suggest that the zones are moving. We find no evidence that this inferred movement can be attributed to habitat changes or to differences in dispersal between the parental species. Other studies suggest that the primary cause of this inferred movement is behavioral dominance of Townsend's males over Hermit males. The shape of the zone in the Washington Cascades also suggests that Townsend's on the east side of the Cascade Mountains are better adapted to dry forests, possibly due to gene flow from populations derived from the Rocky Mountains. The third, isolated zone in the Cascade Mountains of Oregon offers unique opportunities to test this hypothesis.