Influence of breeding phenology and clutch size on hybridization between hermit and townsend's warblers

Abstract

In avian hybrid zones, differences in timing of breeding or clutch size may confer a selective advantage for one species over the other. We compared clutch sizes, arrival dates, pairing dates, and clutch initiation dates among Hermit Warblers (Dendroica occidentalis), Townsend's Warblers (D. townsendi), and their hybrids from areas within and adjacent to their hybrid zones in Oregon and Washington. Adult males of all phenotypes arrived first, with yearling males and females following by 10 and 18 days, respectively. Arrival was significantly later at higher elevations for adult males. The phenotypes did not differ in arrival dates when the effect of elevation was removed. Pairing dates were similar for the three male phenotypes, and clutch initiation dates were similar for the three female phenotypes. Thus, breeding phenology seems not to influence competition between these warblers and their hybrids. However, the clutches of Townsend's Warblers were 0.6 eggs larger than those of Hermit Warblers and 1.15 eggs larger than those of hybrids, suggesting a selective advantage for Townsend's Warblers. Hybrid females produced the smallest clutches we recorded (though not significantly smaller than Hermit Warbler clutches), suggesting that hybrids are inferior to both parental species.