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. Received 16 July 1997, accepted 5 February 1998. HERMIT (DENDROICA OCCIDENTALIS) and Townsend's (D. TOWNSENDI) warblers are sister species that apparently diverged in Rocky Mountain (Townsend's) and coastal (Hermit) refugia during the middle Pleistocene (Bermingham et al. 1992). Three geographically isolated hybrid zones connect the ranges of these species, one in the Olympic Mountains of Washington, another in the southern Cascade Mountains of Washington, and a third in the Cascade Mountains of Oregon south of Mt. Hood (Rohwer and Wood 1998). The two Washington hybrid zones have been described in detail. For both zones, transitions from one parental phenotype to the other occur over 100 to 125 km, a distance only three to four times greater than estimates of root mean square dispersal (Rohwer and Wood 1998). These rapid character transitions imply that strong selective forces prevent these zones from becoming wider (Barton and Hewitt 1985, 1989). Rohwer and Wood (1998) evaluated a variety of models that might explain these narrow zones, and they concluded that both hybrid in3Present address: Department of Zoology, P.O. Box 118525, University of Florida, Gainesville, Florida, 32611, USA. E-mail: spearson@zoo.ufl.edu feriority and parental fitness asymmetries are keeping these zones narrow and, further, that parental fitness asymmetries are causing them to move. Hybrid zones will remain narrow when selection against hybrids is balanced by immigration toward the center of the zone by parentals (Barton and Hewitt 1985, 1989, Barton and Gale 1993). Asymmetric charactertransition curves also suggest that the Washington zones are moving southward, owing to the competitive superiority of Townsend's Warblers over both hybrids and Hermit Warblers (Rohwer and Wood 1998). Because this inference was based only on the shape of the character-transition curves, the competitive ranking of hybrids and Hermit Warblers could not be inferred. Several alternative models that can explain abrupt character transitions are not applicable to these hybrid zones (Rohwer and Wood 1998, Pearson 1997a). These include recent contact (Endler 1977, Barrowclough 1980), hybrid superiority within an ecotone (Moore 1977, Moore and Price 1993) and habitat modification (Gill 1980, Wake et al. 1980, Hewitt 1989: table 4). These warbler hybrid zones are interesting because the inferred fitness asymmetry implies that Townsend's Warblers are replacing Hermit