DISCUSSION
The degree of difference between Golden-winged and Blue-winged warblers is hard to quantify. In regions of sympatry, the two species often nest in old field successional habitat sometimes with over-lapping territories. Yet, when specific habitats are available, the two phenotypes show strong differences in habitat preference. The managed forest in our study attracted 98% Golden-winged Warblers (McNeil et al. 2017, 2018) despite being in a region dominated by Blue-winged Warblers. Swamp forests in southern New York attracted 95% Golden-winged Warblers (Confer et al. 2010) even though both species were about equally abundant in adjacent uplands. Rush and Post (2008) documented a similar differences in habitat preference in a wetland-upland mosaic in the St. Lawrence River valley. In addition to differences in preferred breeding habitat, the winter range differs with Golden-winged Warblers extending farther south into northern South America (Bennett et al. 2017, Kramer et al., 2017). Blue-winged Warblers arrive earlier on their sympatric breeding grounds (Ficken and Ficken 1968, Canterbury and Stover 1999). Golden-winged Warblers weigh more, have larger wing chords, but smaller tarsi (Confer 1992, Gill et al. 2001). The primary song of Golden-winged and Blue-winged warblers, which is used to attract mates, are readily distinguished (Ficken and Ficken 1966, 1968, 1969, Gill and Murray 1972a, Highsmith 1989) with small variation among males of the same phenotype (Gill and Murray 1972b) Differences in habitat preference, range, behavior, and morphology surely have a genetic foundation, but their contribution of the differentiation between these two species is not readily quantified.
Our study provides a measure of the degree of speciation by compiling the pairing frequencies for sympatric populations of Golden-winged and Blue-winged warblers and their hybrids. Among our nine studies at eight study areas some Golden-winged Warbler populations were expanding, others semi-stable, and others declining. Some study areas had a strong preponderance of Golden-winged Warblers over Blue-winged Warblers, others were semi-equal, and others had a preponderance of Blue-winged Warblers. Thus, our results represent a wide range of population conditions. Pooled values show a reproductive isolation of 0.966. Among all of these diverse populations (Table 2) the frequency of hybrid social pairs between the Golden-winged and Blue-winged phenotypes ranged from 0% for three studies to 4% (n = 4) in one study. Given the small sample size for hybrid pairs (n = 0 to 4), the variance of the frequency of hybridization among our study areas is compatible with the hypothesis that primary hybridization occurs with similar frequency among all sympatric populations.
An important limitation of using social pairing data to estimate the strength of behavioral isolation in birds is the presence of extra-pair paternity, wherein either member of a pair may mate and produce offspring with an individual other than their social partner. Vallender et al. (2007) estimated that 32% of young in a contact zone between Golden-winged and Blue-winged warblers were the result of extra-pair paternity among the Golden-winged Warblers. However, social pairing data should only produce biased estimates of behavioral isolation if individuals systematically seek extra-pair partners that differ in phenotype from their social partner. Importantly, there was no evidence of extra-pair paternity between Golden-winged and Blue-winged warblers documented by Vallender et al. (2007). This suggests that behavioral isolation from social pairing data would be minimally confounded by the presence of extra-pair or extra-species copulations in this system.
Hybrid fitness significantly influences our understanding of differentiation between Golden-winged and Blue-winged warblers, and of the factors that may drive speciation. To assess hybrid fitness we used data for only males because they are usually caught near singing posts, which are used by both mated and unmated males, and which seems to provide an unbiased sample of pairing frequency. We exclude females who are most often caught in nets placed near a known nest, which would provide a biased sample of pairing frequency. Ficken and Ficken (1968) compiled data from several sources that showed a significant difference in the ratio of paired to unpaired males for ‘”pures”’ vs.hybrid: 93% (n = 32:3) vs . 46% (n = 6:7) (Chi-square = 11.781, p = 0.018). Confer and Tupper (2000) found that only 1 of 13 resident male Brewster’s Warblers formed a social pair. Experimental manipulation of plumage pattern (Leichty and Grier 2006) showed reduced pairing success for hybrid-looking males. For our pooled results for males, hybrid fitness was significantly lower with a 35% reduction in pairing success rate for hybrids compared to Golden-winged Warblers. Vallender et al. (2007) analyzed male and female pairing success for a study in southeastern Ontario. Based on these data, Kramer et al. (2018) suggest that “there is little evidence of costs to producing hybrid young”. However, considering just males, the data showed a pairing success rate of 42% (55 of 132) for Golden-winged Warblers and 18% (2 of 11) for Brewster’s Warblers (Vallender et al. 2007), a 57% reduction in pairing success for hybrids compared to Golden-winged Warblers. The trend for this data for males agrees with the significant results reported by Ficken and Ficken (1968a) and the extremely low pairing success for hybrids observed by Confer and Tupper (2000), and to the significant reduction in hybrid fitness shown by our pooled results and by the paired-t tests for our individual studies. Consideration of all available data document that male hybrids have a significant loss in reproductive fitness compared to both Golden-winged and Blue-winged warblers.
On average half the progeny of a backcross by genetically pure Golden-winged or Blue-winged warblers with a Brewster’s Warbler will have the Brewster’s phenotype. Our compilation shows that male Golden-winged Warblers are about three times more likely to form a backcross social pair than male Blue-winged Warblers. Consequently, sexual selection against hybrids has a more detrimental effect on Golden-winged than on Blue-winged warblers. This difference contributes to the replacement of Golden-winged Warblers by Blue-winged Warblers.
Despite the near-complete levels of reproductive isolation between Golden-winged and Blue-winged warblers that we document, other studies have documented high levels of introgression (Shapiro et al. 2004, Dabrowski et al. 2005; Vallender et al. 2007) and weak genome-wide differentiation (Toews et al. 2016) in this system. Our estimates of reproductive isolation might underestimate the actual level of gene flow between Golden-winged and Blue-winged Warblers. Nevertheless, our primary data analysis assessed whether divergent plumage phenotypes contribute to non-random mating in this system, not to an assessment of the actual levels of gene flow. The percentage of the total population composed of individuals with hybrid phenotypes averaged across sites (5.2%; lower 95% CI: 4.1; upper 95% CI: 6.7) is reasonably consistent with the probability of gene flow (estimated as 1 - total RI) based on the joint effects of behavioral isolation and sexual selection against hybrids averaged across sites (3.4%; lower 95% CI: 1.1%; upper 95% CI: 5.8%).