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%).