Geographically consistent hybridization dynamics between the Black-crested and Tufted titmouse with evidence of hybrid zone expansion

We studied hybridization between the Black-crested and Tufted titmouse across two geographically distinct transects that differ in the timing of secondary contact by hundreds to thousands of years. We found that hybridization patterns correspond to localized hybrid swarms and that the titmouse hybrid zone is likely slowly expanding over time, a product of short post-natal dispersal distances coupled with weak or absent selection against admixture. We show the southern part of the hybrid zone located in Texas is four times wider than the northern region of hybridization in Oklahoma, which is likely due to geographic differences in hybrid zone age. Despite differences in width, most individuals in both transects are advanced-generation hybrids and backcrosses, suggesting geographically consistent hybridization dynamics. We documented a strong correlation between genotypes and plumage index, suggesting that hybridization has not yet resulted in the decoupling of plumage and genome-wide ancestry as observed in some other avian hybrid zones. Although our results suggest the ongoing expansion of the hybrid zone, the rate of expansion appears to be slow, on the scale of tens of meters a year, and it will likely take hundreds of thousands to millions of years before homogenization of the parental populations. While we did not find support for partial reproductive isolation in the hybrid zone itself, there is the possibility that ecological or sexual selection limits introgression into allopatric regions. Broadly, the results of our study highlight the value of multiple, geographically distant, transects across a hybrid zone for assessing the evolutionary dynamics of hybridizing lineages. We studied hybridization patterns between Black-crested and Tufted titmice in two areas of their contact zone that differ in the age of onset of hybridization by hundreds or thousands of years. We show that northern (younger) parts of the hybrid zone have a genetic transition four times narrower than the southern (older) region, which plausibly reflects the differences in the hybrid zone age or is a result of differences in the breadth of the ecotone between titmouse habitats in the north and south. Our results suggest that the Black-crested and Tufted titmouse hybrid zone might be in the process of slowly expanding following selectively neutral hybridization dynamics, although we cannot completely rule out a possible role of ecological or sexual selection in limiting gene flow ou