Shadow of a doubt: premating and postmating isolating barriers in a temporally complex songbird (Passeriformes: Paridae) hybrid zone

Hybrid zones, where populations with incomplete reproductive isolation interact, are particularly good systems in which to study how isolating barriers evolve during speciation. Examining a hybrid zone over time or with contacts of different ages allows us to understand the relative roles of and interactions between different isolating barriers (such as selection against hybrids, innate preferences for hybrid, or parental types) and how they change with continued contact between the interacting populations. One such temporally complex hybrid zone is that of two oscine songbirds, the black-crested ( Baeolophus atricristatu s) and tufted ( B. bicolor ) titmice (family Paridae). In one region, the two populations have been interbreeding for several thousands of years; elsewhere, populations of the two species have contacted within the past century. We tested (1) if males treat other populations as potential competition; (2) if females show preferences for male phenotype; and (3) if there are reproductive consequences to hybridization. We found that older zone males responded most strongly to conspecifics, whereas younger zone discrimination is weaker; moreover, females responded most strongly to tufted song and plumage. Intrinsic postmating isolation appears to be absent even in the older part of this system. Future studies should focus on potential ecological or behavioral postmating barriers preventing expansion of the hybridization. Significance statement Hybrid zones, areas where populations with distinct differences meet and interbreed, are excellent for studying speciation. Because hybridizing populations can interbreed but do not fully merge, they allow us to understand what behaviors and intrinsic incompatibilities keep populations apart. Hybrid zones that contain different ages of contact can be particularly useful because we can see how such behaviors change with continued interaction. We found differences between behaviors in the younger and older contact zones in one such complex hybrid zone between songbirds, indicating behaviors evolve with continued interaction. Few studies as yet have examined behavior directly in a temporally complex zone as we did here, so our study provide new insights into how behavior evolves during the divergence of new species.