Haley L. Kenyon

Rates of hybridization are increasing globally, but we lack an understanding of both the history and evolutionary outcomes of hybridization for most species. This makes it difficult to understand whether, and to what extent, humans are influencing hybridization. Integrating field studies with whole genome data is a critical next step for understanding hybridization and the extent of human influences on evolution. Here, we combine 3 years of population monitoring with 569 whole genomes to characterize the reproductive ecology of, and hybridization between, two common songbirds, black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees, for which hybridization is correlated with human habitat disturbance across North America. Working within a geographic region that we previously identified as a hotspot of contemporary chickadee hybridization we find that, despite geographic and temporal breeding overlap, few early generation hybrids are produced indicating that reproductive barriers typically prevent contemporary hybridization. Yet, every chickadee we sampled in sympatry possessed heterospecific ancestry, indicating that both contemporary and historical hybridization have occurred during the evolutionary history of chickadees in Colorado. Why contemporary hybridization continues to occur despite evidence for character displacement of chickadee song remains less clear, but urban forests may play a role.

Signal divergence may be pivotal in the generation and maintenance of new biodiversity by allowing closely related species to avoid some costs of co-occurrence. In birds, closely related, sympatric species are more divergent in their colour patterns than those that live apart, but the selective pressures driving this pattern remain unclear. Traditionally, signal divergence among sympatric species is thought to result from selection against hybridization, but broad evidence is lacking. Here, we conducted field experiments on naive birds using spectrometer-matched, painted 3D-printed models to test whether selection against hybridization drives colour pattern divergence in the genus Poecile. To address selection for male colour pattern divergence without the influence of learning or the evolution of female discrimination in sympatry, we simulated secondary contact between Poecile species, and conducted mate choice experiments on naive, allopatric females. We found that female black-capped chickadees (Poecile atricapillus) are equally likely to perform copulation solicitation displays to sympatric and allopatric heterospecific congeners when they are paired with conspecifics, but exhibit a strong preference for less divergent males when presented with paired heterospecific congeners. These results suggest that increased colour pattern divergence among sympatric species can reduce the likelihood of mixed mating in some contexts, and therefore should be favoured by selection against hybridization.

Aggressive interactions help individuals to gain access to and defend resources, but they can be costly, leading to increased predation risk, injury, or death. Signals involving sounds and color can allow birds to avoid the costs of intraspecific aggressive encounters, but we know less about agonistic signaling between species, where fights can be frequent and just as costly. Here, we review photographic and video evidence of aggressive interactions among species of birds (N = 337 interactions documenting the aggressive signals of 164 different bird species from 120 genera, 50 families, and 24 orders) to document how individuals signal in aggressive encounters among species, and explore whether these visual signals are similar to those used in aggressive encounters with conspecifics. Despite the diversity of birds examined, most aggressively signaling birds displayed weapons (bills, talons, wings) used in fighting and placed these weapons closest to their heterospecific opponent when signaling. Most species oriented their bodies and heads forward with their bills pointing towards their heterospecific opponent, often highlighting their face, throat, mouth, and bill. Many birds also opened their wings and/or tails, increasing their apparent size in displays, consistent with the importance of body size in determining behavioral dominance among species. Aggressive postures were often similar across species and taxonomic families. Exceptions included Accipitridae and Falconidae, which often highlighted their talons in the air, Columbidae, which often highlighted their underwings from the side, and Trochilidae, which often hovered upright in the air and pointed their fanned tail downward. Most species highlighted bright carotenoid-based colors in their signals, but highlighted colors varied across species and often involved multiple colors in combination (e.g., black, white, and carotenoid-based colors). Finally, birds tended to use the same visual signals in aggressive encounters with heterospecifics that they use in aggressive encounters with conspecifics, suggesting that selection from aggressive interactions may act on the same signaling traits regardless of competitor identity.

Signal divergence is thought to reduce the costs of co‐occurrence for closely related species and may thereby be important in the generation and maintenance of new biodiversity. In birds, closely related, sympatric species are more divergent in their color patterns than those that live apart, but the selective pressures driving sympatric divergence in color pattern are not well understood. Here, we conducted field experiments on naïve birds using spectrometer‐matched, painted 3D‐printed models to test whether selection against heterospecific aggression might drive color pattern divergence in the genus Poecile. We found that territorial male black‐capped chickadees (P. atricapillus) are equally likely to attack sympatric and allopatric congeners, and wintering flocks are equally likely to visit feeders occupied by sympatric and allopatric congeners, despite sympatric congeners being more divergent in color pattern. These results suggest that either the concerted evolution of additional traits (e.g., discrimination), or interactions in sympatry that promote learning, are required if color pattern divergence among sympatric species is to reduce heterospecific aggression. Alternatively, color pattern divergence among sympatric species may be caused by other selective pressures, such as selection against hybridization or habitat partitioning and secondary signal adaptation.