Extensive local introgression despite rare contemporary hybridization between two backyard songbirds

Area of Science:

• Evolutionary Biology
• Ornithology
• Conservation Genetics

Background:

• Global rates of hybridization are increasing, yet the evolutionary history and outcomes remain poorly understood.
• Human influence on hybridization is difficult to assess due to limited historical and ecological data.
• Understanding hybridization is crucial for comprehending human impacts on evolution.

Purpose of the Study:

• To investigate the reproductive ecology and hybridization between black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees.
• To assess the extent of contemporary and historical hybridization in relation to human habitat disturbance.
• To explore the factors influencing ongoing hybridization in a known hotspot.

Main Methods:

• Combined 3 years of population monitoring with whole-genome sequencing of 569 individuals.
• Analyzed reproductive ecology and genetic ancestry in a region of sympatry.
• Investigated patterns of hybridization correlated with human habitat disturbance.

Main Results:

• Few early-generation hybrids were detected, suggesting reproductive barriers typically prevent contemporary hybridization.
• All sampled chickadees in sympatry showed evidence of heterospecific ancestry, indicating both historical and contemporary hybridization.
• Hybridization rates correlate with human habitat disturbance, particularly in North America.

Conclusions:

• Reproductive barriers are largely effective in preventing contemporary hybridization between these chickadee species.
• Despite barriers, historical and ongoing gene flow occurs, evidenced by widespread heterospecific ancestry.
• The role of urban forests in facilitating contemporary hybridization warrants further investigation.