Extensive local introgression despite rare contemporary hybridization between two backyard songbirds

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Abstract
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.
Funding Information
  • U.S. National Science Foundation (DGE-1650115)
  • V.V.P (NSF IOS-2119824, NSF IOS-1856181)
  • S.A.T (NSF DEB-192889)
  • CU Boulder Research & Innovation Office
  • Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship (PDF-578303-2023)
  • CU Boulder’s Graduate Student Government
  • Graduate School
  • EBIO Department & Museum of Natural History
  • Indian Peaks Wilderness Alliance
  • Garden Club of America
  • Colorado Field Ornithologist Society
  • Denver Field Ornithologists
  • American Ornithological Society
  • American Museum of Natural History
  • Society for the Study of Evolution
  • Society of Systemic Biologists
  • Boulder County Naturalist Association

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