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Black-capped and mountain chickadee hybridization and breeding ecology in Boulder County, CO

Grabenstein, Kathryn1; Theodosopoulos, Angela2; Semenov, Georgy3; Kenyon, Haley3; Pravosudov, Vladimir4; Taylor, Scott3

Published Nov 20, 2025; Updated Dec 03, 2025 on Dryad. https://doi.org/10.5061/dryad.n2z34tnbc

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Abstract

Here, we combine three 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. This dataset includes: reproductive information and genomic information for Colorado chickadees, as well as simulation data for assigning wild birds to genomic classes (i.e., characterizing hybridization).