Hybridization is an important aspect of speciation, yet questions remain about the ecological and environmental factors that influence hybridization among wild populations. We used microsatellite genotyping data and collected land cover and environmental data for four North American chickadee species: black-capped Poecile atricapillus, mountain P. gambeli, chestnut-backed P. rufescens and boreal P. hudsonicus chickadees. Combining these datasets, we sought to examine whether there is evidence of admixture between four widely distributed North American chickadee species; whether admixture takes place more often between more closely related species pairs or between species pairs with more similar ecological preferences; and whether certain habitat types have higher rates of admixture than others. We detected admixture for five of the six species pairs analyzed (chestnut-backed–mountain chickadee pair showed no evidence of admixture), and found rates of admixture varied geographically, and within taxa pairs. Admixture was higher among less closely related species than more closely related species, although habitat similarity was not a significant predictor. Finally, rates of admixture were higher in urban parkland habitats than deciduous, mixed or coniferous forest habitats. Our work indicates admixture occurs frequently among North American parids, and habitat and environmental variation may play an important role in the frequency and geographic distribution of hybridization.

We collected blood samples from 1428 chickadees (representing four common North American species) from 19 geographic data. Individuals were genotyped at five microsatellite loci. We ran species pairs through STRUCTURE at each location to identify individuals with mixed ancestry. We examined the rate of admixture among species pairs and determined whether admixture occurs more frequently between more closely related species pairs (i.e. between species within the black-headed clade or brown-headed clade) or between species pairs with more similar ecological preferences? We also correlate admixture with environment and elevation data to identify habitat and environmental characteristics that may influence admixture among parids. Habitat and environmental data were obtained from the WorldClim dataset for the environmental analyses, and we used COI data to examine and characterize the phylogenetic relationship among the four chickadee species.