Black-capped and mountain chickadee range-wide condition

Both abiotic and biotic drivers influence species distributions. Abiotic
drivers, such as climate, have received considerable attention, even
though biotic drivers, such as hybridization, often interact with abiotic
drivers. We sought to explore the (1) costs of co-occurrence for
ecologically similar species that hybridize and (2) associations between
ecological factors and condition to understand how abiotic and biotic
factors influence species distributions. For two closely related and
ecologically similar songbirds, black-capped and mountain chickadees, we
characterized body condition, as a proxy for fitness, using a 1,358
individual range-wide dataset. We compared body condition in sympatry and
allopatry with several abiotic and biotic factors using species-specific
generalized linear mixed models. We generated genomic data for a subset of
217 individuals to determine the extent of hybridization-driven admixture
in our dataset. Within this data subset, we found that ~11% of the
chickadees had hybrid ancestry, and all hybrid individuals had typical
black-capped chickadee plumage. In the full data set, we found that birds
of both species, independent of demographic and abiotic factors, had
significantly lower body condition when occurring in sympatry than birds
in allopatry.  This could be driven by either the inclusion of
cryptic, likely poor condition, hybrids in our full dataset, competitive
interactions in sympatry, or be due to range edge effects. We are
currently unable to discriminate between these mechanisms. Our findings
have implications for mountain chickadees in particular, which will
encounter more black-capped chickadees as black-capped chickadee ranges
shift upslope and could lead to local declines in mountain chickadee
populations.