Sympatry leads to reduced body condition in chickadees that occasionally hybridize

Abstract

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 1358‐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 dataset, 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 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.

Experimental test of selection against hybridization as a driver of avian signal divergence

“…due to sexual selection) could have allowed these two species to expand into sympatry by reducing the likelihood of hybridization as they became sympatric, preventing introgression and fusion. Alternatively, mountain chickadees that historically resembled both Mexican and black‐capped chickadees in colouration would have been more likely to hybridize with black‐capped chickadees upon secondary contact; however, evidence suggests that hybridization between these species is costly (Grabenstein et al, 2022). In this scenario, any colour variant within mountain chickadees that was more divergent from black‐capped chickadees would have been favoured in sympatry because it would reduce the likelihood of hybridization.…”

“…Poecile species belong to the passerine family estimated to have the largest degree of hybridization (Paridae: Justyn et al, 2020). Black‐capped and mountain chickadees are known to interbreed to a limited extent in parts of their ranges where they co‐occur (Grabenstein et al, 2022; Graham et al, 2021; Grava et al, 2012). Despite the colour pattern differences between these species, hybrid individuals are rarely intermediate in colour phenotype, and some evidence suggests that certain hybrid individuals may be sterile (Grabenstein et al, 2022).…”

“…Black‐capped and mountain chickadees are known to interbreed to a limited extent in parts of their ranges where they co‐occur (Grabenstein et al, 2022; Graham et al, 2021; Grava et al, 2012). Despite the colour pattern differences between these species, hybrid individuals are rarely intermediate in colour phenotype, and some evidence suggests that certain hybrid individuals may be sterile (Grabenstein et al, 2022). This evidence suggests that colour pattern differences that restrict indiscriminate mating between black‐capped and mountain chickadees could serve as an advantageous premating barrier where they co‐occur.…”

“…Black-capped and mountain chickadees are known to interbreed to a limited extent in parts of their ranges where they co-occur (Grabenstein et al, 2022;Graham et al, 2021;Grava et al, 2012).…”

“…Despite the colour pattern differences between these species, hybrid individuals are rarely intermediate in colour phenotype, and some evidence suggests that certain hybrid individuals may be sterile (Grabenstein et al, 2022). This evidence suggests that colour pattern differences that restrict indiscriminate mating between F I G U R E 1 Experimental framework and setup: (a) phylogeographic framework for testing the effects of sympatry (following Noor, 1997).…”