Hybridization between closely related songbirds is related to human habitat disturbance.

Human habitat disturbances can promote hybridization between closely related, but typically reproductively isolated, species. We explored whether human habitat disturbances are related to hybridization between two closely related songbirds, black-capped and mountain chickadees, using both genomic and citizen science data sets. First, we genotyped 409 individuals from across both species' ranges using reduced-representation genome sequencing and compared measures of genetic admixture to a composite measure of human landscape disturbance. Then, using eBird observations, we compared human landscape disturbance values for sites where phenotypically diagnosed hybrids were observed to locations where either parental species was observed to determine whether hybrid chickadees are reported in more disturbed areas. We found that hybridization between black-capped and mountain chickadees positively correlates with human habitat disturbances. From genomic data, we found that (1) hybrid index (HI) significantly increased with habitat disturbance, (2) more hybrids were sampled in disturbed habitats, (3) mean HIs were higher in disturbed habitats versus wild habitats, and (4) hybrids were detected in habitats with significantly higher disturbance values than parentals. Using eBird data, we found that both hybrid and black-capped chickadees were significantly more disturbance-associated than mountain chickadees. Surprisingly, we found that nearly every black-capped chickadee we sampled contained some proportion of hybrid ancestry, while we detected very few mountain chickadee backcrosses. Our results highlight that hybridization between black-capped and mountain chickadees is widespread, but initial hybridization is rare (few F1s were detected). We conclude that human habitat disturbances can erode pre-zygotic reproductive barriers between chickadees and that post-zygotic isolation is incomplete. Understanding what becomes of recently hybridizing species following large-scale habitat disturbances is a new, but pressing, consideration for successfully preserving genetic biodiversity in a rapidly changing world.

Sympatry leads to reduced body condition in chickadees that occasionally hybridize.

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.

A highly invasive malaria parasite has expanded its range to non-migratory birds in North America.

Parasite range expansions are a direct consequence of globalization and are an increasing threat to biodiversity. Here, we report a recent range expansion of the SGS1 strain of a highly invasive parasite, Plasmodium relictum, to two non-migratory passerines in North America. Plasmodium relictum is considered one of the world's most invasive parasites and causes the disease avian malaria: this is the first reported case of SGS1 in wild non-migratory birds on the continent. Using a long-term database where researchers report avian malaria parasite infections, we summarized our current understanding of the geographical range of SGS1 and its known hosts. We also identified the most likely geographical region of this introduction event using the MSP1 allele. We hypothesize that this introduction resulted from movements of captive birds and subsequent spillover to native bird populations, via the presence of competent vectors and ecological fitting. Further work should be conducted to determine the extent to which SGS1 has spread following its introduction in North America.

Breaking Barriers: Causes, Consequences, and Experimental Utility of Human-Mediated Hybridization.

Hybridization between naturally co-occurring species that normally do not interbreed is being documented following anthropogenic habitat modifications for an increasing number of taxa. Here, we evaluate the mechanisms by which disturbance promotes hybridization and highlight the utility of human-caused hybridization for understanding evolution. Monitoring hybridization dynamics before, and following, disturbance over multiple timescales offers a unique opportunity to understand how disturbances alter species interactions and to pinpoint the mechanisms that cause species barriers to fail. Identifying the conditions promoting hybridization in disturbed habitats, the generality of these conditions across taxa, and the taxa most affected by human-mediated change is critical for furthering our understanding of human impacts on evolution and for informing management.