No abstract

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, blackcapped and mountain chickadees, using both genomic and citizen science data sets.First, we genotyped 409 individuals from across both species' ranges using reducedrepresentation 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.

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.

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.

Feather coloration and patterning are major signals influencing mate choice within and between species. However, most studies of the role of plumage in mate choice have focused on colorful species with obvious sexual dichromatism. To better understand how achromatic plumage might influence hybridization, we quantified plumage variation between and within two achromatic songbirds that occasionally hybridize, Black‐capped (Poecile atricapillus) and Mountain (P. gambeli) chickadees. We collected feathers from 43 live birds and photographed 155 prepared museum specimens to measure overall plumage color and the size of the throat and cheek patches. Using principal component analyses and generalized linear mixed models, we characterized plumage patterns within and between Black‐capped and Mountain chickadees from Colorado to examine plumage color variation and differences in throat and cheek patch size. We found that Black‐capped Chickadees (1) were less achromatic and had brighter plumage with more color contrast than Mountain Chickadees, (2) had smaller throat and cheek patches than Mountain Chickadees, and (3) were not sexually dimorphic. We also found that male Mountain Chickadee museum specimens had brighter plumage with more ultraviolet reflectance than female museum specimens (i.e., they are sexually dimorphic). However, we did not observe sexual dimorphism in live Mountain Chickadees, potentially because we did not sample the supercilium. In contrast to previous studies, we found that Black‐capped Chickadees are not sexually dimorphic, potentially because plumage is not used in mating decisions for populations at lower latitudes. Between Black‐capped and Mountain chickadees, differences in plumage color and patch sizes may influence hybridization if female Mountain Chickadees prefer the brighter plumage and greater color contrast of male Black‐capped Chickadees. Our results will guide future work exploring the role plumage might play in hybridization between Black‐capped and Mountain chickadees by informing plumage manipulation experiments investigating the influence of brighter plumage and color contrast in hybridization.