Kathryn C. Grabenstein

Rates of hybridization are increasing globally, but we lack an understanding of both the history and evolutionary outcomes of hybridization for most species. This makes it difficult to understand whether, and to what extent, humans are influencing hybridization. Integrating field studies with whole genome data is a critical next step for understanding hybridization and the extent of human influences on evolution. Here, we combine 3 years of population monitoring with 569 whole genomes to characterize the reproductive ecology of, and hybridization between, two common songbirds, black-capped (Poecile atricapillus) and mountain (P. gambeli) chickadees, for which hybridization is correlated with human habitat disturbance across North America. Working within a geographic region that we previously identified as a hotspot of contemporary chickadee hybridization we find that, despite geographic and temporal breeding overlap, few early generation hybrids are produced indicating that reproductive barriers typically prevent contemporary hybridization. Yet, every chickadee we sampled in sympatry possessed heterospecific ancestry, indicating that both contemporary and historical hybridization have occurred during the evolutionary history of chickadees in Colorado. Why contemporary hybridization continues to occur despite evidence for character displacement of chickadee song remains less clear, but urban forests may play a role.

Cryptic genetic differentiation is being increasingly documented in birds and other organisms using genome‐wide variation. A recent example of cryptic genetic differentiation in a widespread species with conserved morphology is the northern house wren Troglodytes aedon . We found that, despite extremely similar morphology and no documented vocal differences, the two subspecies of the northern house wren, T. a. aedon (eastern) and T. a. parkmanii (western), exhibited both nuclear and mitochondrial genomic differentiation. Individuals present along the Front Range of the Colorado Rocky Mountains possessed nuclear genetic variation intermediate between T. a. aedon and T. a. parkmanii ; additionally, both divergent mitochondrial lineages, corresponding to the western and eastern northern house wren populations, occur in Colorado. However, the dynamics of this putative contact zone (i.e. amount of hybridization or introgression) and the degree of differentiation between the two subspecies remain uncharacterized. To expand our understanding of northern house wren population genetic differentiation and explore the possibility of hybridization, we used a double digest restriction‐site associated (ddRAD) approach and sequenced 127 northern house wrens, including 109 individuals from across Colorado and Wyoming, as well as nine individuals each from eastern and western allopatric regions. Our results highlight that T. a. aedon occur significantly further west than previously thought, and provide evidence for weak population structure within the northern house wren, while generally setting the stage for future investigations of northern house wren population genomics and the genetic basis of cryptic speciation.

Character displacement theory predicts that closely-related co-occurring species should diverge in relevant traits to reduce costly interspecific interactions such as competition or hybridization. While many studies document character shifts in sympatry, few provide corresponding evidence that these shifts are driven by the costs of co-occurrence. Black-capped (Poecile atricapillus) and mountain chickadees (P. gambeli) are closely-related, ecologically similar, and broadly distributed songbirds with both allopatric and sympatric populations. In sympatry, both species appear to suffer costs of their co-occurrence: 1) both species are in worse body condition compared to allopatry and 2) hybridization sometimes yields sterile offspring. Here, we explored character displacement in the songs of black-capped and mountain chickadees by characterizing variation in male songs from sympatric and allopatric populations. We found that mountain chickadees sing differently in sympatry versus allopatry. Specifically, they produced more notes per song, were more likely to include an extra introductory note, and produced a smaller glissando in their first notes compared to all other populations. Combined with previous research on social dominance and maladaptive hybridization between black-capped and mountain chickadees, we posit that differences in sympatric mountain chickadee song are population-wide shifts to reduce aggression from dominant black-capped chickadees and/or prevent maladaptive hybridization.

Haemosporidian parasite communities are broadly similar in Boulder County, Colorado, between two common songbirds –– the Black-capped Chickadee (Poecile atricapillus) and Mountain Chickadee (Poecile gambeli). However, Mountain Chickadees appear more likely to be infected with Plasmodium, and potentially experience higher infection burdens with Leucocytozoon in contrast to Black-capped Chickadees. We found that elevation change (and associated ecology) drives the distributions of these parasite genera. For Boulder County chickadees, environmental factors play a more important role in structuring haemosporidian communities than host evolutionary differences. However, evolutionary differences are likely key to shaping the probability of infection, infection burden, and whether an infection remains detectable over time. We found that for recaptured birds, their infection status (i.e., presence or absence of detectable parasite infection) tends to remain consistent across capture periods. We sampled 234 chickadees between 2017 and 2021 across a ~1,500-m elevation gradient from low elevation (i.e., the city of Boulder) to comparatively high elevation (i.e., the CU Boulder Mountain Research Station). It is unknown whether long-term haemosporidian abundance trends have changed over time in our sampling region. However, we ask whether potentially disparate patterns of Plasmodium susceptibility and Leucocytozoon infection burden could be playing a role in the negative population trends of Mountain Chickadees.

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.

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.

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.

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.