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When different species experience similar selection pressures, the probability of evolving similar adaptive solutions may be influenced by legacies of evolutionary history, such as lineage-specific changes in genetic background. Here we test for adaptive convergence in hemoglobin (Hb) function among high-altitude passerine birds that are native to the Qinghai-Tibet Plateau, and we examine whether convergent increases in Hb–O₂ affinity have a similar molecular basis in different species. We documented that high-altitude parid and aegithalid species from the Qinghai-Tibet Plateau have evolved derived increases in Hb–O₂ affinity in comparison with their closest lowland relatives in East Asia. However, convergent increases in Hb–O₂ affinity and convergence in underlying functional mechanisms were seldom attributable to the same amino acid substitutions in different species. Using ancestral protein resurrection and site-directed mutagenesis, we experimentally confirmed two cases in which parallel substitutions contributed to convergent increases in Hb–O₂ affinity in codistributed high-altitude species. In one case involving the ground tit (Parus humilis) and gray-crested tit (Lophophanes dichrous), parallel amino acid replacements with affinity-enhancing effects were attributable to nonsynonymous substitutions at a CpG dinucleotide, suggesting a possible role for mutation bias in promoting recurrent changes at the same site. Overall, most altitude-related changes in Hb function were caused by divergent amino acid substitutions, and a select few were caused by parallel substitutions that produced similar phenotypic effects on the divergent genetic backgrounds of different species.

Montane birds represent a large proportion of global avian biodiversity and are highly sensitive to seasonal climatic shifts. Many montane species undertake elevational migration in response to seasonal climate change, but how they cope with disparate environmental pressures along the elevational gradient is poorly understood. Here, we investigate phenotypic plasticity in two closely related elevational migrants that differ in their seasonal movements (small-scale vs. large-scale elevational migrants). Through common-garden experiments with adult male birds of these species, we compared gene and protein expression plasticity in response to changes in temperature and partial pressure of oxygen (PO 2 ). The small-scale elevational migrant exhibited greater plasticity in response to changes in temperature, and the large-scale elevational migrant exhibited greater plasticity in response to changes in PO 2 . These findings correspond to annual variation in environmental conditions experienced by each species, suggesting that phenotypic plasticity may co-vary with the seasonal elevational distributions of these elevational migrants. Many montane birds seasonally migrate between elevations. This study shows two bird species exhibit phenotypic plasticity in response to this altitude shift: the small-scale elevational migrant shows greater temperature-driven plasticity, while the large-scale migrant displays stronger hypoxia-driven plasticity.

Migratory birds often form flocks during spring migration and compete for territories once they arrive at breeding sites. Understanding the emergence and structure of these flocks has important implications for avian ecology and conservation, as flocking can influence migration success, resource distribution, and population resilience. In this study, we propose a sequential game model to examine if and when flocking behavior emerges and the types of flocks that form as a result of strategic decision‐making. Specifically, in the context of this study, we refer to migratory birds as “agents,” who make asynchronous decisions about their arrival time to breeding grounds. While earlier arrival can help individuals secure higher quality territories, traveling in a flock can improve foraging efficiency and lower flight energy consumption. Our model captures the competing dynamics between the benefits of arriving early (competition) and the benefits of flocking (cooperation). We study this model in the single‐species case and then extend it to a mixed‐species scenario. By analyzing the subgame perfect equilibrium (SPE) of these games, we find that when differences between territory quality are small relative to the benefits of flocking, agents tend to form larger flocks, favoring cooperation over competition. The opposite is also true—when the differences between territory quality are high, intense competition for high‐quality territories leads to smaller, less cohesive flocks, with stronger agents advancing their arrival time. Our model offers new theoretical insights on how flocking behaviors emerge that can help us understand the drivers of variation in migratory behavior observed in nature.

Atmospheric black carbon has long been recognized as a public health and environmental concern. More recently, black carbon has been identified as a major, ongoing contributor to anthropogenic climate change, thus making historical emission inventories of black carbon an essential tool for assessing past climate sensitivity and modeling future climate scenarios. Current estimates of black carbon emissions for the early industrial era have high uncertainty, however, because direct environmental sampling is sparse before the mid-1950s. Using photometric reflectance data of >1,300 bird specimens drawn from natural history collections, we track relative ambient concentrations of atmospheric black carbon between 1880 and 2015 within the US Manufacturing Belt, a region historically reliant on coal and dense with industry. Our data show that black carbon levels within the region peaked during the first decade of the 20th century. Following this peak, black carbon levels were positively correlated with coal consumption through midcentury, after which they decoupled, with black carbon concentrations declining as consumption continued to rise. The precipitous drop in atmospheric black carbon at midcentury reflects policies promoting burning efficiency and fuel transitions rather than regulating emissions alone. Our findings suggest that current emission inventories based on predictive modeling underestimate levels of atmospheric black carbon for the early industrial era, suggesting that the contribution of black carbon to past climate forcing may also be underestimated. These findings build toward a spatially dynamic emission inventory of black carbon based on direct environmental sampling.

Aim: To identify hotspots of endemic and non-endemic avian diversity in the mountains of south-west China and delineate biodiversity corridors that connect the faunas of northern and southern Asia. To understand how biodiversity and endemism in this region has been maintained through palaeoclimate change. Location: The mountains of south-west China, spanning an elevational gradient > 7000 m. Methods: We used the distributional data of 752 breeding birds to investigate current patterns of diversity across elevational and geographical space. We simulated species richness under palaeoclimate models of global temperature change, assessing changes in species richness. Results: Contemporary species richness of non-endemic birds peaked at 800-1800 m elevation, while endemic richness peaked at 2000-3000 m. Richness of non-endemic birds was highest in the southern Hengduan Mountains and Yungui Plateau, while endemic richness peaked further north, extending into the mountains along the western edge of the Sichuan Basin. Under global warming models, species richness remained high throughout the Hengduan Mountains region. Under global cooling models, the Sichuan Basin showed increased richness. Conclusions: Endemism peaked in the mountains along the western edge of the Sichuan Basin, highlighting the importance of this region in promoting and maintaining diversity. This region has likely functioned as a biodiversity corridor, bridging the Palaearctic and Oriental biotas to the north and south. Climate simulations suggest that the mountains of south-west China can accommodate upslope range shifts in response to warming, but low elevation specialists may have experienced increased extinction probabilities during cold periods in the recent past, which may in part explain the current mid-elevation diversity peak. During glacial periods the Sichuan Basin likely served as a warm refugium for montane birds. Steep environmental heterogeneity has been a key to maintaining high diversity and endemism in the region during palaeoclimate change. These same features will likely shape the effects of future climate change on biodiversity in the region.

Geographic turnover in community composition is created and maintained by eco-evolutionary forces that limit the ranges of species. One such force may be antagonistic interactions among hosts and parasites, but its general importance is unknown. Understanding the processes that underpin turnover requires distinguishing the contributions of key abiotic and biotic drivers over a range of spatial and temporal scales. Here, we address these challenges using flexible, nonlinear models to identify the factors that underlie richness (alpha diversity) and turnover (beta diversity) patterns of interacting host and parasite communities in a global biodiversity hot spot. We sampled 18 communities in the Peruvian Andes, encompassing ∼1,350 bird species and ∼400 hemosporidian parasite lineages, and spanning broad ranges of elevation, climate, primary productivity, and species richness. Turnover in both parasite and host communities was most strongly predicted by variation in precipitation, but secondary predictors differed between parasites and hosts, and between contemporary and phylogenetic timescales. Host communities shaped parasite diversity patterns, but there was little evidence for reciprocal effects. The results for parasite communities contradicted the prevailing view that biotic interactions filter communities at local scales while environmental filtering and dispersal barriers shape regional communities. Rather, subtle differences in precipitation had strong, fine-scale effects on parasite turnover while host–community effects only manifested at broad scales. We used these models to map bird and parasite turnover onto the ecological gradients of the Andean landscape, illustrating beta-diversity hot spots and their mechanistic underpinnings.

Variation in grassland vegetation structure influences the habitat selection of insectivorous birds. This variation presents a trade‐off for insectivorous predators: Arthropod abundance increases with vegetation height and heterogeneity, but access to arthropod prey items decreases. In contrast, grazing by large herbivores reduces and homogenizes vegetation, decreasing total arthropod abundance and diversity. However, the presence of livestock dung may help counteract the overall reduction in invertebrates by increasing arthropods associated with dung. It is unclear, however, how the presence of arthropod prey in dung contributes to overall habitat selection for insectivorous birds or how dung‐associated arthropods affect trade‐offs between vegetation structure, arthropod abundance, and access to prey. To explore these relationships, we studied habitat selection of the Black‐necked Crane (Grus nigricollis), a large omnivorous bird that breeds on the Qinghai–Tibet Plateau. We assessed the relationships between habitat selection of cranes and vegetation structure, arthropod abundance, and the presence of yak dung. We found that Black‐necked Cranes disproportionately foraged in grassland patches with short sward height, low sward height heterogeneity, and high numbers of dry yak dung, despite these habitats having lower total arthropod abundance. Although total arthropod abundance is lower, these habitats are supplemented with dry yak dung, which are associated with coleopteran larvae, making dung pats an indicator of food resources for breeding Black‐necked Cranes. Coleopteran adults and larvae in yak dung appear to be an important factor influencing the habitat selection of Black‐necked Cranes and should be considered when assessing grassland foraging trade‐offs of insectivorous birds. This research provides new insights into the role of livestock dung in defining foraging habitats and resources for insectivorous predators. Although total arthropod abundance is lower, these habitats are supplemented with dry yak dung, which are associated with coleopteran larvae, making dung pats an indicator of food resources for breeding Black‐necked Cranes. Coleopteran adults and larvae in yak dung appear to be an important factor influencing the habitat selection of Black‐necked Cranes and should be considered when assessing grassland foraging trade‐offs of insectivorous birds.

Functional traits are the essential phenotypes that underlie an organism's life history and ecology. Although biologists have long recognized that intraspecific variation is consequential to an animals’ ecology, studies of functional variation are often restricted to species‐level comparisons, ignoring critical variation within species. In birds, interspecific comparisons have been foundational in connecting flight muscle phenotypes to species‐level ecology, but intraspecific variation has remained largely unexplored. We asked how age‐ and sex‐dependent demands on flight muscle function are reconciled in birds. The flight muscle is an essential multifunctional organ, mediating a large range of functions associated with powered flight and thermoregulation. These functions must be balanced over an individual's lifetime. We leveraged within‐ and between‐species comparisons in a clade of small passerines (Tarsiger bush‐robins) from the eastern edge of the Qinghai–Tibet Plateau. We integrated measurements of flight muscle physiology, morphology, behaviour, phenology and environmental data, analysing trait data within a context of three widespread, adaptive life‐history strategies—sexual dichromatism, age and sex‐structured migration, and delayed plumage maturation. This approach provides a framework of the selective forces that shape functional variation within and between species. We found more variation in flight muscle traits within species than has been previously described between species of birds under 20 g. This variation was associated with the discovery of mixed muscle fibre types (i.e. both fast glycolytic and fast oxidative fibres), which differ markedly in their physiological and functional attributes. This result is surprising given that the flight muscles of small birds are generally thought to contain only fast oxidative fibres, suggesting a novel ecological context for glycolytic muscle fibres in small birds. Within each species, flight muscle phenotypes varied by age and sex, reflecting the functional demands at different life‐history stages and the pressures that individuals face as a result of their multi‐class identity (i.e. species, age and sex). Our findings reveal new links between avian physiology, ecology, behaviour and life history, while demonstrating the importance of demographic‐dependent selection in shaping functional phenotypic variation. The authors found more variation in avian flight muscle traits within species of Tarsiger bush‐robins than has been previously described between other species of small birds. This variation was associated with the discovery of mixed fibre types (glycolytic and oxidative) in the flight muscle of all three study taxa, suggesting a novel ecological context for glycolytic fibres in small birds.

The extent to which species ranges reflect intrinsic physiological tolerances is a major, unsolved question in evolutionary ecology. To date, consensus has been hindered by the limited tractability of experimental approaches across most of the tree of life. Here, we apply a macrophysiological approach to understand how hematological traits related to oxygen transport shape elevational ranges in a tropical biodiversity hotspot. Along Andean elevational gradients, we measured traits that affect blood oxygen-carrying capacity—total and cellular hemoglobin concentration and hematocrit—for 2,355 individuals of 136 bird species. We used these data to evaluate the influence of hematological traits on elevational ranges. First, we asked whether hematological plasticity is predictive of elevational range breadth. Second, we asked whether variance in hematological traits changed as a function of distance from the midpoint of the elevational range. We found that the correlation between hematological plasticity and elevational range breadth was slightly positive, consistent with a facilitative role for plasticity in elevational range expansion. We further found reduced local variation in hematological traits near elevational range limits and at high elevations, patterns consistent with intensified natural selection, reduced effective population size, or compensatory changes in other cardiohematological traits with increasing distance from species-specific optima for oxygen availability. Our findings suggest that constraints on hematological plasticity and local genetic adaptation to oxygen availability promote the evolution of the narrow elevational ranges that underpin tropical montane biodiversity.