Abiotic conditions shape spatial and temporal morphological variation in North American birds

Elevational constraints on flight efficiency shape global gradients in avian wing morphology

Wings with an elongated shape or larger surface area are associated with increased flight efficiency in a wide range of animals from insects to birds.1,2,3,4 Inter- and intra-specific variation in these attributes of wing shape is determined by a range of factors-including foraging ecology, migration, and climatic seasonality5,6,7,8-all of which may drive latitudinal gradients in wing morphology.9,10 A separate hypothesis predicts that wing shape should also follow an elevational gradient5,11 because air density declines with altitude,12 altering the aerodynamics of flight and driving the evolution of more efficient wings in high-elevation species to compensate for reduced lift.13,14,15 Although previous analyses have shown a tendency for longer or larger wings at higher elevations, at least locally,16,17,18,19,20 it is difficult to rule out a range of alternative explanations since we currently lack a global synthesis of elevational gradients in wing shape for any taxonomic group. In this study, we use phylogenetic models to explore elevational effects on metrics of wing morphology linked to aerodynamic function in 9,982 bird species while simultaneously controlling for multiple climatic factors and ecological attributes of species. We found that relative wing elongation (hand-wing index) and wing area increase with elevation, even when accounting for latitude, temperature seasonality, body mass, habitat, aerial lifestyle, and altitudinal migration. These results confirm a pervasive elevational gradient in avian wing morphology and suggest that aerodynamic constraints linked to air density, perhaps coupled with oxygen deficiency, contribute to global patterns of trait evolution in flying animals.

Eye catching light: Anthropogenic light at night and its evolutionary influence on the avian eye

Variation among avian species in their responses to artificial night lighting was recently linked to differences in dim light vision, but whether dim light vision is under selection from human-caused night lighting is unexplored. Here, we approximated dim light vision using eye geometries from museum specimens of six species collected across 100+ years and sought to determine whether proxies for artificial night lighting were related to within-species variation in dim light vision. We found variation in dim light vision was strongly linked to artificial night lighting proxies for three species and weakly linked for a forth, but the relationship varied by species. This variation is likely related to differences in ecological traits and may also reflect whether artificial night lighting facilitates increased temporal activity or results in physiological costs for each species. These results suggest that altered sensory environments are likely a source of selection shaping the sensory abilities of animals in the anthropocene.

Understanding mechanisms of avian flight by integrating observations with tests of competing hypotheses

A long-standing problem in the study of avian flight is determining how biomechanics and physiology are associated with behaviour, ecological interactions and evolution. In some avian clades, flight mechanisms are strongly linked to ecology. Hummingbirds, for example, exhibit traits that support both hovering flight and nectar foraging. In most avian clades, however, features such as wing shape are highly variable among taxa without clear relationships to biomechanics, energetics or ecology. In this Commentary, we discuss challenges to understanding associations between phenotype and performance in avian flight. A potential pitfall in studies that attempt to link trait specialization with performance is that the most relevant traits and environments are not being considered. Additionally, a large number of studies of the mechanisms of avian flight are highly phenomenological. Although observations are essential for hypothesis development, we argue that for our discipline to make progress, we will need much more integration of the observational phase with developing crucial tests of competing hypotheses. Direct comparison of alternative hypotheses can be accomplished through analytical frameworks as well as through experimentation.

Genetic and morphological shifts associated with climate change in a migratory bird

BACKGROUND

Rapid morphological change is emerging as a consequence of climate change in many systems. It is intuitive to hypothesize that temporal morphological trends are driven by the same selective pressures that have established well-known ecogeographic patterns over spatial environmental gradients (e.g., Bergman's and Allen's rules). However, mechanistic understanding of contemporary morphological shifts is lacking.

RESULTS

We combine morphological data and whole genome sequencing from a four-decade dataset in the migratory bird hermit thrush (Catharus guttatus) to test whether morphological shifts over time are accompanied by genetic change. Using genome-wide association, we identify alleles associated with body size, bill length, and wing length. Shifts in morphology and concordant shifts in morphology-associated alleles over time would support a genetic basis for the observed changes in morphology over recent decades, potentially an adaptive response to climate change. In our data, bill size decreases were paralleled by genetic shifts in bill size-associated alleles. On the other hand, alleles associated with body size showed no shift in frequency over time.

CONCLUSIONS

Together, our results show mixed support for evolutionary explanations of morphological response to climate change. Temporal shifts in alleles associated with bill size support the hypothesis that selection is driving temporal morphological trends. The lack of evidence for genetic shifts in body size alleles could be explained by a large role of plasticity or technical limitations associated with the likely polygenic architecture of body size, or both. Disentangling the mechanisms responsible for observed morphological response to changing environments will be vital for predicting future organismal and population responses to climate change.

Long-term increases in wing length occur independently of changes in climate and climate-driven shifts in body size

Recent widespread reductions in body size across species have been linked to increasing temperatures; simultaneous increases in wing length relative to body size have been broadly observed but remain unexplained. Size and shape may change independently of one another, or these morphological shifts may be linked, with body size mediating or directly driving the degree to which shape changes. Using hierarchical Bayesian models and a morphological time series of 27 366 specimens from five North American migratory passerine bird species, we tested the roles that climate and body size have played in shifting wing length allometry over four decades. We found that colder temperatures and reduced precipitation during the first year of life were associated with increases in wing length relative to body size but did not explain long-term increases in wing length. We found no conclusive evidence that the slope of the relationship between body size and wing length changed among adult birds in response to any climatic variable or through time, suggesting that body size does not mediate shifts in relative wing length. Together, these findings suggest that long-term increases in wing length are not a compensatory adaptation mediated by size reductions, but rather are driven by non-climatic factors.

Integrating Spatial Analyses of Genomic and Physiological Data to Understand Avian Responses to Environmental Change

Projected rates of climate change over the next century are expected to force species to shift ranges, adapt, or acclimate to evade extinction. Predicting which of these scenarios may be most likely is a central challenge for conserving biodiversity in the immediate future. Modeling frameworks that take advantage of intraspecific variation across environmental gradients can be particularly important for meeting this challenge. While these space-for-time approaches are essential for climatic and genomic modeling approaches, mechanistic models that incorporate ecological physiology data into assessing species vulnerabilities rarely include intraspecific variation. A major reason for this gap is the general lack of empirical data on intraspecific geographic variation in avian physiological traits. In this review, we outline the evidence for and processes shaping geographic variation in avian traits. We use the example of evaporative water loss to underscore the lack of research on geographic variation, even in traits central to cooling costs in birds. We next demonstrate how shifting the focus of avian physiological research to intraspecific variation can facilitate greater integration with emerging genomics approaches. Finally, we outline important next steps for an integrative approach to advance understanding of avian physiological adaptation within species. Addressing the knowledge gaps outlined in this review will contribute to an improved predictive framework that synthesizes environmental, morphological, physiological, and genomic data to assess species specific vulnerabilities to a warming planet.

Shorebirds Are Shrinking and Shape-Shifting: Declining Body Size and Lengthening Bills in the Past Half-Century

Animals are predicted to shrink and shape-shift as the climate warms, declining in size, while their appendages lengthen. Determining which types of species are undergoing these morphological changes, and why, is critical to understanding species responses to global change, including potential adaptation to climate warming. We examine body size and bill length changes in 25 shorebird species using extensive field data (> 200,000 observations) collected over 46 years (1975-2021) by community scientists. We show widespread body size declines over time, and after short-term exposure to warmer summers. Meanwhile, shorebird bills are lengthening over time but shorten after hot summers. Shrinking and shape-shifting patterns are consistent across ecologically diverse shorebirds from tropical and temperate Australia, are more pronounced in smaller species and vary according to migration behaviour. These widespread morphological changes could be explained by multiple drivers, including adaptive and maladaptive responses to nutritional stress, or by thermal adaptation to climate warming.

Advances in breeding phenology outpace latitudinal and elevational shifts for North American birds tracking temperature

Terrestrial species can respond to a warming climate in multiple ways, including shifting in space (via latitude or elevation) and time (via phenology). Evidence for such shifts is often assessed independent of other temperature-tracking mechanisms; critically, no study has compared shifts across all three spatiotemporal dimensions. Here we used two continental-scale monitoring databases to estimate trends in the breeding latitude (311 species), elevation (251 species) and phenology (111 species) of North American landbirds over 27 years, with a shared pool of 102 species. We measured the magnitude of shifts and compared them relative to average regional warming (that is, shift ratios). Species shifted poleward (1.1 km per year, mean shift ratio 11%) and to higher elevations (1.2 m per year, mean shift ratio 17%), while also shifting their breeding phenology earlier (0.08 days per year, mean shift ratio 28%). These general trends belied substantial variation among species, with some species shifting faster than climate, whereas others shifted more slowly or in the opposite direction. Across the three dimensions (n = 102), birds cumulatively tracked temperature at 33% of current warming rates, 64% of which was driven by advances in breeding phenology as opposed to geographical shifts. A narrow focus on spatial dimensions of climate tracking may underestimate the responses of birds to climate change; phenological shifts may offer an alternative for birds-and probably other organisms-to conserve their thermal niche in a warming world.

Can internal range structure predict range shifts?

Poleward and uphill range shifts are a common-but variable-response to climate change. We lack understanding regarding this interspecific variation; for example, functional traits show weak or mixed ability to predict range shifts. Characteristics of species' ranges may enhance prediction of range shifts. However, the explanatory power of many range characteristics-especially within-range abundance patterns-remains untested. Here, we introduce a hypothesis framework for predicting range-limit population trends and range shifts from the internal structure of the geographic range, specifically range edge hardness, defined as abundance within range edges relative to the whole range. The inertia hypothesis predicts that high edge abundance facilitates expansions along the leading range edge but creates inertia (either more individuals must disperse or perish) at the trailing range edge such that the trailing edge recedes slowly. In contrast, the limitation hypothesis suggests that hard range edges are the signature of strong limits (e.g. biotic interactions) that force faster contraction of the trailing edge but block expansions at the leading edge of the range. Using a long-term avian monitoring dataset from northern Minnesota, USA, we estimated population trends for 35 trailing-edge species and 18 leading-edge species and modelled their population trends as a function of range edge hardness derived from eBird data. We found limited evidence of associations between range edge hardness and range-limit population trends. Trailing-edge species with harder range edges were slightly more likely to be declining, demonstrating weak support for the limitation hypothesis. In contrast, leading-edge species with harder range edges were slightly more likely to be increasing, demonstrating weak support for the inertia hypothesis. These opposing results for the leading and trailing range edges might suggest that different mechanisms underpin range expansions and contractions, respectively. As data and state-of-the-art modelling efforts continue to proliferate, we will be ever better equipped to map abundance patterns within species' ranges, offering opportunities to anticipate range shifts through the lens of the geographic range.

Seasonality Structures Avian Functional Diversity and Niche Packing Across North America

Assemblages in seasonal ecosystems undergo striking changes in species composition and diversity across the annual cycle. Despite a long-standing recognition that seasonality structures biogeographic gradients in taxonomic diversity (e.g., species richness), our understanding of how seasonality structures other aspects of biodiversity (e.g., functional diversity) has lagged. Integrating seasonal species distributions with comprehensive data on key morphological traits for bird assemblages across North America, we find that seasonal turnover in functional diversity increases with the magnitude and predictability of seasonality. Furthermore, seasonal increases in bird species richness led to a denser packing of functional trait space, but functional expansion was important, especially in regions with higher seasonality. Our results suggest that the magnitude and predictability of seasonality and total productivity can explain the geography of changes in functional diversity with broader implications for understanding species redistribution, community assembly and ecosystem functioning.

Differential changes in the morphology and fuel loads of obligatory and partial migrant passerines over half a century in Britain

Migratory distances and stopover locations are changing for many passerines in response to climate change. Morphological changes have been linked to rising global temperatures in both migrants and residents, but the implications of these changes on fuel loads, and associated flight ranges are little studied. Wing length and body mass changes between 1964 and 2020 were calculated for 15 migrant and partially migrant passerines in Britain. Changes in fuel load and lean body mass were also estimated and used to predict flight ranges. Twelve of the species have undergone morphological changes and eight species, estimated fuel load changes. Nine species were estimated to have reduced flight ranges, indicating that the morphological changes have not compensated fully for the reduction in flight range experienced since 1964. Partial migrants showed greater decreases in flight ranges than did full migrants, which may indicate greater behavioural plasticity in the former. Those species which do not adapt morphologically or behaviourally may be unable to complete long migrations, resulting in restriction to sub-optimal breeding/wintering habitats, or a need for a sooner first stop and more stops en route. This highlights the importance of conserving migratory stopover sites, particularly in the Mediterranean and North Africa that immediately precede major geographical barriers, as-well-as breeding and wintering grounds.

A chromosome-level genome of Chenghua pig provides new insights into the domestication and local adaptation of pigs

As a country with abundant genetic resources of pigs, the domestication history of pigs in China and the adaptive evolution of Chinese pig breeds at different latitudes have rarely been elucidated at the genome-wide level. To fill this gap, we first assembled a high-quality chromosome-level genome of the Chenghua pig and used it as a benchmark to analyse the genomes of 272 samples from three genera of three continents. The divergence of the three species belonging to three genera, Phacochoerus africanus, Potamochoerus porcus, and Sus scrofa, was assessed. The introgression of pig breeds redefined that the migration routes were basically from southern China to central and southwestern China, then spread to eastern China, arrived in northern China, and finally reached Europe. The domestication of pigs in China occurred ∼12,000 years ago, earlier than the available Chinese archaeological domestication evidence. In addition, FBN1 and NR6A1 were identified in our study as candidate genes related to extreme skin thickness differences in Eurasian pig breeds and adaptive evolution at different latitudes in Chinese pig breeds, respectively. Our study provides a new resource for the pig genomic pool and refines our understanding of pig genetic diversity, domestication, migration, and adaptive evolution at different latitudes.

Selective effect of winter weather on a songbird's morphology depends on individual sex and winter condition

Knowledge of the effect of harsh weather on the phenotypic traits of organisms is essential for understanding the environmental influence on phenotype evolution and holds implications for predicting how species respond to current climate change. For many birds, harsh weather in winter often imposes a strong selective effect on their survival, and only the individuals with certain phenotypes may survive. However, whether the selective effect on phenotype varies with winter weather conditions has been poorly investigated. Here, we explored the selective effect of winter weather on black-throated tit's (Aegithalos concinnus) morphological traits under winters with and without severe snowstorms. We found that for males, the sizes of their bills, heads and wings significantly affected their overwinter survival, but the effects varied with winter conditions. In relatively benign winters, males with smaller bill depths, smaller bill surface areas, and greater head lengths survived better; whereas, in winters with severe snowstorms, a reverse pattern was found. This phenomenon was likely driven by selection pressures from heat retention and foraging requirements, with their relative importance depending on winter conditions. Additionally, wing length was positively correlated with male survival and the relationship was stronger in harsher winters, which was probably due to longer wings' higher flight efficiency in adverse weather. By contrast, we found no correlation between morphological traits and survival in females. These results suggest a sex-specific and condition-dependent selective effect of environment on bird phenotypes, implying complicated interactions between different selection pressures and phenotype evolution.

Context-dependent bird body mass responses to climate change

Previous studies demonstrated decreasing body size of birds in response to rising temperatures. Recently, Neate-Clegg et al. documented that birds have been becoming larger in an Afromontane forest over four decades. This highlights the complexity of morphological responses to climate, the importance of context, and the need to study phenomena in a diversity of regions.

Endocrine flexibility can facilitate or constrain the ability to cope with global change

Global climate change has increased average environmental temperatures world-wide, simultaneously intensifying temperature variability and extremes. Growing numbers of studies have documented phenological, behavioural and morphological responses to climate change in wild populations. As systemic signals, hormones can contribute to orchestrating many of these phenotypic changes. Yet little is known about whether mechanisms like hormonal flexibility (reversible changes in hormone concentrations) facilitate or limit the ability of individuals, populations and species to cope with a changing climate. In this perspective, we discuss different mechanisms by which hormonal flexibility, primarily in glucocorticoids, could promote versus hinder evolutionary adaptation to changing temperature regimes. We focus on temperature because it is a key gradient influenced by climate change, it is easy to quantify, and its links to hormones are well established. We argue that reaction norm studies that connect individual responses to population-level and species-wide patterns will be critical for making progress in this field. We also develop a case study on urban heat islands, where several key questions regarding hormonal flexibility and adaptation to climate change can be addressed. Understanding the mechanisms that allow animals to cope when conditions become more challenging will help in predicting which populations are vulnerable to ongoing climate change. This article is part of the theme issue 'Endocrine responses to environmental variation: conceptual approaches and recent developments'.

The incredible shrinking puffin: Decreasing size and increasing proportional bill size of Atlantic puffins nesting at Machias Seal Island

Climate change imposes physiological constraints on organisms particularly through changing thermoregulatory requirements. Bergmann's and Allen's rules suggest that body size and the size of thermoregulatory structures differ between warm and cold locations, where body size decreases with temperature and thermoregulatory structures increase. However, phenotypic plastic responses to malnutrition during development can result in the same patterns while lacking fitness benefits. The Gulf of Maine (GOM), located at the southern end of the Labrador current, is warming faster than most of the world's oceans, and many of the marine species that occupy these waters exist at the southern edge of their distributions including Atlantic puffins (Fratercula arctica; hereafter "puffin"). Monitoring of puffins in the GOM, at Machias Seal Island (MSI), has continued annually since 1995. We asked whether changes in adult puffin body size and the proportional size of bill to body have changed with observed rapid ocean warming. We found that the size of fledgling puffins is negatively related to sea surface temperature anomalies (warm conditions = small fledgers), adult puffin size is related to fledgling size (small fledgers = small adults), and adult puffins have decreased in size in recent years in response to malnutrition during development. We found an increase in the proportional size of bill to wing chord, likely in response to some mix of malnutrition during development and increasing air temperatures. Although studies have assessed clinal variation in seabird morphology with temperature, this is the first study addressing changes in seabird morphology in relation to ocean warming. Our results suggest that puffins nesting in the GOM have morphological plasticity that may help them acclimate to ocean warming.

Space-for-time substitutions in climate change ecology and evolution

In an epoch of rapid environmental change, understanding and predicting how biodiversity will respond to a changing climate is an urgent challenge. Since we seldom have sufficient long-term biological data to use the past to anticipate the future, spatial climate-biotic relationships are often used as a proxy for predicting biotic responses to climate change over time. These 'space-for-time substitutions' (SFTS) have become near ubiquitous in global change biology, but with different subfields largely developing methods in isolation. We review how climate-focussed SFTS are used in four subfields of ecology and evolution, each focussed on a different type of biotic variable - population phenotypes, population genotypes, species' distributions, and ecological communities. We then examine the similarities and differences between subfields in terms of methods, limitations and opportunities. While SFTS are used for a wide range of applications, two main approaches are applied across the four subfields: spatial in situ gradient methods and transplant experiments. We find that SFTS methods share common limitations relating to (i) the causality of identified spatial climate-biotic relationships and (ii) the transferability of these relationships, i.e. whether climate-biotic relationships observed over space are equivalent to those occurring over time. Moreover, despite widespread application of SFTS in climate change research, key assumptions remain largely untested. We highlight opportunities to enhance the robustness of SFTS by addressing key assumptions and limitations, with a particular emphasis on where approaches could be shared between the four subfields.

Spatial and seasonal variation in thermal sensitivity within North American bird species

Responses of wildlife to climate change are typically quantified at the species level, but physiological evidence suggests significant intraspecific variation in thermal sensitivity given adaptation to local environments and plasticity required to adjust to seasonal environments. Spatial and temporal variation in thermal responses may carry important implications for climate change vulnerability; for instance, sensitivity to extreme weather may increase in specific regions or seasons. Here, we leverage high-resolution observational data from eBird to understand regional and seasonal variation in thermal sensitivity for 21 bird species. Across their ranges, most birds demonstrated regional and seasonal variation in both thermal peak and range, or the temperature and range of temperatures when observations peaked. Some birds demonstrated constant thermal peaks or ranges across their geographical distributions, while others varied according to local and current environmental conditions. Across species, birds typically demonstrated either geographical or seasonal adaptation to climate. Local adaptation and phenotypic plasticity are likely important but neglected aspects of organismal responses to climate change.

Temperature-dependent Developmental Plasticity and Its Effects on Allen's and Bergmann's Rules in Endotherms

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species diversity on our planet. Among the most well-known ecogeographical rules are Bergmann's rule and Allen's rule, with each correlating ambient temperature to the size and shape of endotherms within a species. In recent years, these two rules have attracted renewed research attention, largely with the goal of understanding how they emerge (e.g., via natural selection or phenotypic plasticity) and, thus, whether they may emerge quickly enough to aid adaptations to a warming world. Yet despite this attention, the precise proximate and ultimate drivers of Bergmann's and Allen's rules remain unresolved. In this conceptual paper, we articulate novel and classic hypotheses for understanding whether and how plastic responses to developmental temperatures might contributed to each rule. Next, we compare over a century of empirical literature surrounding Bergmann's and Allen's rules against our hypotheses to uncover likely avenues by which developmental plasticity might drive temperature-phenotype correlations. Across birds and mammals, studies strongly support developmental plasticity as a driver of Bergmann's and Allen's rules, particularly with regards to Allen's rule. However, plastic contributions toward each rule appear largely non-linear and dependent upon: (1) efficiency of energy use (Bergmann's rule) and (2) thermal advantages (Allen's rule) at given ambient temperatures. These findings suggest that, among endotherms, rapid changes in body shape and size will continue to co-occur with our changing climate, but generalizing the direction of responses across populations is likely naive.

Decoupling pioneering traits from latitudinal patterns in a north American bird experiencing a southward range shift

Ecogeographic rules describe spatial patterns in biological trait variation and shed light on the drivers of such variation. In animals, a consensus is emerging that 'pioneering' traits may facilitate range shifts via a set of bold, aggressive and stress-resilient traits. Many of these same traits are associated with more northern latitudes, and most range shifts in the northern hemisphere indicate northward movement. As a consequence, it is unclear whether pioneering traits are simply corollaries of existing latitudinal variation, or whether they override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation. The tree swallow Tachycineta bicolor is a songbird undergoing a southward range shift in the eastern United States, in direct opposition of the poleward movement seen in most other native species' range shifts. Because this organic range shift countervails the typical direction of movement, this case study provides for unique ecological insights on organisms and their ability to thrive in our changing world. We sampled female birds across seven populations, quantifying behavioural, physiological and morphological traits. We also used GIS and field data to quantify a core set of ecological factors with strong ties to these traits as well as female performance. Females at more southern expansion sites displayed higher maternal aggression, higher baseline corticosterone and more pronounced elevation of corticosterone following a standardized stressor, contrary to otherwise largely conserved latitudinal patterning in these traits. Microhabitat variation explained some quantitative phenotypic variation, but the expansion and historic ranges did not differ in openness, distance to water or breeding density. This countervailing range shift therefore suggests that pioneering traits are not simply corollaries of existing latitudinal variation, but rather, they may override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation.