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The ability of animals to sync the timing and location of molting (the replacement of hair, skin, exoskeletons or feathers) with peaks in resource availability has important implications for their ecology and evolution. In migratory birds, the timing and location of pre-migratory feather molting, a period when feathers are shed and replaced with newer, more aerodynamic feathers, can vary within and between species. While hypotheses to explain the evolution of intraspecific variation in the timing and location of molt have been proposed, little is known about the genetic basis of this trait or the specific environmental drivers that may result in natural selection for distinct molting phenotypes. Here we take advantage of intraspecific variation in the timing and location of molt in the iconic songbird, the Painted Bunting (Passerina ciris) to investigate the genetic and ecological drivers of distinct molting phenotypes. Specifically, we use genome-wide genetic sequencing in combination with stable isotope analysis to determine population genetic structure and molting phenotype across thirteen breeding sites. We then use genome-wide association analysis (GWAS) to identify a suite of genes associated with molting and pair this with gene-environment association analysis (GEA) to investigate potential environmental drivers of genetic variation in this trait. Associations between genetic variation in molt-linked genes and the environment are further tested via targeted SNP genotyping in 25 additional breeding populations across the range. Together, our integrative analysis suggests that molting is in part regulated by genes linked to feather development and structure (GLI2andCSPG4) and that genetic variation in these genes is associated with seasonal variation in precipitation and aridity. Overall, this work provides important insights into the genetic basis and potential selective forces behind phenotypic variation in what is arguably one of the most important fitness-linked traits in a migratory bird.

Studies of Antarctic paleo‐archives have produced conflicting hypotheses on the relative impact of long‐term climate change and historic exploitation of marine mammals on Southern Ocean krill predator foraging ecology. We disentangle these hypotheses using amino acid stable isotope analysis on a 7000‐yr Holocene archive of Adélie penguin (Pygoscelis adeliae) eggshells to differentiate variation in diet and trophic dynamics from baseline biogeochemical cycling as drivers of the rapid decline in krill predator bulk tissue δ¹⁵N values in recent centuries. Contrary to previous hypotheses suggesting solely trophic dynamic mechanisms as drivers of this decline, we identified an abrupt decline in source amino acid δ¹⁵N values, indicative of major changes in biogeochemical cycling at the base of the Southern Ocean food web that mirrored the decline in penguin bulk tissue δ¹⁵N values. These abrupt shifts in penguin δ¹⁵N values and associated biogeochemical cycling aligned with climatic events during the Little Ice Age that decreased surface δ¹⁵N_NO3−, likely connected to a proposed increase in Ekman upwelling via a southward migration of the Westerlies. This baseline shift was in addition to a long‐term, gradual decline in penguin trophic position over the Holocene that began prior to both recent anthropogenic climate change and a proposed “krill‐surplus” following historic marine mammal exploitation in the 19^thand 20^thcenturies. In resolving these outstanding hypotheses about drivers of Southern Ocean food web dynamics, this study emphasizes the fundamental importance of climate‐induced variability in biogeochemical cycling on ecological processes and improves the ability of paleo‐archives to inform the ecological consequences of future environmental change in the Southern Ocean.

Geographic distribution models of environmentally stable isotopes (the so‐called “isoscapes”) are widely employed in animal ecology, and wildlife forensics and conservation. However, the application of isoscapes is limited to elements and regions for which the spatial patterns have been estimated. Here, we focused on the ubiquitous yet less commonly used stable sulfur isotopes (δ³⁴S). To predict the European δ³⁴S isoscape, we used 242 feather samples from Eurasian Reed Warbler (Acrocephalus scirpaceus) formed at 69 European wetland sites. We quantified the relationships between sample δ³⁴S and environmental covariates using a random forest regression model and applied the model to predict the geographic distribution of δ³⁴S. We also quantified within‐site variation in δ³⁴S and complementarity with other isotopes on both individual and isoscape levels. The predicted feather δ³⁴S isoscape shows only slight differences between the central and southern parts of Europe while the coastal regions were most enriched in³⁴S. The most important covariates of δ³⁴S were distance to coastline, surface elevation, and atmospheric concentrations of SO₂gases. The absence of a systematic spatial pattern impedes the application of the δ³⁴S isoscape, but high complementarity with other isoscapes advocates the combination of multiple isoscapes to increase the precision of animal tracing. Feather δ³⁴S compositions showed considerable within‐site variation with highest values in inland parts of Europe, likely attributed to wetland anaerobic conditions and redox sensitivity of sulfur. The complex European geography and topography as well as using δ³⁴S samples from wetlands may contribute to the absence of a systematic spatial gradient of δ³⁴S values in Europe. We thus encourage future studies to focus on the geographic distribution of δ³⁴S using tissues from diverse taxa collected in various habitats over large land masses in the world (i.e., Africa, South America, or East Asia).

Methods for inferring geographic origin from the stable isotope composition of animal tissues are widely used in movement ecology, but few computational tools and standards for data interpretation are available.

Stable hydrogen and oxygen isotopic compositions (δ²H and δ¹⁸O, respectively) of animal tissues have been used to infer geographical origin or mobility based on the premise that the isotopic composition of tissue is systematically related to that of local water sources. Isotopic data for known‐origin samples are required to quantify these tissue–environment relationships. Although many of such data have been published and could be reused by researchers, differences in the standards used for calibration and analytical procedures for different datasets limit the comparability of these data.

In stressful environments, facilitation often aids plant establishment, but invasive plant pathogens may potentially disrupt these interactions. In many treeline communities in the northernRockyMountains of theU.S. andCanada,Pinus albicaulis, a stress‐tolerant pine, initiates tree islands at higher frequencies than other conifers – that is, leads to leeward tree establishment more frequently. The facilitation provided by a solitary (isolated)P. albicaulisleading to tree island initiation may be important for different life‐history stages for leeward conifers, but it is not known which life‐history stages are influenced and protection provided. However,P. albicaulismortality from the non‐native pathogenCronartium ribicolapotentially disrupts these facilitative interactions, reducing tree island initiation. In twoRockyMountain eastern slope study areas, we experimentally examined fundamental plant–plant interactions which might facilitate tree island formation: the protection offered byP. albicaulisto leeward seed and seedling life‐history stages, and to leeward krummholz conifers. In the latter case, we simulated mortality fromC. ribicolafor windwardP. albicaulisto determine whether loss ofP. albicaulisfromC. ribicolaimpacts leeward conifers. Relative to other common solitary conifers at treeline, solitaryP. albicaulishad higher abundance. More seeds germinated in leeward rock microsites than in conifer or exposed microsites, but the odds of cotyledon seedling survival during the growing season were highest inP. albicaulismicrosites. Planted seedling survival was low among all microsites examined. Simulating death of windwardP. albicaulisbyC. ribicolareduced shoot growth of leeward trees. Loss ofP. albicaulisto exotic disease may limit facilitation interactions and conifer community development at treeline and potentially impede upward movement as climate warms.