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Synopsis 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. 

Animals developing at high elevation experience a suite of environmental challenges, most notably the low partial pressure of oxygen ( P O 2 ) in ambient air. In low P O 2 , bird species with high-elevation ancestry consistently demonstrate higher hatching success than lowland counterparts, suggesting highland birds are adapted to restricted O 2 (hypoxia) in early development. Haemoglobin (Hb), the critical oxygen-transport protein, is a likely target of P O 2 -related selection across ontogeny since Hb isoforms expressed at distinct developmental stages demonstrate different O 2 affinities. To test if Hb function is under P O 2 -related selection at different ontogenetic stages, we sampled a songbird, the hooded siskin ( Spinus magellanicus ), across two approximately 4000 m elevational transects. We sequenced all of the loci that encode avian Hb isoforms, and tested for signatures of spatially varying selection by comparing divergence patterns in Hb loci to other loci sampled across the genome. We found strong signatures of diversifying selection at non-synonymous sites in loci that contribute to embryonic ( α π , β H ) and definitive ( β A ) Hb isoforms. This is the first evidence for selection on embryonic haemoglobin in high-elevation Neoaves. We conclude that selection on Hb function at brief, but critical stages of ontogeny may be a vital component to high elevation adaptation in birds.