Birds With Distinct Ecological Traits Show Varied Haemoglobin Adaptations Along Elevation Gradients

Ecological systems are highly dynamic, with organisms continually adapting to various environmental stressors along natural gradients. Birds along elevation gradients serve as excellent models for examining physiological adaptations, such as elevated haemoglobin concentrations at high altitudes due to lower oxygen availability. This study aimed to examine how various ecological traits influence the haemoglobin concentration responses of multiple bird taxa to an elevation gradient. We measured haemoglobin concentration in 920 birds of 133 species at six sites representing an elevation gradient spanning from 60 to 1,600 m above sea level. Using MCMC Bayesian mixed models, we identified important ecological determinants of haemoglobin concentration and further ran separate models to test whether haemoglobin concentration responses to elevation differ between various functional groups of birds. Our results showed that haemoglobin concentration increased significantly with elevation and was strongly influenced by wing morphology, body mass, season, and primary lifestyle. The rate of increase with elevation varied by lifestyle: terrestrial and perching birds exhibited a steeper increase in haemoglobin concentration with elevation, while aerial birds also increased haemoglobin but at a more gradual rate. However, the remaining traits did not alter how species respond to hypoxia; for example, birds increased haemoglobin at the same rate in both the dry and wet seasons, meaning seasonal changes did not strongly impact elevation-driven haemoglobin adjustments. Elevation is the primary driver of variation, while lifestyle influences baseline levels rather than the rate of change. Despite differences in lifestyle-driven oxygen demands, birds exhibit a similar haemoglobin response to hypoxia at moderate elevations, where adjustments remain within physiological limits, indicating that hypoxia-driven haemoglobin adjustments occur independently of baseline oxygen demands. These findings demonstrate how birds regulate oxygen transport relative to ecological constraints, providing insights into their physiological flexibility across environmental gradients.