Haemosporidian infections limit physiological adaptation to elevation in birds: evidence from haemoglobin and fat reserves

Haemosporidian parasites are widely recognised as important drivers of avian physiology, with potential effects on oxygen transport and energy balance. These parasites infect red blood cells and consume host haemoglobin, reducing oxygen‐carrying capacity. At higher elevations, birds face reduced oxygen availability and are expected to increase haemoglobin to offset hypoxia, which may also influence energy allocation and fat reserves. Here, we tested whether haemosporidian infections constrain this response and whether their effects on haemoglobin and fat reserves become more pronounced at higher elevations compared to lower elevations. We quantified infection presence, intensity, haemoglobin concentration, and fat reserves in 926 birds from 135 species sampled along a 1600 m elevational gradient in a subtropical ecosystem. Haemoglobin concentration was lower in infected birds than in uninfected individuals overall. Haemoglobin increased with elevation in uninfected but not infected birds, implying that infection constrains the ability to cope with lower oxygen availability. Among infected individuals, higher parasite intensities were associated with further reductions in haemoglobin at higher elevations. In contrast, fat reserves were unaffected by infection or elevation and varied only with season, being lower during the wet season regardless of infection status. These findings show that haemosporidian infection imposes a baseline constraint on oxygen transport, with additional costs of parasite load emerging under hypoxic conditions. The absence of effects on fat reserves further indicates that the impacts of infection vary by trait and may go undetected if only energy‐based measures are used.