The aim of this study was to determine whether blood oxygen capacity of waders varies with respect to migration at both inter-specific and individual level. To verify this hypothesis we measured hemoglobin concentration in 875 waders from 14 species during their autumn migration through central Poland. In most of the species we found an increase in the hemoglobin levels along with increasing fat loads during the stopover period, which suggests that individual birds are able to elevate their oxygen-carrying capacity of blood prior to departure on a migratory flight. Positive relationship between hemoglobin concentrations of waders and their fat loads was confirmed at the inter-specific level by the comparative analysis of independent contrasts. Comparative analysis also demonstrated that hemoglobin concentrations were positively related with theoretical flight range and mean refueling rate during stopovers. The results indicate that species traveling according to the strategy of energy-minimization (short-distance migrants, low fat reserves, low refueling rates) have lower blood oxygen capacity in comparison to time-selected species (long-distance migrants, high fat reserves, high refueling rates). It remains uncertain whether high hemoglobin levels in long-distance migrants are a fixed evolutionary trait or a temporal physiological adaptation associated with carrying considerable fat load.
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