BUILDUP OF RED-BLOOD-CELLS IN REFUELING BAR-TAILED GODWITS IN RELATION TO INDIVIDUAL MIGRATORY QUALITY

We examined hemoglobin concentration, hematocrit and mean cell hemoglo bin concentration (MCHbC) in Bar-tailed Godwits Limosa lapponica durin g the latter half of their staging period in May in the Wadden Sea of The Netherlands. Here they recover from a 4,000-5,000 km long flight f rom West African wintering areas and refuel in preparation for a fligh t of similar length toward the arctic Siberian breeding grounds. Hemog lobin concentration and hematocrit levels showed no differences betwee n the sexes, but were higher than expected for a bird species of their size. Bar-tailed Godwits that were on schedule with regard to molt an d body mass gains, had elevated concentrations of red blood cells all the time during 19-30 May. In contrast, late-arriving, nutritionally s tressed individuals, recognizable by the absence of contour feather mo lt, showed an increase in both hemoglobin concentration and hematocrit over time before scheduled departure on 1 June, converging to similar values as the group of high migratory quality. There were no time-rel ated and categorical variations in MCHbC. We interpret the relatively low concentrations of red blood cells in late arriving light-weight in dividuals to indicate that long-distance migration flights may result in light anemia. The quick recovery and maintenance of high hemoglobin and hematocrit levels throughout the latter half of the staging perio d indicates that high concentrations of red blood cells are as necessa ry for rapid refuelling as for long-distance flights. The increased vi scosity of blood with high hematocrit values was reflected in cooccurr ing increases in the size of the heart.