Build-up of red blood cells in refuelling Bar-tailed Godwits in relation to individual migratory quality

We examined hemoglobin concentration, hematocrit and mean cell hemoglobin concentration (MCHbC) in Bar-tailed Godwits Limosa lapponica during 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 from West African wintering areas and refuel in preparation for a flight of similar length toward the arctic Siberian breeding grounds. Hemoglobin concentration and hematocrit levels showed no differences between 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 and body mass gains, had elevated concentrations of red blood cells all the time during 19-30 May. In contrast, late-arriving, nutritionally stressed individuals, recognizable by the absence of contour feather molt, 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-related and categorical variations in MCHbC. We interpret the relatively low concentrations of red blood cells in late arriving light-weight individuals 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 period indicates that high concentrations of red blood cells are as necessary for rapid refuelling as for long-distance flights. The increased viscosity of blood with high hematocrit values was reflected in co-occurring increases in the size of the heart