Blood parameter changes during stopover in a long-distance migratory shorebird, the bar-tailed godwit Limosa lapponica taymyrensis

Bar-tailed godwits migrate from West African wintering sites to breeding areas in northern Russia with only one stopover. We compared hematocrit (Hct), blood hemoglobin concentration (Hb), and mean cell hemoglobin concentration (MCHb; a measure of the relative proportion of Hb in the cellular blood fraction) between arriving godwits lured to land 60 km short of the stopover site and godwits during subsequent refueling. The Hct and Hb of arriving godwits was low when compared to that of refueling birds. On the stopover site, Hct and Hb correlated positively with size-corrected body mass. In addition, Hb and MCHb reached peak levels in the last days of stopover. We explored the possibility of regenerative anemia in arriving godwits by comparing the fraction of reticulocytes (young red blood cells) between arriving and refueling birds. No differences were found. Therefore, we suggest that the increase in Hct, Hb, and MCHb during refueling is not in response to a severe anemic state at arrival. Rather, we suggest that the increase in blood parameters may anticipate the increased aerobic requirements of impending migratory flight and possibly satisfy heightened oxygen demands of the larger body mass of fattened birds. The Hct increase on the stopover site may also serve to buffer the red blood cell population against possible red blood cell breakdown during long-distance flight.