Leukocyte profiles indicate nutritional, but not moulting stress in a migratory shorebird, the Common Snipe (Gallinago gallinago)

Abstract

The relative ratio of the two main leukocyte types, heterophils (H) and lymphocytes (L), is known to change proportionally to the concentrations of stress hormones in the circulating blood. Thus, analysis of leukocyte profiles serves as a reliable proxy of stress in vertebrates, as high H/L ratios indicate stronger stress response. Moult and migration are among the most energetically demanding and nutritionally stressful processes in the annual cycle of wild birds. Although most birds separate these two activities in time, a moult–migration overlap has been reported for several avian species. The aim of this study was to examine whether overlapping energy requirements of moult and migration produce an increased organismal stress in a shorebird species, the Common Snipe (Gallinago gallinago). For this purpose, we assessed leukocyte profiles and nutritional condition in more than 350 moulting and non-moulting snipe during autumn migration through central Europe. We found negative correlations between H/L ratios and different measures of condition, including size-corrected body mass, fat load, and plasma concentrations of triglycerides and total protein. However, no evidence was found for the effect of moult status on H/L ratios. Our study indicates that while migration in a poor nutritional state can be associated with elevated stress response, the process of feather replacement may not constitute a serious stress for migrating Common Snipe. We suggest that these results may help to explain the occurrence of moult–migration overlap in the Common Snipe and possibly in other avian species.