Evaluating physiological condition as a driver of oversummering in Red Knots Calidris canutus: insights from hematological and biochemical markers

Migration decisions in shorebirds may be influenced by their physiological condition. We evaluated whether hematological and biochemical parameters are associated with the decision to oversummer in adult Red Knots Calidris canutus rufa on non-breeding grounds in Argentina. We analyzed hematocrit, leukocyte profiles, and plasma metabolites (total protein, glucose, triglycerides, and cholesterol) in 54 individuals sampled during either the oversummering period or northward migration. Adult migrants exhibited significantly higher hematocrit and nutritional markers (total proteins, cholesterol, triglycerides, glucose) than oversummering adults, while immune parameters did not differ substantially between the two groups. Moreover, oversummering adults and one-year-old birds showed similar physiological profiles, suggesting a shared non-migratory condition. These results suggest that oversummering may reflect reduced metabolic readiness or suboptimal physiological condition, although it may not necessarily be linked to pathological or immune constraints. While the differences observed may reflect phase-specific physiological demands, they could also indicate that individuals in poorer condition forego migration to preserve physiological stability. Further longitudinal studies, integrating physiological data with ecological and behavioral variables, will be needed to clarify the drivers of oversummering. Understanding these dynamics is essential for informing conservation strategies in the context of global change.