Over-summering causes in a long distance migrant shorebird: Red Knot (Calidris canutus rufa) in SE South America

DAMICO V; ISACCH J; ROJAS P; MARTINEZ CURCI NS; CASTRESANA GJ

Congreso; VII Western Hemisphere Shorebird Group Meeting; 2017

The over-summering phenomenon ─by which boreal-breeding birds fail to migrate north and remain far away from their breeding areas during boreal summer / austral winter─ is particularly frequent in shorebirds. The most widely accepted hypothesis to explain it, proposes that it involves immature birds that do not return to breeding areas until reaching sexual maturity. Whilst, a less explored hypothesis proposes that it involves both mature and immature individuals, which do not migrate to breeding areas because of an absent or delayed pre-migratory physiological conditioning (possibly due to sterility, senility or poor health status). We studied the causes of over-summering in the Red Knot (Calidris canutus rufa). We captured 145 over-summerers during 2012, 2013 and 2014 at Punta Rasa, Buenos Aires, Argentina, determine the percentage of adults and juveniles, and characterize their physical and physiological condition (through weight, moult, leukocyte profile and presence of blood parasites). Our results indicate that Punta Rasa is used as an over-summering area by juvenile and adult Red Knots, and thus the hypothesis of sexual immaturity is not enough to explain the over-summering phenomenon. Otherwise, our data support the hypothesis of absent or delayed pre-migratory physiological conditioning. Most birds showed, at the beginning of the over-summer, no fat accumulation. Almost all juveniles exhibited some or all worn flight feathers, while adults showed incomplete alternate plumage. None Red Knots showed symptoms of disease or anomalies. Based on cellular immune components, it is not possible to conclude that over-summerers were undergoing pathological and / or stressful processes.