Unraveling over-summering causes throughout a long distance migrant shorebird: Red Knot (Calidris canutus rufa) at southern South America

The over-summering phenomenon ─by whichboreal-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. While a less explored hypothesis proposes that over-summering birds 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) a near threatened shorebird species. For this purpose we captured145 over-summerersduring 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 thatPunta 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, over-summerers are birds with absent or delayed pre-migratory physiological conditioning. Most of them showed little or no pre-migratory fat accumulation. Almost all juveniles exhibited absent or delayed moulting of primary feathers, while adults showed delayed moulting of body plumage. This work shed light on the causes of over-summering andhighlights the importance of Punta Rasa both for the recruitment of juveniles and the recovery of adult Red Knots.