CONICET | Buscador de Institutos y Recursos Humanos

The over-summering, understood as the phenomenon by which boreal-breeding birds fail to migrate north and remain in the south far away from their breeding areas during boreal summer / austral winter, was reported in at least 15 families of birds, but it is particularly frequent in shorebirds. Several hypotheses were proposed to explain it mostly by using shorebirds as study systems. The most widely accepted hypothesis to explain it proposes that it involves immature birds. The arrest of migration could happen when the costs of the journey are higher than the benefits of gaining experience in breeding areas. 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; sterility, senility or poor health status (e. g. illness, injuries, starvation) are some of the causes that may explain it. The latter hypothesis predicts that, over-summerers would be both mature and immature individuals which have not reached the moulting and fattening conditions that enable them to face the flight to breeding areas and/or reproduce successfully. We studied the causes of over-summering in the Red Knot (Calidris canutus rufa) a ?near threatened? shorebird species with decreasing population trends. Deep knowledge about over-summering behavior in an endangered species is essential to achieve sound conservation efforts, whether it involves mature or immature individuals. On the one hand, an increased incidence of mature birds that are failing to contribute to reproduction might be a symptom of a population in trouble. On the other hand, if most individuals are immature a single catastrophic event or habitat degradation in a site supporting large numbers of over-summering birds could decimate future breeding stock. For this purpose we captured 145 over-summering Red Knots during the austral winters of 2012, 2013 and 2014 at Punta Rasa, Bahía Samborombón, Buenos Aires Argentina. We determine the percentage of adults and juveniles over-summering in different years 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 for approximately four months a year (mid-May to September), 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, indicating that the costs associated with migration are greater for juveniles than for adults. Adults showed incomplete alternate body plumage, considered an honest signal of sexual selection which completeness is related to the conditions faced at the time of molt. None Red Knots showed symptoms of disease or anomalies. Based on cellular immune components considered in this work, it is not possible to conclude that over-summerers were undergoing pathological and / or stressful processes. This work shed light on the causes of over-summering in shorebirds and highlights the importance of Punta Rasa both for the recruitment of juveniles and the recovery of adult Red Knots. Consequently, it emphasizes the need to guarantee the environmental conditions that allow Red Knots to rest and feed at Punta Rasa during the austral winter. In this sense it is important to consider a management of the area that protects the main feeding areas located in estuarine intertidal flats during low tide and limits tourism and recreation activities on the oceanic beaches during high tide.