To migrate or not: drivers of over-summering in a long-distance migratory shorebird

The phenomenon of over-summering in southern non-breeding areas by boreal-breeding birds is particularly prevalent among shorebirds. Despite its frequency, it is understudied compared with most other aspects of shorebird ecology. Our aim was to expand knowledge of this subject through a study of Red Knots Calidris canutus rufa over-summering at a site in Argentina during the austral winter. We measured the proportion of one-year-old and adult over-summerers and evaluated the roles of flight-feather condition and physiological status (through leukocite profile, physiological stress index, and presence of blood parasites) as triggers for over-summering. We also explore sex-ratio, the evolution of body mass and extent of breeding plumage within each age-class. Over-summerers were 57% yearlings and 43% adults, 46% females and 54% males. Almost all yearlings exhibited incomplete molt of primary feathers and some were in active primary molt. This suggests that the condition of flight-feathers...