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

Abstract

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 and the timing of molt are likely to be important factors selecting for deferred migration during the first year of life. Other factors, not associated with flight-feather molt, seem to trigger over-summering in adults, which had completed flight feather molt but had low fat loads and/or incomplete alternate plumage. We found no evidence of a weakened immune system, high loads of blood parasites or high stress levels that can explain this poor migratory conditioning and therefore over-summering. Our data indicates that our adult age-class comprises both young individuals postponing first breeding until they are at least two years old and sexually mature individuals with prior reproductive experience skipping a breeding opportunity. Breeding propensity and age at first breeding are both poorly known, but key demographic parameters that determine population growth. This study suggests that potentially they can be estimated from mark–recapture at non-breeding areas and this warrants further study.