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

MARTÍNEZ?CURCI, NATALIA S.; ISACCH, JUAN P.; D'AMICO, VERÓNICA L.; ROJAS, PABLO; CASTRESANA, GABRIEL J.

JOURNAL OF AVIAN BIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2020 vol. 51 p. 1 - 11

0908-8857

The phenomenon of over-summering in southern non-breeding areas by borealbreeding birds is particularly prevalent among shorebirds. Despite its frequency, it isunderstudied compared with most other aspects of shorebird ecology. Our aim was toexpand knowledge of this subject through a study of red knots Calidris canutus rufaover-summering at a site in Argentina during the austral winter. We measured theproportion of one-year-old and adult over-summerers and evaluated the roles of flightfeather condition and physiological status (through leukocite profile, physiologicalstress index and presence of blood parasites) as triggers for over-summering. We alsoexplore sex-ratio, the evolution of body mass and extent of breeding plumage withineach age-class. Over-summerers were 57% yearlings and 43% adults, 46% females and54% males. Almost all yearlings exhibited incomplete molt of primary feathers andsome were in active primary molt. This suggests that the condition of flight-feathersand 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 moltbut had low fat loads and/or incomplete alternate plumage. We found no evidenceof a weakened immune system, high loads of blood parasites or high stress levels thatcan explain this poor migratory conditioning and therefore over-summering. Our dataindicates that our adult age-class comprises both young individuals postponing firstbreeding until they are at least two years old and sexually mature individuals with priorreproductive experience skipping a breeding opportunity. Breeding propensity and ageat first breeding are both poorly known, but key demographic parameters that determine population growth. This study suggests that potentially they can be estimatedfrom mark?recapture at non-breeding areas and this warrants further study