Mortality within the annual cycle: seasonal survival patterns in Afro-Siberian Red Knots Calidris canutus canutus

Estimates of seasonal mortality for long-distance migrant birds are extremely challenging to collect and con- sequently reports are scarce. Determining when and where mortality occurs within the annual cycle is important for an understanding of population dynamics and the evolutionary drivers of long-distance migration. We collected data on seasonal survival in a mark-recapture study of colour-marked Red Knots Calidris canutus canutus in their main wintering area at tropical Banc d'Arguin, Mauritania, West Africa. Our study population breeds 9,000 km to the northeast on Taymyr Peninsula, central northern Siberia. Our results show that annual apparent survival decreased from 0.87 ± 0.01 (SE) in 2002-2005 to 0.78 ± 0.02 in 2006-2009. During the 3-year time-window between 2006 and 2009, additional resightings just before migration and after return to the wintering grounds allowed us to partition the year into two periods: the non- breeding period on the Banc d'Arguin, and the migration and breeding period away from it. We estimated that, on the Banc d'Arguin, the 2-month apparent survival rate was 0.94 ± 0.01, whereas 2-month survival approached unity during the rest of year. Hence, most mortality occurred on the tropical wintering grounds. We review the possible physio- logical and ecological stressors involved and discuss the generality of these results.