Sex –specific mortality associated with the drastic decline of Red Knot Calidris canutus rufa in 2000?

P. M. GONZÁLEZ; A. BAKER; V. D'AMICO

Irlanda

Congreso; Wader Study Group 2005 Meeting; 2005

Adult sex ratio in catches of Red Knots at Rio Grande in Tierra del Fuego in November 2000 was biased in favour of females (2:1) p<0.0416, N = 87, following the onset of a severe population decline after the northern migration that year.  Knots from the Tierra del Fuego population captured at San Antonio Oeste on northern migration in 1998 had a balanced sex ratio (ns, N = 219), but subsequent recaptures from these and other knots banded before 2000  showed the same bias in favour of females   p<0.0045, N = 60 as in Río Grande.  This suggests that males suffered higher mortality than females over this period, and thus contributed to the population decline. Sex-specific differences in breeding behavior, migratory patterns and molt strategies might contribute to stronger selective pressures operating on males.  Males tend to migrate earlier than females on average, arrive earlier on the arctic breeding grounds, and take care of the young, whereas females leave the nest after eggs hatch and begin to migrate south 3-4 weeks earlier than males.  If the quality of breeding plumage honestly signals the condition of individuals and plays a role in sexual selection as has been suggested, then the redder breeding plumage of males exerts a selective pressure to molt earlier.  Males arriving in Río Grande after the southern migration were initially behind females in getting winter plumage (p<0.0000, N = 367), but they were ahead of females in acquiring breeding plumage when the following northern migration began (p<0.0015, N = 164). Similarly adult males captured at San Antonio Oeste in March 1998 had 0.50 times the probability of females to be resighted or caught in following years (p<0.000), and this was negatively correlated with date of capture (p<0.0003) and positively with active body molt  (p<0.0000).  In seasons when conditions for migration are unfavourable these factors are likely to significantly increase the risk of mortality in males, and contribute to population decline and unbalanced sex ratios in adults.