Sex-specific aggression and sex ratio in wintering finch flocks: serins and siskins differ

Males are dominant over females in many bird species. This may lead to male monopolisation of resources whenever food is scarce or clumped and secondarily to lower female survival rates. As a result of the consequent male-biased sex ratio in the adult population, competition may arise either (1) between males and females, as males attempt to exclude females from feeding patches, or (2) between males because females do not pose a competitive threat. We recorded agonistic interactions between males and females in wintering foraging flocks of serins (Serinus serinus) and siskins (Carduelis spinus) to test for inter-specific differences. Most of the aggressive interactions in serins were between males and females, whereas in siskins they were between males. We also compared sex ratios for each species during the winter, determined from separate trapping efforts over an 11-year period, to test whether the direction of aggression by males (i.e. male/male; male/female) relates to variations in female survival rates. The proportion of females was smaller in winter than in autumn for serins, but differences in siskins were negligible. Results are interpreted in relation to the social organization displayed by both species studied.