Aggressive behaviours and correlates of dominance in Purple Sandpipers Calidris maritima at a communal winter roost

Aggression and the competitive ability of individual Purple Sandpipers were studied at a communal winter roost in northeast England. Aggression was most intense in high winds and more frequent in moderate breezes than when wind speeds were high or low. Higher wind speeds resulted in a drop in aggression rate, as birds had to face into the wind to maintain their balance. The ability of an individual to secure a sheltered roost position was related to its size and sex, larger individuals being dominant over those smaller than themselves. Purple Sandpipers exhibit reversed sexual size dimorphism and females were thus dominant over males. There was also weak evidence that adults were dominant over first-winter birds. The importance of size and sex in determining dominance at the roost is discussed in relation to the evolution of reversed sexual size dimorphism in Purple Sandpipers and similar shorebirds.