Lack of social hierarchy in wintering white-tailed eagles (Haliaeetus albicilla) when scavenging

Abstract

We studied agonistic interactions and the influence of dominance relationships on access to food in groups of wintering white-tailed eagles (Haliaeetus albicilla) when scavenging fish carcasses in Eastern Poland. The data did not conform to the arrangements of a linear hierarchy. No winner-loser effect on fight success was observed: winners of aggressive encounters could turn into losers at repeated interactions during the same day. There was no reduction in overt aggression rate and no increase in linearity of dominance hierarchy over time. Immature and adult birds did not differ significantly in the rate of initiated aggressive interactions or in the time spent feeding. The outcome of agonistic contests did not depend on the estimated prior residence in the study area or time spent feeding beforehand, however older birds better defended their feeding positions against younger conspecifics. We suggest that unstable dominance relationships in white-tailed eagles may result from (a) some attributes of social organization of wintering flocks, (b) the effect of hunger on the outcomes of agonistic interactions masking the true competitive ability of the contestants, and (c) the ontogenetic tendency of young birds to challenge dominant opponents.