Group size and vigilance behaviour in chimango caracaras feeding on experimental carcasses

ESRELLA BARREIRO; ALEJANDRO TRAVAINI; ALEJANDRO RODRIGUEZ

Granada

Congreso; Congreso Internacional de la Sociedad Española de Etología y Ecología Evolutiva; 2016

Sociedad Española de Etología y Ecología Evolutiva

Vigilance has been traditionally considered as an antipredatory behavior, but it may also beassociated with protection against conspecific interference. Individuals often reduce theirinvestment in vigilance with increasing group size so they may spend more time foraging (the socalled ?group size effect?). However, building up numbers of foragers in clumped food necessarilyincreases the chances for interference competition. We investigated patterns of vigilance inchimango caracaras (Milvago chimango) feeding upon experimental mammal carcasses. Foodaggregation allows the formation of feeding groups in this otherwise territorial and solitary raptor,favouring the occurrence of agonistic interactions. We examine two hypotheses: (1) if individualsneed to frequently check the location of other flock members either to initiate or avoid agonisticinteractions, we expect vigilance time will augment with increasing group size; (2) alternatively, ifflocks do not engage into agonistic interactions, we expect individuals to devote less time to scanas groups grow. The proportion of vigilance time decreased with increasing group size, and thisreduction was achieved through decreasing the frequency of vigilance. The duration of vigilanceevents was not correlated with frequency of vigilance suggesting that frequency and durationcould play different functions. A decrease in frequency of vigilance was associated with longerbouts of food handling and eventually a higher probability of detaching a piece of meat from thecarcass. The negative effect of group size on the frequency of vigilance was lineal, probablybecause caracaras never formed very large groups where the frequency and costs of agonisticencounters were expected to be significant. We found little evidence for intraspecific interactionsin chimango caracara groups as well as support for the group size effect. This suggests potentialcooperation as well as relatedness between group members. Our findings open new questionsabout caracara social structure.