Sex-biased aggression and male-only care at sea in Brconnich's Guillemots Uria lomvia and Razorbills Alca torda

摘要

In several groups in the order Charadriformes, biparental care is followed by a period of male-only care. Several hypotheses attempting to explain extended male parental care in shorebirds do not fit the Alcini. In a previous study of Brconnich's Guillemots Uria lomvia and Razorbills Alca torda, we did not find support for female-biased parental effort at the breeding site that would lead to males being in better condition to care for chicks at sea. However, in both species, males spent more off-duty time at the breeding site than females, suggesting greater involvement in the defence of egg or chick, breeding site and mate. We predicted that there would be a male bias in size and aggressive behaviour associated with parental roles. To test this, body size and aggression of attending male and female Brconnich's Guillemots and Razorbills were measured during incubation and brooding on the Gannet Islands, Labrador. Parental aggression was measured using natural observations of all agonistic interactions and, in Razorbills only, in situ responses to presentations of a predator model. In both species, males were significantly larger than females in culmen and gape length. Guillemot males initiated agonistic interactions more frequently than females during incubation. In contrast, female Guillemots were subjected to aggression more frequently than males and as a result were involved in more fights. In addition, the few chicks that were seen to die were being attended by single females. During the brooding period, Razorbill males responded aggressively to intruders more frequently than females, made more aggressive responses than females, and responded aggressively more frequently and more intensely than females to a predator model. In both species there was a similar male bias in morphology and behaviour that is consistent with male parents being more capable of protecting their chick, a probable advantage to chick survival during the uniparental care phase of some Charadriformes.