Training siblings to be submissive losers: dominance between booby nestlings

Abstract

Second-hatched chicks in two-chick broods of the blue-footed boody, Sula nebouxii, usually remain subordinate to their siblings throughout the nestling period, even those that outgrow the firsthatched chick. Initially, the superior size and maturity of the first-hatched chick enable it to dominate, but subsequently other factors could be involved. The idea that a chick's own social experience determines whether it behaves aggressively or submissively was tested by pairing unfamiliar chicks in the field. In brief trials, dominants (chicks with a history of dominance) tended to behave aggressively and subordinates (chicks with a history of subordination) tended to behave submissively, whatever the prior social experience of the partner. However, in chicks lacking prior social experience with a sibling, direction of dominance depended on relative size or age. When experience was pitted against size, eight of 12 experimental subordinates permanently paired with younger and smaller dominants initially behaved aggressively, although only one subordinate succeeded in becoming dominant. None of the nine control subordinates that were paired with older and larger dominants showed aggressive behaviour. In conclusion, the results of this study demonstrate that agonistic roles are determined by experience, although subordinates may become aggressive when they detect a personal advantage. When subordinates do become aggressive they are competitively inferior to smaller dominants.