Aggression, incubation behaviour and egg-loss in macaroni penguins, Eudyptes chrysolophus, at South Georgia

In crested penguins Eudyptes spp., the first-laid (A) egg is very much smaller than the second (B) egg and, although viable, rarely survives to hatching. I tested the recent hypothesis that this is an adaptation to cope with the high degree of egg loss resulting from selection for male aggression. In 60 nests of macaroni penguins 60% of A-eggs were lost on the same day and 23% on the day before the B-egg was laid. During the laying interval adults spent increasing time incubating prone (i.e. providing maximum protection for the A-egg). Aggression rates peaked before A-eggs were laid. Both results are contrary to the suggestion that intra-specific aggression and poor egg protection cause high A-egg loss and are the main factors favouring egg-size dimorphism. It is hypothesised that successful laying of the B-egg may have a causal effect on A-egg loss and that the A-egg may function as insurance against unsuccessful laying (immediate loss) of the B-egg.