Growth and survival in macaroni penguin, Eudyptes chrysolophus, A- and B-chicks: do females maximise investment in the large B-egg?
In crested penguins, Eudyptes spp., the first-laid (A) egg is smaller than the second-laid (B) egg and although viable rarely survives to hatching. I studied survival and growth to fledging of A- and B-chicks at experimentally-manipulated nests, and individual variation in relative investment by adult females in A- and B-eggs, in macaroni penguins, E. chrysolophus. At hatching, A-chicks averaged 55% of B-chick weight. However, in one year there was no difference in fledging weight between A- and B-chicks and in the second year A-chicks were only 7% lighter than B-chicks at fledging. A- and B-chicks did not differ in physical size at fledging. Although B-egg weight varied by up to 73% between individuals the relative size of the two eggs within the clutch remained the same, the A:B-egg weight ratio being 0.62. These results do not support the idea that birds maximise investment in the large B-egg. It is suggested that, in contrast to the assumption made by most hypotheses, the cost of egg production in crested penguins is very low, and that consequently the selection pressure required to maintain a two-egg clutch may also be low. A physiological basis for the intra-clutch egg dimorphism is proposed. It is suggested that during egg development the rate of yolk deposition increases with time and that the small A-egg, which is initiated first, develops for a period when the rate of yolk deposition is below maximum. Some evidence is presented to support this hypothesis.