The Ecological Significance of Clutch Size in the South African Gannet (Sula capensis (Lichtenstein)

Abstract

Lack (1954) suggests that clutch size in birds is regulated to enable the maximum possible number of chicks to survive to breeding age. In the South African gannet, Sula (bassana) capensis (Lichtenstein), the most dangerous phase of the life history is soon after birds fledge (leave the nesting area). These birds have to learn to fly and fish, apparently without aid from adults. Possibly because of this a high percentage die during the first few weeks of independence. If Lack is correct, it is probable that the body weight of chicks when they fledge has some relationship to their chances of post-fledging survival; evolutionary selection pressures will probably have acted to ensure that the maximum possible number of young leave the nest at the optimum weight for survival. One possible adaptation, that might aid chicks to survive after fledging, could be the regulation of clutch size to ensure that the chicks hatched could be reared to an optimum fledging weight. In addition, or alternatively, the fledging period (time spent in the nest) could be lengthened and chick's growth slowed down. This would spread the parent's feeding burden over a longer period and make the rearing of a chick less strenuous. The chick would probably benefit by receiving a greater total amount of food. If evolution had proceeded in a different direction, it could have led to parents feeding their young after fledging, until they learned to fish. However, at present, this would conflict with the juveniles' behaviour of migrating to the tropics as soon as they can fly. In this paper the chief problem investigated is the significance of the gannet's clutch size. This was investigated at Lambert's Bay, South Africa (320 05' S, 180 18' E) in the following ways. Firstly, I tried to see if birds ever laid two-egg clutches. Secondly, adults were tested to see if they could hatch two eggs and rear two young, and the breeding success of these birds was compared with that of birds with one egg. Thirdly, a comparison was made between the rates of development and the weights at fledging of chicks from single-chick broods and two-chick broods (twins). The twins had been experimentally created in most cases by donating a second egg of equal age to several marked nests; similar experiments had previously been undertaken by several workers with other species, including Nelson (1964) who worked with the North Atlantic gannet (S. (bassana) bassana). Fourthly, the post-fledging survival rates of juveniles were analysed to see which fledging weight seemed to be optimum. The relationship between date of fledging and post-fledging survival, was tested by analysing recovery and recapture data. Possible detrimental effects on parents rearing two chicks were also investigated.