FACTORS INFLUENCING CLUTCH‐SIZE AND CHICK GROWTH IN THE NORTH ATLANTIC GANNET SULA BASSANA

• Published in: Ibis
• Main Author: Nelson, J. B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1964
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1964.tb03681.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1964.tb03681.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1964.tb03681.x

Summary The Gannet lays only one egg, and replacements are common. In 1962 in Scotland the Gannet was shown to be capable of hatching two eggs and rearing two young of the same age. If one of the brood of two was younger it did not survive. In striking contrast, S. leucogaster and S. dactylatra hardly ever manage to rear two young even though they normally lay two eggs. The results of artificial “twinning” showed that hatching success was as high for two eggs as for one, though the incubation period was two days longer, and that fledging success was 83% against 94% (of eggs hatched). This gave the parents of twins a 76% greater output than singles. The twins grew almost as well as singles, though weighing slightly less than a single of the same age, and taking on average four days longer to fledge. These figures refer only to twins of approximately the same age. The twin growth curves are compared in detail with normal single‐chick curves and with twin‐Shag growth curves, The results are discussed in terms of a possible causal mechanism related to feeding behaviour, since, though twin Gannets grow equally as well as each other, and, in the later stages, as well as singles, they are always slightly behind due to a lag which occurred early in growth when the actual amount of food consumed was extremely small. The results are thus apparently at striking variance with the theory that the Gannet rears the maximum number of young that it can feed; but it is still necessary to know how well the twinned Gannet young survive to breeding, and whether the rearing of an extra chick imposes a real strain on the adults (signs of which were beginning to show by the time the young fledged) which in long‐lived birds might offset the reproductive advantage of producing twins. It is suggested that, particularly in sea‐birds, this problem is worth exploring more systematically than has yet been done. Catastrophic years are thought unlikely to weed out twins, since to offset the 76% reproductive advantage would require such years to ...