The breeding of the Sooty tern Sterna fuscata in the Seychelles and the effects of experimental removal of its eggs

In the Seychelles the eggs of the Sooty tern Sterna fuscata are collected for human consumption. A reduction in the number of Sooty terns breeding in exploited colonies has been attributed to over‐exploitation of the eggs. However, the population decline has been over‐estimated and factors other than exploitation have led to a reduction in the land area available to breeding colonies. In the study colony, on Bird Island, egg laying was highly synchronous, and birds which laid at peak laying time had a higher reproductive success than those that laid early or late. The higher success of these birds was due to their being at the same stage of the reproductive cycle as their neighbours: this reduced losses of eggs to predators and losses of chicks due to pecking by adults and other chicks. Birds which laid late, which included young birds and birds which had lost their first egg and laid a replacement, were therefore penalized. The ability to lay a replacement egg after loss of the first, was found experimentally to be much greater than previous estimates had suggested, and some individuals were induced to lay a third egg. Both the ability to relay and the success of replacement eggs declined seasonally. The laying of a replacement egg did not seem to place extra strain on the parents, and the lower weights of most replacements compared with first eggs was due to seasonal factors rather than to the inability of adults to produce a large second egg. The food brought by the parents to the chicks consisted mainly of fish and squid, but frequent and abrupt changes in the species composition of the food suggested that food supplies were unpredictable. In 1973 chick growth was interrupted by two periods of food shortage, and although late hatched chicks experienced this food shortage early in their lives, starvation did not appear to be a major source of mortality. The regulation currently in force in the Seychelles imposing a close season for egg collecting protects only late‐laid eggs, and since these have a very low success rate and can contribute little to future generations, there is no biological foundation for continuing this restriction. Adult mortality and age at first breeding are not known for Seychelles Sooty tern populations, and exploitation should be restricted to 10–20% of the birds that lay during the main laying period.