Consequences of brood size and hatching sequence for prefledging mortality in sandwich terns: Why lay two eggs?

Mortality of Sandwich Tern Sterna sandvicensis chicks held in enclosures was studied in colonies on Griend, Dutch Wadden Sea, from 1992-1999 and Hirsholm, Danish Kattegat, in 1997. Survival until fledging of chicks amounted to 73% for chicks hatching from first-laid eggs or single egg clutches and 59-64% for partially hatched 2-egg clutches, whereas 6% of the second hatchlings survived until fledging. Less than 2% of all 2-chick broods actually fledged 2 chicks. Because 18% of the 2-egg clutches only hatched one egg, still 7% of the fledglings of 2-egg clutches originated from a secondlaid egg. In nests where both eggs hatched, the number of chicks was generally reduced soon after hatching. Within 5 days from hatching, more than 50% of the second hatchlings died of starvation or were preyed upon. It appears that overproduction commonly occurs in Sandwich Terns and that it mainly serves as an insurance mechanism. On Griend and Hirsholm, chick productivity of 2-egg clutches was somewhat higher compared to 1-egg clutches. Undernourishment was an important cause of death, either directly by starvation or by selective predation of chicks in poor condition. This in combination with earlier published studies suggests that Sandwich Tern parents on Griend are exposed to a high food stress.