Chick Survival in Terns (Sterna Spp.) with Particular Reference to the Common Tern

Abstract

The differential survival of young birds within a brood prior to fledging occurs in many bird species. This phenomenon is particularly common in raptors, storks and corvids where it has been well documented (e.g. Schmaus 1938; Schuiz 1942, 1957; Lockie 1955), but it also occurs in various other species. This differential survival within a brood reflects the asynchronous hatching of the eggs which occurs when the parent bird commences incubation before the clutch is complete. It has been suggested that this asynchronous hatching is an adaptation to a variable food supply (Lack 1954, 1966), so that if the parents find difficulty in obtaining food for all their brood, food will go to the first hatched and largest nestlings which are higher in the peck order. Only when food is plentiful and the first chicks become satiated do the smaller and weaker chicks obtain food. When the food supply is limited, the last hatched chicks soon starve to death, so that food goes only to the survivors and is not wasted. Asynchronous hatching occurs in terns, and differential survival of chicks within a brood was found during a study of the breeding biology of the common tern (Sterna hirundo L.), Arctic tern (S. paradisaea Pontopp.), roseate tern (S. dougallii Montague) and the sandwich tern (S. sandvicensis Latham). In all four species, the survival of single chicks and first chicks from broods of two or three tended to be similar within a species. However, second chicks from broods of two and three had a lower fledging success and this was much lower still in third chicks. Since differential fledging success was most marked in broods of three, it was decided to examine chick mortality in the common tern where this brood size was most frequent. During this investigation carried out on Coquet Island, 55038' N, 01037' W, from 1965 to 1967, inclusive, about 59?4 of the common terns laid three eggs, 37Y/. two eggs and 4XO one egg. Normally, the second egg in a clutch of two hatches half a day after the first, but in a clutch of three the first and second eggs hatch closer together whilst the third egg hatches about 2 days later. The demands of the brood and the parental care were examined to detect the method by which this differential mortality occurs in common tern chicks.