The Role of Environmental Factors in the Growth of Tern Chicks

Abstract

Seabirds have to forage under conditions of exceptionally large and rapid environmental fluctuations. In particular, those that plunge from air to water in search of prey are exposed to complex interactions between weather and sea surface conditions, and those that dive close inshore are further subjected to changes in their feeding sites caused by the daily tidal movements. We might therefore anticipate considerable day-to-day variation in the amount of food that such birds can obtain, and this should be most evident in the breeding season when adult birds are feeding chicks. A number of studies on the daily growth rates of chicks have suggested that this is the case for certain plunge-diving terns. Hawksley (1950, 1957) showed that chicks of Arctic terns (Sterna paradisaea Pont.) gained weight on clear days but generally lost weight on foggy days and Lemmetyinen (1972) has produced some supporting evidence. Langham (1968) has shown that daily weight changes in chicks of Arctic terns, common terns (S. hirundo L.) and roseate terns (S. dougallii Montagu) were affected by windspeed, duration of sunshine and amount of rainfall. Some of these relationships obtained by Langham and others have been briefly summarized by Anon. (1968). In these studies, observations at the nest-site were not accompanied by observations at sea, and consequently interpretation of results has been speculative. To gain a fuller understanding of variation in growth rates of tern chicks, it is desirable to know (a) whether the factors correlating with growth rates are also determinants of fishing ability in the adults, and (b) the nature of any relationship between environmental factors and fishing ability. In an attempt to fulfil these requirements, a study was therefore designed to interrelate environmental factors, fishing ability and growth of chicks. The work was carried out from 1968 to 1970 at Coquet Island (55?20'N,1?32'W) in Northumberland. The island supports breeding populations of Arctic terns, common terns, roseate terns and Sandwich terns (S. sandvicensis Latham); this study is concerned with the last three of these. Observations near the colony showed that fishing ability of common and Sandwich terns was influenced by windspeed and sea surface conditions (Dunn 1973) and also by tide (Dunn 1972; see also Boecker 1967). To determine whether these factors also affected chick development, they were included in a multivariate analysis of growth in chicks of the three species. In the present paper the results of this analysis are presented and examined in the context of parental fishing ability.