Eggshells of Arctic Terns from Finland: Effects of Incubation and Geography

Abstract

-Seventy-four eggs from seven colonies of Arctic Terns (Sterna paradisaea) in the Quark and the Bothnian Bay of Finland were collected in 1981 shortly after laying and immediately before hatching. Shell thickness, weight, thickness index, and egg weight index were determined and compared with the same characteristics of 200 eggs collected between 1874 and 1935. We found no significant differences in these measures of egg thickness between recent and museum shells from the same geographical areas. Shells of museum specimens from different geographical regions did show significant variations. The weight and the wing and tarsus length of the embryos correlated negatively and significantly with all measured characteristics of the shell except its thickness when the shell membranes were present. During the incubation period, the shell's thickness (without membranes) decreased 8%; thickness index and weight decreased 4%; and the shell's thickness with shell membranes present decreased 12%. In this paper, we discuss reasons for these changes. Pesticide-related reproductive failures have been reported in both American and European terns (Sterna spp.; Switzer and Lewin 1971, Koeman and van Genderen 1972, Switzer et al. 1973, Gochfield 1975, Fox 1976). For example, high levels of chlorinated hydrocarbons were found in the tissues of marine animals from the Baltic Sea (Jensen et al. 1969, Koivusaari et al. 1972, Anderson and Hickey 1974), an area where eggshell thinning of 1117% was reported in White-tailed Eagles (Haliaeetus albicilla) and Ospreys (Pandion haliaetus; Koivusaari et al. 1980, OdsjO 1982). In contrast, Lemmetyinen and Rantamiki (1980) reported low pesticide contamination in the eggs of Arctic Terns (S. paradisaea) from the archipelago of southwestern Finland. The thickness of eggshells from these terns has recently increased significantly (5.2%, P < 0.05) in Finland (Gulf of Bothnia; Pulliainen and Marjakangas 1980). Several studies concerning geographic variations in eggshells have been published (e.g., Anderson and Hickey 1970, 1972; Sutcliffe 1978; Olsen 1982), but there are almost no studies of this kind from Europe (e.g., Svensson 1978). Besides effects of pesticides and geography, the mobilization of eggshell calcium for the developing embryo is known to affect eggshell thickness (e.g., Kreitzer 1972). Our objectives, ther fore, were (1) to describe recent changes in the thickness and size of Arctic Tern eggshells in comparison to museum material; (2) to check the reliability of available museum material for use as a standard; and (3) to find a method for measuring shell variables that is independent of the shell thinning that occurs naturally during embryonic development. MATERIALS AND METHODS We collected Arctic Tern eggs in 1981 from three colonies in Quark (6 310'N, 2 12 5'E) and from four colonies in Bothnian Bay (65003'N, 25010'E) in the Gulf of Bothnia (Fig. 1). At ach nest, one egg was chosen randomly at an early stage of incubation (little or no embryonic development). Nine of these nests in the Quark colony were marked and two more eggs were taken from each shortly before hatching was expected. Where Arctic and Common terns (S. hirundo) bred in the same colonies, we confirmed identification of Arctic Tern nests by observation from a blind or by flushing a parent from its nest before taking an egg. Eggs were kept in a refrigerator until prepared. Their length and breadth were measured with a vernier caliper to the nearest 0.1 mm. A piece of shell, 16-18 mm in diameter, was cut out from the equator of each egg. The contents of the egg were then removed, the shells were rinsed with water, and the shell