| Summary: | In 1992-1994, we measured clutch and egg sizes in Arctic, Common and Sandwich Terns, nesting at the island Griend in the Dutch Wadden Sea. For all three tern species, first-laid eggs were larger than last-laid eggs. Clutches with relatively large first-laid eggs also had relatively large last-laid eggs, although the extra investment in the latter was smaller than in the former. Eggs of Common Terns were on average larger than eggs of Arctic Terns. However, the size of first-laid eggs in Arctic Terns was equal to that of last-laid eggs in Common Terns. In 1994, Arctic Terns laid larger a-eggs than in 1992-93. Between-year variation in egg size in the Sandwich Tern was due to different sampling dates. Sandwich Tern eggs became smaller as the laying season progressed. Only for Sandwich Terns, we found differences in clutch size between years. On the basis of egg size, we estimated that 20% of the one-egg clutches originated from two-egg clutches. Nests with three or more eggs were mainly clutches of two pairs (the second pair starting to lay in the same nest after eggs of the first pair had been preyed upon), but occasionally clutches with two females were found.
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