Tim Birkhead, Jens-Keld Jensen, Jamie Thompson, Sjurdur Hammer, Paul Thompson and Robert Montgomerie
Abstract Members of the seabird family Procellariidae (albatrosses, petrels and shearwaters) typically produce a single-egg clutch. Two-egg clutches have been recorded occasionally in some of those species, but it is not known whether they were laid by a single female. In this study we examined eight two-egg clutches of the Fulmar Fulmarus glacialis in the Faroes, to assess whether those eggs may have been laid by the same female. Using data from eggs laid in different years by the same 100 females on Eynhallow, Orkney, we first confirmed that the egg measurements were repeatable from year to year. Second, using egg length, breadth, volume and three indices of shape, we compared the eggs from the eight two-egg clutches with (i) 100 pairs of eggs sampled at random from 111 single-egg clutches from the Faroes, and (ii) eggs laid in different years by 100 females on Eynhallow. Our analyses focused on differences between eggs in each pair. Differences in the eggs of two-egg clutches were more similar to those of pairs of eggs taken at random than to pairs of eggs from the same female in different years. We infer from this that the eggs in two-egg clutches were laid by different females.
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