Consequences of a female‐biased sex‐ratio in a socially monogamous bird: female‐female pairs in the Roseate Tern Sterna dougallii

• Published in: Ibis
• Main Authors: Nisbet, Ian C.T., Hatch, Jeremy J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1999
• Subjects: Bird Island
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1999.tb07553.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1999.tb07553.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1999.tb07553.x

In the socially monogamous gulls and terns, female‐biased sex ratios are sometimes revealed by the occurrence of ‘supernormal clutches’, which are usually attended by female‐female pairs or other multi‐female associations. We studied these phenomena in the endangered Roseate Tern Sterna dougallii at Bird Island, USA, from 1970 to 1995. DNA‐techniques were used to sex breeding adults in 1992–94. Supernormal clutches (with three or four eggs) have comprised 1–7% of all Roseate Tern clutches at Bird Island since at least 1970, probably increasing in frequency since 1980. Supernormal clutches were spatially clustered; most were laid late in the peak period of nesting during each season. More than 80% of supernormal clutches and at least 7% of normal clutches were attended by multi‐female associations; most of these were female‐female pairs, with a few trios (male + two females, or three females) and one quartet (four females). More than half of the multi‐female associations attended normal clutches. Some female‐female pairs were maintained for up to five years. The age‐distribution of females mated to females did not differ significantly from that of females mated to males. Females mated together usually laid eggs synchronously (±2 days). Such females laid fewer eggs than females mated to males (means 1.20 versus 1.73), and had lower fertility and hatching success (about 46% versus 98%); they were less successful in raising young from eggs that did hatch (means 58% versus 73%), but this difference was not significant. Their overall breeding success was much lower (about 0.34 fledglings per female versus 1.35). The sex‐ratio of breeders was about 127 females to 100 males; about 20% of breeding females did not have male mates. Female Roseate Terns that do not obtain male mates appear to be of low phenotypic ‘quality’ ‐ based on late laying, small clutches and small eggs. Our data support the hypothesis that such females have a higher fitness if they mate with each other and raise a few young than if they do not breed ...