Female-Female Pairings in Caspian Terns

Abstract

The frequency and cause of clutches were investigated in two Caspian Tern (Sterna caspia) colonies in the Pacific Northwest. Over 4% of the nests contained four eggs and 0.8% contained five to six eggs. Trapping and sexing all attendants at two three-egg nests, two four-egg nests, and one fiveegg nest revealed that the three-egg nests were incubated by heterosexual pairs and the clutches (four to six eggs) by female-female pairs. Although clutches in Caspian Terns were rare throughout North America before 1940, they have increased in frequency since 1950 in the U.S. but not in Canada. Female pairings occur when one female associates with another instead of with a male during the breeding season. Both females lay eggs in the same nest, often producing a supernormal clutch of four to six eggs, twice the normal number. The two females share parental responsibilities including incubation duties, territorial defense, and care of any chicks resulting from promiscuous matings with males (Hunt and Hunt 1977). This phenomenon has been discovered in four gull species: Western Gull (Larus occidentalis; Hunt and Hunt 1977), Ring-billed Gull (L. delawarensis; Ryder and Somppi 1979, Conover et al. 1979a), California Gull (L. californicus; Conover et al. 1979a) and Herring Gull (L. argentatus; Shugart 1980). Heretofore, female pairs have been reported only in gulls. In this study, I surveyed Caspian Tern (Sterna caspia) colonies in eastern Washington and Oregon in order to determine the relative frequency of clutches and if they resulted from female pairings. I also examined whether the frequency of clutches had increased since 1950, an expected occurrence if DDT contributed to the frequency of female pairings in this species by feminizing male embryos, as Fry and Toone (198 1) have hypothesized, or by causing higher male mortality.