Year: 1980
Type: article
Abstract: In 1968, Schreiber (1970) found that 11.3 per cent of the clutches of Western Gulls (Larus \noccidentalis wymani) on San Nicolas Island had more than three eggs. He believed that the low \nhatching success (0.13 per cent) of these clutches was the result of insufficient heat transfer in \nincubating four or more eggs. On Santa Barbara Island in 1972, 11 per cent of clutches were \nfound to consist of more than three eggs, with 4 . 9 per cent hatching success (Hunt and Hunt \n1973). It was believed, at first, that these eggs all came from the same female, as the eggs were \nof similar color and only two adults could be identified on each territory. Between 1972 and \n1976, the frequency of clutches with supernumerary eggs varied between 7 and 13 per cent. It \nalso became apparent that these were laid by female-female pairs. \nEvidence for female-female pairing found by Hunt and Hunt (1977) is as follows: \n(1) In three-egg clutches, 67 per cent of eggs were laid two days apart, whereas in the larger \nclutches many were laid on consecutive days and 11 per cent on the same day. This suggests that \nat least two females were laying in the same nest. \n(2) Most eggs in the clutches with more than three eggs were infertile. While 81.5 percent of \nthe eggs of normal clutches showed development, zero to 14 per cent developed in the larger \nclutches. The few eggs that were fertile resulted from promiscuous, heterosexual matings. \nIndeed, marked birds have been observed mounting or being mounted by birds other than their \nmate. In such situations males were off their own territory, and in five of 18 cases females were \nalso off their own territory. \n(3) The sex of gulls trapped on nests perhaps provides the most convincing evidence. Hunt \nand Hunt (1977) trapped 10 male and 15 female adult gulls incubating normal clutches (one to \nthree eggs), and one male and 74 females on larger clutches. Most important, on 23 nests \ncontaining larger clutches, two females were captured. \nThus, it appears that on territories where nests contained large clutches, two females are in \nattendance with no apparent permanent association of males for at least one to two weeks prior \nto clutch initiation. These females essentially act as though they are heterosexually paired and \nremain together from one year to the next, although in one instance a female paired to another \nfemale in 1976 was found paired to a male in 1977. They engage in most of the usual courtship \nand territorial behavior but relatively rarely show mounting and copulation behavior normally \nattributable to males. If given hatching eggs, they are capable of raising young (G. Hunt, M. \nHunt, and S. Anthony, unpubl. field notes). \nThe existence of female-female pairs raises many questions. If the quality of a mate is \nassessed in courtship, how is the mechanism failing, and how widespread and important is \nPromiscuous, heterosexual mating? Why do female-female pairs form? For the last question we \nhave two working hypotheses, which are not mutually exclusive. These hypotheses are the \nsubject of this paper.
Authors: John C. Wingfield, A. Martin, Molly W. Hunt, George L. Hunt, Donald S. Farner
Language: English
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2402915337