Sex Ratios of Prefledging Western Gulls

Western Gull (Larus occidentalis) chicks on Santa Barbara Island, California, had a sex ratio at hatching of 1.12 M/F (n = 609); the sex ratio of chicks ->35 days of age was 0.89 (n = 189). The sex ratio at hatching and fledging did not vary significantly from 1.0 or from each other, but the data suggest that male mortality before fledging exceeded that of females. Depressed growth rates of male chicks hatched third may be responsible for these higher male mortality rates. We found no evidence for seasonal or hatching-order effects on sex ratios at hatching. We suggest that postfledging differences in mortality be- tween the sexes are in part responsible for the skewed sex ratio (0.67 males/female) observed in the adult breeding population. Received 4 October 1985, accepted 11 June 1986. biased sex ratios in species in which young are sexually dimorphic in size at the termination of parental care. Numerous recent studies have investigated the hatching and fledging sex ratios of sexually dimorphic birds. Newton and Marquiss (1979), working with the Eurasian Sparrowhawk (Ac- cipiter nisus), Fiala (1981) with Red-winged Blackbirds (Agelaius phoeniceus), Lombardo (1982) with Eastern Bluebirds (Sialia sialis), and Harmsen and Cooke (1983) with Lesser Snow Geese (Chen caerulescens) found no deviation from a 1:1 sex ratio at hatching or fledging. Others found variation in sex ratios at hatching as a function of hatching order (Ankney 1982, Lesser Snow Goose; Davies and Payne 1982, domestic fowl, Gallus domesticus; Ryder 1983, Ring-billed Gull, Larus delawarensis; Weather- head 1985, Red-winged Blackbird) or season (Howe 1977, Common Grackle, Quiscalus quis- cula). Differential chick mortality resulting in biased sex ratios at fledging were found for Common Grackles (Howe 1976), Common Cap- ercaillie (Tetrao urogallus; Wegge 1980), North- ern Harriers (Circus cyaneus; Picozzi 1980), and Red-winged Blackbirds (Blank and Nolan 1983). 33 In Western Gulls (L. occidentalis), males are larger than females. The breeding population of Western Gulls on Santa Barbara Island (SBI), California, has an estimated adult sex ratio of 0.67 (males/female) (Hunt et al. 1980). The fe- male-biased sex ratio in the adult population could result from differential mortality after fledging. Alternatively, differential mortality of chicks could initiate sex-ratio bias at hatching, particularly if males required greater parental investment and an adjustment of the sex ratio before the termination of parental care result- ed. We tested the hypothesis that a female bias in the sex ratio of chicks before fledging was responsible for the observed bias in the adult breeding population on SBI. Additionally, we sought evidence for mechanisms that might re- suit in a biased prefledging sex ratio.