The reproductive performance and body condition of silver gulls (Larus novaehollandiae) during a protracted breeding season

Silver Gulls Lams novaehollandiae were studied on Penguin Island, south-western Western Australia, during three peaks of laying over the eight month breeding seasons of 1987 to 1990. It was hypothesized that seasonal variation in their natural, food supply, would be less during the middle laying-peak, when there is no mass stranding of littoral vegetation on beaches, and that this would be reflected in the body condition of adults as well as characteristics affecting their reproductive success. Clutch-size, egg-size and egg mass did not vary annually, seasonally, or from island to island. Egg-size and mass, however, decreased during the laying sequence. The lipid and protein content of eggs was greatest during the early laying-peak and earlier in the laying sequence, and was more variable in first-laid eggs. There appeared to be no trade-off between clutch-size, egg-size and egg quality, suggesting that the first two may be heritable, whereas egg composition may reflect the body condition of the laying female. The duration of rapid yolk deposition did not vary seasonally, but did vary between females, and averaged ten days. Less yolk was deposited, and at a slower rate, in laterlaid eggs within a clutch. Day-specific growth rings in sibling eggs differed in size, suggesting that daily, yolk deposition was not maternally influenced and unique to each follicle. Embryonic growth followed the von Bertalanffy curve, was constant seasonally and between eggs, and appeared under fixed, probably genetic, control. Eggs containing developing embryos appeared to lose less water via conductance through the shell than other larids, possibly as an adaptation to the warm, mediterranean climate. Hatching success did not vary seasonally, but did vary within clutches. Since female Silver Gulls allocated more reserves to their first-laid eggs, later, smaller and less massive eggs hatched less frequently. The mass, but not the sex, of a hatchling could be predicted from egg mass. Although sex was related to egg sequence, the overall ratio was unity, and suggested that sex determination is a random, meiotic event in Silver Gulls. Hatching success was independent of egg quality, but post-hatching growth and survival were directly related to egg quality. Chicks which died had exhausted their yolk sacs. Surviving hatchlings grew exponentially during the first month of life as the water index of tissues decreased in the entire chick and in its leg musculature. Contrary to initial expectations, adult condition was maximal during the middlelaying peaks, reproductive performance was not proximately constrained by food availability to females, and the gulls were omnivorous. This study showed that the reduced clutch-size and size of later-laid eggs within clutches of Silver Gulls were not proximately controlled by the body condition of adults. Double-brooding and long breeding lifespans mean that a relatively low reproductive effort within a single laying-bout would not necessarily decrease the lifetime reproductive output of Silver Gulls relative to shorter-lived, single-brooded species of temperatenesting larids that lay clutches of three eggs