Reproductive Ecology of Silver Gulls, Larus-Novaehollandiae Stephens, in South-Australia

Abstract

Aspects of the reproductive ecology of silver gulls, Larus novaehollandiae Stephens, were studied in South Australia during 1968-78. Some 15 950 pulli and 597 breeding adults were banded on 15 colonies, using numbered and coloured leg bands designed for identification of free-living individuals without the necessity of recapture. Nine colonies were on marine islands, four were on lacustrine islands, one was on a port breakwater, and one was on causeways in a saltworks complex. Fidelity to colony, mate and nest-site, and timing of breeding are considered in detail. The spatial relationship of adult and immature gulls, on the colony studied in greatest detail, and reproductive longevity of the gulls are also discussed. The gulls nested from June to December inclusive, although the commencement and length of breeding seasons varied between colonies and between years. There was a marked absence of 1-year-old gulls on breeding colonies. Of 3530 pulli banded on the major study colony, Penguin Island (37º30''S., 140º01''E.) at Beachport, in 1968, 950 were known to be alive in the 1970 breeding season. Almost 40% of these 2-year-olds were seen on breeding colonies, and mostly on the natal colony; however, only 0.3% were known to raise pulli, and a further 0.7% to produce eggs. Significant breeding activity therefore did not commence until the end of the third year, or later. The older, previously established breeders on Penguin Island were significantly more successful, and had significantly earlier laying dates, than younger birds. Potential reproductive longevity appears to be at least 11 breeding seasons. In this study, silver gulls showed considerably less fidelity to nest-sites and colonies than has been suggested previously. Significant shifts of nest-site, or change of colony, with or without the previous mate, were not uncommon. Of gulls banded on nests as established breeding aduls, 7-44% of gulls found breeding in subsequent years were at colonies other than the one where banded. In 1970, over 80% of 2-year-old gulls recorded present on any colony were only recorded on the natal one. Of the pulli banded on Penguin Island in 1968 and 1969 and later found actually nesting or breeding, 18% were recorded on other colonies. A small percentage of gulls were seen to have high fidelity to nest-site and to mate over 3 or 4 seasons, but it is suggested that, for some colonies, over 50% of silver gulls which eventually breed may do so at colonies other than natal. Significant spatial separation was found on Penguin Island between areas generally occupied by established breeders, which were known to be at least 4-year-olds, and by the 2-year-olds. The older birds may select and hold the territories most suitable for nesting, where vegetation or topography minimise intraspecific aggression which often leads to high mortality rates of eggs and pulli, and assist protection of eggs and pulli from extreme weather. Non-breeding gulls associating with the colonies occasionally move into the nesting areas, but are aggressively forced away and tend to congregate on the periphery of the colony or on exposed areas not suitable for successful breeding.