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The study was made on Somes Island in Wellington Harbour during the 1961-62 breeding season. Incubation behaviour, development of the incubation drive and methods of nest relief are discussed. The average incubation period is 27 days, with extremes of 23 and 30 days. The average breaking period of the eggs is three days, with extremes of one and six days. Two-egg clutches hatch in one to five days, three-egg clutches in two to six days. In most two- and three-egg clutches incubation becomes effective on arrival of the second egg. Of all the eggs from first clutches, 66.1% hatched successfully. Half of those failing to hatch were addled or contained dead embryos. Three-egg clutches had a higher hatching percentage than two-egg clutches, which were in turn more successful than one-egg clutches. Egg losses are correlated with nesting density; greatest losses occurring in areas of highest density.

Brooding and defensive behaviour are outlined, and the feeding of chicks discussed. A wide variety of foods is offered the chicks, but in general they are fed whatever happens to be handy and available in quantity. Chicks leave the nest two to three days after hatching, are able to swim at five to six days of age, and fly at about seven weeks. The egg tooth is lost on the eighth or ninth day, the yolk-sac scar disappears by the end of the third week, and the beak becomes wholly black after five weeks. Young birds leave the colony within a month of fledging.

Minimum chick mortality to the flying stage was 19.8% – heaviest losses being sustained in the first week after hatching. The minimum overall mortality of eggs and chicks to the flying stage was 46.9%, and a mean of 1.3 chicks per breeding pair reached the flying stage. The majority of breeding adults found dead were males, most of which died from wounds inflicted by other gulls. A few immature birds in the colony showed incomplete breeding behaviour.