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Breeding of Australian White Ibis, Straw-necked Ibis and Silver Gulls on Winter Swamp, Ballarat from 1990-1997


Breeding populations of Silver Gull Larus novaehollandiae, Australian White Ibis Threskiornis molucca and Straw-necked Ibis Threskiornis spinicollis were studied, from 1990 to 1997, on Winter Swamp, Ballarat, Victoria. Silver Gulls and Australian White Ibis bred from early July to early February but there were differences amongst years. Factors affecting the variation in the starting date and length of breeding season are discussed. Australian White Ibis had a mean clutch size of 2.25 (S.D. = 0.14) and a mode of three. There was a 6.8 times increase in the number of Australian White Ibis pulli banded each year from 1990 to 1996. Breeding effort of the population, measured by the number of ibis pulli banded per month, was influenced by the depth of water in the swamp during the previous month (r2 = 0.23, F1,18 = 5.00, P = 0.040). Silver Gull mean clutch size was 1.92 (S.D, = 0.10) and the mode, two eggs. Between 1991 and 1995 there was a 15 fold increase in the number of gull pulli banded. There was a weak relationship between the number of gull pulli banded and evaporation during the previous month (r2 = 0.11, F1,36 2, = 4.33, P = 0.044)

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