Population increase and nesting patterns of the black noddy Anous minutus in Pisonia forest on Heron Island: Observations in 1978, 1979 and 1992

Abstract Estimates of the population size of black noddy Anous minutus on Heron Island were made by counting nests in permanently marked plots in Pisonia grandis forest in 1978, 1979 and 1992. The results, and published data, indicate that the population has been increasing at ca 7% per annum since early this century and currently is ca 63 000 ± 7000 pairs. The continued exponential increase in bird numbers poses interesting ecological questions and potential management problems, as more of the island is occupied by the birds and interaction with the vegetation intensifies. Pisonia is by far the most common tree on the island and most nests are in this species. However, there is evidence that Ficus opposita is preferred over Pisonia and the high mortality of this species in the marked plots may be due to excessive use by noddies. Pisonia trees which reach the forest canopy and are in the 40–60 cm stem diameter class have more nests than smaller trees. Larger stems (>60 cm diameter) are also underutilized relative to their size, and it is suggested that this is because they are more liable to windthrow in cyclones.