Population trends over 50 years of a Pied Imperial-Pigeon breeding colony on North Brook Island, a tropical Great Barrier Reef island, Australia

The Pied Imperial-Pigeon (Ducula bicolor) is a summer migrant to northern Australia, and nests colonially on many islands of the Great Barrier Reef, Queensland. Numbers are perceived to have declined as a result of large-scale shooting in the past and clearing of their mainland feeding grounds. To obtain evidence of any population changes, 1-10 counts were made each breeding season of the evening flight from the mainland to a nesting colony on North Brook Island. The counts began in 1965 and have continued for 50 years. The count represents one individual of a pair that visits the mainland to feed and returns in the evening. In the 1965-66 breeding season, the December count (the count used to compare years) was 4692 birds. Over the following three seasons, numbers declined to a low of 1451 birds in 1968-69; this decline was attributed to shooting of up to 1100 birds at a time. Large-scale shooting stopped in 1968 and was followed by a steady exponential increase (y=3370.4e(0.0908x)) in numbers over 23 years to 29 818 birds in the 1992-93 season. Numbers have fluctuated between similar to 25 000 and 35 000 individuals over the following 18 years. A tropical cyclone of extreme intensity affected the pigeons' breeding site on North Brook Island and their mainland feeding areas in February 2011. In the following breeding season of 2011-12, numbers fell to 5311, but recovered to 21 088 in the 2012-13 season. The dramatic drop in numbers was attributed to the pigeons moving elsewhere for the 2011-12 season. It is predicted that the post-cyclonic number of 21 088 will take 4 years to recover to 30 000. The counts are a simple and somewhat coarse measure of population trends, but provide a population baseline for more sophisticated studies on the movements of pigeons and their responses to long-term changes in habitat and climate