Long-term variation in the distribution of the White-bellied Sea-Eagle (Haliaeetus leucogaster) across Australia
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The White-bellied Sea-Eagle Haliaeetus leucogaster (Accipitridae) is widespread within Australia. However, in a number of states it is thought to be declining locally in response to human induced disturbance. Here we analyse the Australian Bird Atlas data to identify the extent and pattern of change in range and density of the species between three Atlas Periods (1901-1976, 1977-1981 and 1998-2001) using a new standardized frequency measure, the Occupancy Index (OI) for 1ࠢlocks (approx. 100 km2) across the continent. At the continental scale, there was no significant difference in the spatial extent of occupancy between Atlas Periods. However, there were considerable changes in frequency and range extent between defined regions, and there were distinct differences in the pattern of change in OI between coastal and inland blocks over time. Coastal blocks showed much more change than inland blocks, with a clear increase in the use of coastal blocks, accompanied by a decrease in inland blocks, during the 1977-1981 Atlas Period, relative to both other Atlas Periods. While there were slight (and not statistically significant) trends for OI to increase in areas containing dams, and to decrease in urbanized coastal areas, the over-riding factor associated with distributional shifts and frequency changes was apparently climatic fluctuation (the 1977-1981 period showing the influence of El Niᯠassociated drought). Within this study, the impression of abundance was strongly dependent on both the temporal and spatial scale of analysis. This highlights the importance of large-scale analysis in interpreting change in distribution and abundance of widespread species.