Distribution and foraging by non-breeding Caspian Terns on a large temperate estuary of south-western Australia-preliminary investigations

Abstract

This study investigates the distribution, abundance, and foraging ecology of Caspian Terns, Hydroprogne caspia, during 5 months of their non-breeding season, in the Peel-Harvey Estuary, south-western Australia. Observations were carried out at 20 sites around the estuary and 6 main areas (13 sites) where terns were abundant. Terns were observed every hour over 5 h time-blocks in the morning, midday, and afternoon, and the number of birds, number of birds foraging and time spent foraging were recorded for 10 min on the hour. From the 760 h of observation, a single overnight roosting site was identified in November, where a maximum of 147 birds were counted in February, after which time the roosting site appeared to shift. The total number of terns, foragers and proportion of time foraging varied amongst the six areas and foraging activity differed amongst times of day. Two areas, both characterised by large, sandy spits adjacent to shallow water, one adjacent to a river mouth and one near an ocean channel, were particularly important for terns and their foraging. Foraging activity was higher in the morning than at other times of day. Although salinity, air temperature, water temperature and wind speed were correlated with the total terns, foragers and proportion of time foraging, the correlations accounted for <25% of the total variation explained. The results of this study provide information for evaluating the use of Caspian Terns as bio-indicators of the Peel-Harvey Estuary and highlights the importance of this system during the non-breeding period.