Home range and territoriality of Australian owlet-nightjars Aegotheles cristatus in diverse habitats
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Sedentary Australian owlet-nightjars Aegotheles cristatus roost in cavities year-round, and are thought to mate for life and maintain stable home ranges throughout the year. These factors lead to the prediction that they should be highly territorial, yet nothing is known about their home range requirements or level of site fidelity. I used radio-telemetry coupled with GPS recordings to determine home range size in the semi-arid zone of central Australia (13 birds over two winters) and in a eucalypt woodland in the Northern Tablelands of NSW (14 birds over one calendar year). The mean home range in the eucalypt woodland was 17.7 ha based on the minimum convex polygon (MCP) method and 17.4 ha based on the fixed kernel contour (95%), whereas it was 23.8 ha and 24.1 ha based on the MCP and kernel methods in the desert respectively. With the exception of mated pairs (range overlap 41.9%), there was little overlap among individuals in home range areas (<13.0%), even in the densely populated woodland. Home range size did not differ significantly between seasons or study sites; nor was it correlated with arthropod abundance. Owlet-nightjars exhibited high site fidelity, using the same home range throughout the year with a 68% overlap between seasons. These data, plus anecdotal records of aggression and vocalizations, suggest that this species is highly territorial, yet individuals live in close proximity to conspecifics. The degree of territoriality may be due to the obligate cavity roosting behaviour of owlet-nightjars and the need to defend areas with known roost hollows.