Abstract and Summary
In contrast to their behaviour at sea, Manx shearwaters (Puffinus puffinus) are highly-vocal at their breeding colonies during their nocturnal visits. The vocal activity shows seasonal changes that suggest the immature population is responsible for the majority of the calling. This was confirmed by catching birds in traps playing calls and also in mistnets. Male calling predominates at ground level and female calling predominates in flight. The trapping results and observations on immatures marked with small luminescent lights confirm this is due to males being responsible for establishing and defending burrows. The mistnetting also revealed a proportion of males that are comparatively silent in flight. It is argued that these represent birds as yet without burrows. As well as calling in flight, the marked birds called from within their burrows and from on the surface. Other characteristics of flighting activity are presented, which together with playback results, suggest that the process facilitates signalling, listening and assessment of breeding areas in a nocturnal species nesting in an open habitat and with poor terrestrial locomotion. The results suggest that burrow calling serves both sexual and territorial functions, and that aerial calling seems to be mainly concerned with sexual advertisement, but may also discourage intruders.