ABSTRACT
Observations of flight orientation have shown that marine birds utilise a variety of local gradients in atmospheric circulation. Dynamic soaring depends upon vertical accelerations of the wind w/ z, and possibly on shearing of the wind v/ z (Idrac, 1925; Cone, 1964). Sweeping flight in updrafts has been noted frequently (Dixon, 1933; Routh, 1949; Wilson, 1975). Rotary movement upward in thermal convection cells occurs at wind speeds less than 7m·s-1 (Woodcock, 1940, 1975). This behaviour is commonplace at low latitudes with tropical species, notably sooty terns Sterna fuscata (Dinsmore, 1971) and frigate birds Fregatta spp. Linear upward movements in roll vortices occur at windspeeds of 7-13 m·s-1 when the air is unstable i.e., colder than the ocean; soaring ceases at wind speeds greater than 13 m·s-1 (Woodcock, 1975). At high wind speeds gliding occurs in auks (Pennycuick, 1987) as well as procellariids such as shearwaters. Oblique streaming has been reported at mid-latitudes (Kuroda, 1957; Cheshire, 1981) often with the wind to the right in the Northern Hemisphere (Blomqvist & Peterz, 1984) and to the left in the Southern Hemisphere (Norris, 1965). The mechanism postulated by Blomqvist & Peterz (1984) was movement along wave troughs, which tend to be orientated to the left of the wind (or right in the Southern Hemisphere) in the wake of passage of mid-latitude low pressure systems. This orientation will diverge populations away from storms. It will also result in clockwise rotation along the coast of an enclosed sea in the Northern Hemisphere (Blomqvist & Peterz, 1984). This mechanism is consistent with reports of movement along wave troughs in high winds (Lockley, 1974; Jansen, 1981). This mechanism does not require a change in behaviour by trans-equatorial migrants such as greater shearwaters Puffinus gravis.
