FLIGHT SPEED OF SEABIRDS IN RELATION TO WIND-SPEED AND DIRECTION

We studied flight speed among all major seabird taxa. Our objectives w ere to provide further insight into dynamics of seabird flight and to develop allometric equations relating ground speed to wind speed and d irection for use in adjusting seabird density estimates (calculated fr om surveys at sea) for the effect of bird movement. We used triangulat ion at sea to estimate ground speeds of 1562 individuals of 98 species . Species sorted into 25 ''groups'' based on similarity in ground spee ds and taxonomy. After they were controlled for differences in ground speed, the 25 groups sorted into eight major ''types'' on the basis of response to wind speed and wind direction. Wind speed and direction e xplained 16-64% of the variation in ground speed among seabird types. For analyses on air speed (ground speed minus apparent wind speed), we divided the 25 groups according to four night styles: gliding, flap-g liding, glide-napping and napping. Tailwind speed had little effect on air speed of gliders (albatrosses and large gadfly petrels), but spec ies that more often used napping decreased air speed with increase in tailwinds. All species increased air speeds significantly with increas ed headwinds. Gliders showed the greatest increase relative to increas e in headwind speed and flappers the least, With tailwind night, air s peeds were greatest among species with highest wing loading for each f light style except gliders, which showed no relationship. For headwind flight, species with higher wing loading had higher air speeds; howev er, the relation was weaker in flappers compared with species using so me amount of gliding. In contrast, analyses for air speed ratio (i.e. difference between air speed in acrosswinds [with no apparent wind] an d speed flown into headwinds, or with tailwinds, divided by speed acro sswind) revealed that among species using some napping, and with lower wing loading (surface-feeding shearwaters, small gadfly petrels, stor m petrels, phalaropes, gulls and terns), adjusted air speeds more than those with higher wing loading (alcids, ''diving shearwaters''. ''Man x-type shearwaters'', pelicans, boobies and cormorants). As a result, most flappers of low wing loading flew much faster than V-mr (the most energy efficient air speed per distance flown) when flying into headw inds. We suggest that better-than-predicted gliding performance with a crosswinds and tailwinds of large gadfly petrels, compared with albatr osses, resulted from a different type of ''soaring'' not previously de scribed in seabirds.