DIVING SPEEDS AND ANGLES OF A GYRFALCON (FALCO-RUSTICOLUS)

An optical tracking device recorded the three-dimensional paths of 11 dives by a 1,02kg gyrfalcon, trained to dive to a falconer, The dives started at altitudes up to 500 m above the ground and were inclined at angles of 17-63 degrees from the horizontal. The falcon controlled it s speed during the dives, rather than simply falling from the sky, and the dives had three phases. During the first (acceleration) phase, th e falcon accelerated to speed limits between 52 and 58 m s(-1) in the seven fastest dives, evidently with minimum drag, because its accelera tions were close to those predicted from theory for minimum drag, The falcon then began a constant-speed phase by increasing drag by a facto r of 1.3-4.8 while still 100-350 m above the ground in most dives. The constant-speed phase lasted no more than a few seconds, and the falco n then began a deceleration phase by increasing its drag further, this time by factors of 1,7-3,2, and decelerating with a mean value of -0. 95 times gravitational acceleration. During all three phases, the dive angle was nearly constant or increased during the deceleration phase, and the falcon made no changes in its body shape that mere obvious th rough the tracking device telescope except to reduce its wing span as it accelerated. The falconer, however, was close to the falcon at the end of the dive and could see that, during the deceleration phase, the falcon held its wings in a cupped position, apparently with a high an gle of attack and therefore high drag. At the end of the deceleration phase, the falcon dropped its legs, spread its toes and finally spread its wings as it approached the falconer, Although the speeds reported here are the fastest ever measured with known accuracy in animals, th e falcon could theoretically have reached more than 70ms(-1) if it had continued to accelerate with minimum drag until close to the ground. Even at this speed, it would have had enough altitude to pull out of t he dive before crashing into the ground. Several authors have estimate d that diving falcons reach speeds of more than 70 m s(-1), and wild f alcons may reach such speeds when they make long, steep dives upon bir ds flying high in the air.