数理解析研究所講究録
第 1900 巻 2014 年 52-55

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Scaling of Dynamic Soaring Flight in Procellariiformes Seabirds
ミズナギドリ目海鳥におけるダイナミックソアリングのスケーリング則米原善成 (Yoshinari Yonehara) 佐藤克文 (Katsufumi Sato)
大気海洋研究所 (Atmosphere and Ocean Research Institute)、yonehara@aori.u-tokyo.ac.jp

Introduction
Some of the large albatrosses, shearwaters, and petrels have an extreme travelling
performance by covering large distances in their foraging trips and migration (Croxall et al.,2005; Jouventin and Wiemerskirch, 1990). The reason for this large distance and long time
travelling performance is a unique flight mode termed dynamic soaring (Wilson, 1975). Using
dynamic soaring, birds gain energy from the wind above the ocean surface to maintain flight.

Wind speed is high far above the ocean but lower near the surface because of friction with theocean surface and a wind gradient is made. Some studies examine the mechanism of how
birds gain energy from the wind (Sachs et al., 2013; Pennycuick, 2002; Lissaman, 2005) andthey all refer to the use of wind gradient. When dynamic soaring is observed in fine scale, it

can be divided into 4 characteristic phases: upwind climb, turn to downwind, downwind glide,and turn to upwind (Fig. 1). The bird can fly sustainably by repeating this cycle. As a result, itis said that the bird is able to fly with less cost close to basal level when resting(Weimerskirch et al., 2000). Procellariiformes seabirds have long pointed wings with highaspect ratio which is suited for high speed gliding (Videler, 2006). Thus, dynamic soaring isfundamental for the extreme travelling performance of large Procellariiformes seabirds.
Dynamic soaring of large albatrosses, especiallywandering albatross Diomedea exulans, attracted attention
because of their energy efficient sustained flight almost
without flapping, and most of the studies of dynamicsoaring flight of seabirds investigate the flight ofwandering albatross (Richardson, 2011; Sachs et al., 2013).

Fig. 1. Dynamic soaring

Some smaller species also perform dynamic soaring withsome degrees of flapping included. Time percentage for flapping decreased with body size
(Sato et al., 2009; Pennycuick, 1982), indicating that not all the energy for flight is gained

from wind energy in small species. Therefore, it can be considered that there is an optimal
flight style for each bird in response to their size and morphological character (Suryan et al.,2008). However, only few studies examine their flight performance in fine scale, due to thedifficulty to observe their flight performance at open sea. Recent development of miniaturizedanimal borne data loggers enable fine scale and long duration recordings of bird movement

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during flight. The aim of this study is to compare dynamic soaring flight of Procellariiformes
species of different size.

Fig. 2. Studied seabirds. Streaked shearwater, $white\cdot$ chinned petrel, sooty albatross,black browed albatross, wandering albatross (from left to right).

Results
Scaling of cyclic rolling movement
Body mass of birds were $569\pm 51g(mean\pm s.d.)$ for streaked shearwater, $1343\pm 83g$ forwhite-chinned petrel, $2240\pm 10g$ for sooty albatrosses, $3500\pm 257g$ for black-browedalbatrosses, and $9600\pm 1205g$ for wandering albatrosses. Rolling cycle of five species of54
Procellariiformes detected by PSDs calculated from the acceleration data were 2. $5\pm 0.5s$$(mean\pm s.d.)$ for streaked shearwaters, $4.3\pm 0.5s$ for $white\cdot$ chinned petrels, $5.2\pm 0.4s$ forsooty albatrosses, 7. $3\pm 1.0s$ for black-browed albatrosses, and $12.3\pm 1.8s$ for wandering
albatrosses. Rolling cycle of five species of Procellariiformes showed clear relationship withbody mass, with rolling cycle being longer in larger species.
(rolling cycle)$\alpha$$($body $mass)^{0.57}$

From the streaked shearwater with GPS and acceleration logger, 9 flights longer
than 10 minutes were observed during 8 hours of recording. The flight track of streaked
shearwater showed fine scale zigzag movement associated with dynamic soaring. The zigzag
is consisted of slow speed phase which is assumed to be the windward climb and fast speed
phase assumed to be the downwind descent. However, wind speed and wind direction were
not recorded so speed represents the ground speed ofthe bird. Most of the flaps were observed
when speed decreased where it is assumed to be the windward climb. Fewer flaps were seen
during fast speed phase. The cyclic rolling movement started right after the turn fromwindward to downwind and ended at the bottom of downwind descent. This was also
confirmed by video data.

Discussion
From the GPS data, dynamic soaring cycle of streaked shearwaters and wandering, respectively. However, rolling cycle of streaked
andalbatrosses were aroundshearwaters and wandering albatrosses were 2. $5s$ and 12. $3s$ , respectively. Note that dynamic$8\cdot 10s$

$10\cdot 15s$

soaring cycle and rolling cycle were different. While dynamic soaring cycle represents thecyclic change of heading direction, rolling cycle represents the cyclic change of roll angle.
Therefore, flapping, gliding, rolling, and other movements could be included in one dynamic

soaring cycle. The rolling movement of large wandering albatross (12s) covered almost theentire dynamic soaring cycle $(10\cdot 15s)$ . This suggests that wandering albatrosses obtainedenergy from wind to sustain flight solely by rolling movement. On the other hand, the rolling
movement of small streaked shearwaters did not cover the entire dynamic soaring cycle andcould only be seen starting from the turn to downwind and through downwind glide. Whilethe upwind climb, streaked shearwaters were flapping frequently and the body angle might
be kept horizontal, so the rolling movement was not present. Thus, the rolling movement of
streaked shearwaters was considerably shorter than the dynamic soaring cycle. These

differences in flight styles might explain the scaling relationship of the rolling cycle indynamic soaring seabirds.

We can consider that dynamic soaring is consisted of two phases: a phase where thebird gain energy from the wind and a phase where the bird itself produces energy by flapping.

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In fact, from GPS and acceleration data, flight of streaked shearwater could be roughly

divided into two phases: flapping phase during upwind climb and gliding during downwind
descent. Rolling movement recorded in this study corresponded to the downwind descentphase where the bird gains energy from the wind. Dynamic soaring as a whole might be a

combination of rolling, gliding, and flapping, however, when considering energy gain from thewind, rolling movement might have an essential role. Rolling cycle of five species ofProcellariiformes showed clear relationship with body mass and there might be a definite
explanation of this scaling relationship based on physical mechanism.

References