Katsufumi Sato is curious to know how birds get about: how they swim; how they dive; how they fly; how they glide. Which is why he was on a boat in the South Indian Ocean in 2006 studying albatrosses. At the time he wanted to know how much time these intrepid voyagers spend gliding. Sato remembers that he happened to glance out of the boat's window one day and saw a shag flying past. According to Sato he was instantly struck by how stable the flight was. It occurred to him that he might be able to estimate the bird's body mass from the frequency of the wing beat(p. 58), and what was more he already had the data that he needed to test this theory.

But the shag flight data had been collected to answer a completely different question. Sato explains that at the time he was intrigued by how diving birds propel themselves underwater with their feet and was looking for a cooperative bird on which to strap tiny accelerometers so that he could monitor their foot beat frequency. Sato struck up a collaboration with Francis Daunt and Sarah Wanless when Yutaka Watanuki and Akinori Takahashi suggested that shag would `make good subjects for the accelerometer' says Sato.

Travelling to the Isle of May off the Edinburgh coast Sato, Watanuki and Takahashi monitored the diving birds with Daunt and Wanless's help. The team weighed each bird before fitting an accelerometer and depth gauge to their backs and releasing them. As all of the birds were feeding chicks at the time,Sato recalls that the team had no problems retrieving the accelerometer at the end of a day's foraging and could clearly see from the depth gauge records and acceleration patterns when the birds had been airborne, diving and moving on land.

Knowing that he would have plenty of time on his hands monitoring the albatrosses off Crozet Island, Sato had taken the data with him. So, when inspiration struck about a link between wing beat frequency and body mass,Sato was quickly able to put his new idea to the test.

Calculating the wing beat frequency from the acceleration data, Sato found that once steady flight had been established each bird had its own distinctive wing beat frequency. And when the birds embarked on a foraging trip, Sato noticed that the wing beat frequency increased gradually after diving. Knowing the bird's initial mass, Sato was able to calculate the increase each time the bird returned to the air, finding that some birds only gained a few grams after diving while one gained 150 g after multiple dives. Having watched some of the birds and knowing that they usually shook themselves free of water before taking to the wing, Sato and colleagues are confident that the mass gain could be accounted for by the bird's fish load.

`I didn't expect this result' Sato admits and he is optimistic that it could help ecologists better understand birds' interactions with their environments. For example, he explains that there is currently a conflict between Japanese fisherman and the great cormorant. The fisherman accuse the birds of plundering commercial fish stocks. Sato hopes that his new technique could lay this matter to rest by showing just how much fish a hungry cormorant consumes.

References

Sato, K., Daunt, F., Watanuki, Y., Takahashi, A. and Wanless,S. (
2008
). A new method to quantify prey acquisition in diving seabirds using wing stroke frequency.
J. Exp. Biol.
211
,
58
-65.