wind and waves | 115is lower above the wing than below. The effect, enhanced by the cam-
bered shape of the albatross wing, is analogous to the generation of lift
over an aircraft wing. Then, as the combination of drag, the downward
pull of the bird’s weight, and the diminishing wind- speed gradient pre-
vent further useful ascent, the bird reaches the phase of the cycle when
it has minimum energy. It turns to leeward and glides downward, gain-
ing energy before turning into the wind and beginning the cycle afresh.
The overall track is characteristically somewhere between downwind
and across the wind. It needs wind speeds of at least 30 km/h, which of
course explains why albatrosses are mostly found in the world’s windier
oceans where they are spared the energetic costs of flapping.
The techniques described rely on the fact that albatrosses are very
efficient gliders that can travel a considerable distance horizontally for
every metre dropped vertically. Their glide ratios are around 22– 23. This
means that, on the descent from a height of, say, 8 m, an albatross can
travel 22 times that distance, about 180 m, before rising upwards once
more; worse than a man- made glider (glide ratio 40– 45), but better than
an eagle or vulture (about 15). However, the track is necessarily zigzag,
and so albatrosses and indeed other petrels are travelling further than
the simple straight- line distance between start and end points. In the
larger albatrosses, the zigzag track is about 50% further than the straight-
line distance.
Languid is the word that comes to mind when watching a gliding
albatross. And the efficiency of albatross flight is confirmed by modern
heart rate measurements of gliding Wandering Albatrosses. Combining
information from externally- attached heart beat monitors, leg- mounted
immersion recorders that reported whether bird was afloat or airborne
and a satellite pack for relaying the information back to Toulouse in
France, French scientist Henri Weimerskirch discovered that the heart
rate of flying Wandering Albatrosses, around 80 beats/min, was barely
faster than the rate of birds resting on the water, around 60 beats/min.^4
But take- off, involving flapping, was seriously hard work. The heart rate
then topped 200 beats/min.
If the Wandering Albatrosses seem well- monitored, spare a thought
for Red- footed Boobies, also studied by Weimerskirch. The boobies nest
on Europa in the Mozambique Channel from which, over the course of