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than expected, simply because they are travelling further than the
straight- line distance. Exactly this has been observed by Tim Guilford
and colleagues tracking Manx Shearwaters between Skomer and their
feeding areas further north in the Irish Sea. The speed of the shear-
waters’ directional travel was about 40 km/h, rather less than 50 km/h,
which is the speed at which Manx Shearwaters are calculated to cover
most ground (or, more precisely, salty water) per unit of energy ex-
pended during powered flight. The discrepancy is very probably due to
the shearwaters’ use of slope and/or dynamic soaring, and hence a zig-
zag track, to reduce the cost of their journey.^10
This increase in the tortuosity of the track is likely to be greatest
when the bird is travelling across the wind.* When the wind is astern,
the bird can spread its wings and celebrate on the downwind roller-
coaster. When it is necessary to travel into wind, harder work and a
substantially increased heart rate are in prospect. They are unwelcome
and best avoided. No wonder the full circuits of the Southern Ocean
made by albatrosses are from west to east, driven downwind by the
westerlies. Illustrating the same point, Cory’s Shearwaters nesting on
the Berlengas islands off Portugal mostly headed north- west or south-
west when departing the colony to collect food for their chicks. Since
the prevailing winds during this September 2006 project were from the
north- east, the birds apparently favoured crosswinds or tail winds, and
avoided journeys into wind.^11 But perhaps this study needs viewing
through the spectacles of scientific caution since, had the birds headed
north- east into wind, they might have crashed into the coast of main-
land Portugal barely 20 kilometres from the Berlengas!
While zigzag seabird tracks can be a consequence of energy- saving
exploitation of the wind, there is another possibility. Animals spanning
the range of complexity from the simplest worms to magnificent mam-
mals show a behaviour known as area- restricted search. On encounter-
ing food, they tend to turn more frequently than previously. This keeps
- (^) Even if tortuous zig zagging tracks create complications, the difference between a bird’s air-
speed, known fairly accurately, and its ground speed, measured by attached GPS devices, could
provide a measure of wind speed and direction at the bird’s location. The possibility of using
this information to supplement that from weather buoys and so improve weather forecasting is
now being discussed. http://www.pnas.org/content/113/32/9039.abstract (accessed 14 June 2017).