122 | Chapter 6
kilogram heavier in the 20 years, 1990 to 2010,* without any increase in
the lengths of their bones. Whether the weight increase is due to better,
plumper body condition, or to the stronger winds allowing dynamic
soaring by heavier – and hence faster – albatrosses, remains unresolved.
Equally uncertain is whether these favourable circumstances will persist
as wind patterns continue to alter under climate change.
***
Not only do seabirds adjust their routes in response to prevailing wind
patterns when foraging from the colony, they also do so on a global
scale, during migration. That is the cue for a brief sketch of global wind
patterns, due in part to the Coriolis effect which is a result of the earth’s
rotation and the alleged cause of the proverbial tendency of the water
draining down the bath’s plughole to rotate clockwise in the northern
hemisphere and anti- clockwise in the southern. Thus in the north-
ern hemisphere westerly winds dominate across the North Pacific and
Atlantic at latitudes of 40– 50°N. Further south, at the latitude of
the Tropic of Cancer (23.5°N), the air is moving back east to west, the
North- east Trades. Combine the westerlies to the north and the North-
east Trades, and a clockwise circulation becomes established in the
Northern Hemisphere.
In contrast, in the Southern Hemisphere the westerlies blow most
steadily at 40– 50°S, with barely any land except Cape Horn to inter-
rupt their flow. But the return flow, east to west, is here the South- east
Trades. Combine the westerlies to the south and the South- east Trades,
and an anti- clockwise circulation becomes established in the southern
hemisphere.
These patterns were well- known to mariners in the nineteenth cen-
tury and reflected in the routes taken by wool clippers plying between
England and Australia or New Zealand. Leaving England, the route was
southerly as far as the Canaries or Cape Verdes, off the Sahara. At that
point, the North- east Trades were encountered. The ships headed west
- (^) In 1990 females weighed about 8 kg and males 9.5 kg.