68 | Chapter 4
between males and females, as already outlined for Wandering Alba-
trosses? And how does the birds’ behaviour in the non- breeding season
differ from that during breeding?
Mention has already been made of the speed of documented jour-
neys, for instance 300– 500 km/day by Arctic Terns, up to 950 km/day
by circumnavigating Grey- headed Albatrosses. At first gasp these sus-
tained speeds are stupendous. At a second and more considered glance,
they might be anticipated. Depending on species, a seabird in still air
prob ably flies at around 30– 60 km/h. Multiply that speed by 20 flying
hours, allow the bird four hours of rest, and it is easy to generate a day’s
track of 500– 1,000 km, even without the benefit of a following wind.
Exactly that sort of schedule has emerged from geolocator work I
undertook with Tommy Clay of the University of Cambridge. We stud-
ied the Murphy’s Petrels of Henderson Island in the remotest blue of
the South Pacific (24°S). The petrels from Henderson spend the non-
breeding period in the central North Pacific around 40– 45°N. Twice a
year they migrate between the two areas, each time taking only some
10 days to complete the 8,000 km journey.
Now the cunning bit. Geolocators are routinely attached to seabirds’
legs, much like a traditional metal ring that can provide survival and
movement information if the ringed bird is later found, alive or dead.
But, in addition to collecting information on light levels that translates
into data on the bird’s position, the tiny device can also record whether
it is wet or dry. Potentially this allows the researcher to assess as often
as every three seconds whether the bird is on or under the sea (device
wet), or whether the bird is flying or standing with its feet dry. Using
these immersion data, Clay, the analytical force behind our project and
a man who confirms that a twinkling eye and statistical expertise are
by no means incompatible, worked out that the migrating petrels spent
about 75% of the daylight hours and 80% of darkness on the wing. If the
petrels’ flight speed is around 40 km/h, something of a guess, travelling
the necessary 800 km/day is well within their grasp. Our data yielded
no hint of any stopovers en route.
While the Murphy’s Petrel migration follows a north- south axis, equally
rapid journeys are the norm for at least one species following a west- east
route. That species is the Westland Petrel, one of the largest burrowing