adUlt migrations | 65two groups of Cook’s Petrel. These signatures were different and, in both
cases, matched the signatures of 100- year- old Cook’s Petrel skins residing
in the drawers of the California Academy of Sciences and the American
Museum of Natural History, and collected either off Baja California or
off Peru. Rayner concluded that the very different migratory tracks of
the two Cook’s Petrel populations have persisted for at least 100 years,
probably longer, and this is now reflected in clear genetic differences.
While birds from different colonies may mingle or remain apart out-
side the breeding season, that, alas, doesn’t exhaust the possibilities. Birds
from a single colony could potentially use distinct non- breeding areas –
and the case of the male and female of a single Sabine’s Gull pair head-
ing south from Canada to spend the winter far apart in the Pacific and
South Atlantic was mentioned at the start of the chapter.
Consider South Polar Skuas, aggressive attendants at the seabird
throngs of the far south, ever ready to snatch an unguarded petrel egg or
an unattended penguin chick. The skuas quit Antarctica when winter
approaches and, roughly, head north from their breeding area. But on
that northward journey, they may encounter a metaphorical fork in the
road, a decision point. Geolocator studies of the skuas of Terre Adélie,
the section of the Antarctic continent south of Australia, have identi-
fied the birds’ alternative decisions. They either veered east (right) along
the east of Australia to spend the southern winter off Japan, or they
veered west (left) to enter the northern Indian Ocean.^15 Just the same
parting of the migratory ways was discovered among skuas nesting on
the Antarctic Peninsula. There the skuas’ decision is either to pass east
of Cape Horn into the Atlantic and travel north to seas off Newfound-
land, or to veer left into the Pacific where the main non- breeding area is
the span of ocean between Hawaii and British Columbia. (See Map 13.)
They did both, but individual birds, tracked over 2– 3 consecutive years,
consistently followed one or other route.^16
Such individual consistency has emerged repeatedly in recent sea-
bird migration studies. It is a topic to which I shall return in Chapter 7.
But while the skuas provide examples of different birds from a single
population adopting different migratory strategies, there is another
species where individuals from a single population not only head to very
different destinations but sometimes shift between different destinations