adUlt migrations | 73Species such as Wandering Albatrosses where immature and adult
males and females utilise different sea areas are clearly not candidates.
But how about a species where, as far as is known, male and female mi-
grate to similar regions? Scopoli’s Shearwater is one such, and work at a
large Italian colony addressed the question of togetherness outside the
breeding season^27 when the shearwaters leave the Mediterranean and head
south along the western margin of the African continent, some going
no further than Mauretania, others reaching as far south as Namibia.
Members of a pair tend to spend a like number of days travelling and so
reach similar wintering areas. But the geolocator data provide no hint
that they were journeying together. How they come to use similar – but
not identical – wintering areas remains unknown. Conceivably the col-
ony is structured such that genetically similar birds, with a tendency to
undertake similar migrations, breed in the same part of the colony and
sometimes with each other.
Let us turn the question on its head. If the evidence that pairs of sea-
birds travel together is, at best, inconclusive, are there species where
males and females are likely apart when not breeding? The separation of
female Wandering Albatrosses to the north and males to the south could
be related to structural differences between the sexes. Because males are
heavier, the extent of wing area available to support each kilogram of
bird is lower; in the jargon, the wing- loading is greater. Males might
then be able more efficiently to exploit zones of the Southern Ocean
where the winds are stronger, namely the Furious Fifties to the south of
the (merely) Roaring Forties.
Another reason for males and females to have different migrations
might be their contrasting roles early in the breeding season. It nor-
mally falls to the male to secure a nesting site, maybe a burrow in a
dense colony. To prevent usurpation of that site, it could pay to arrive
early. Exactly this argument is used for migrant landbirds where, in
many species, males winter north of females (in the northern hemi-
sphere), and arrive on the breeding grounds before females.
Those Cory’s Shearwaters nesting on the Salvages mostly head to the
southern hemisphere after breeding. When they return in spring, the
journey starts slowly, around 400 km/day with about half the 24 hours
spent in flight. But, north of the Equator, the pace picks up to around
1,000 km/day, with three- quarters of the day in flight. Digesting the data