stick or twist? | 139example have chosen the areas according to what biologists term the
ideal free distribution, akin to the choice of various routes between home
and workplace that was mentioned at the chapter’s outset.
In an ever- changing world, there can be no assurance that the relative
merits of areas A and B are forever fixed. One may improve, the other
deteriorate. Putting it anthropomorphically, the bird does not want its
fixed routine to cause it to miss out on a bonanza elsewhere, or to be
stranded in an area where fish stocks have plummeted or the sea unex-
pectedly frozen over. There may be advantages in occasional sampling
visits to unfamiliar areas. Possibly this happens. Studies of consistency
over several years typically report lower consistency values than those
conducted over a couple of years. Even among shorter studies, there can
be a divergence between birds showing striking consistency and those,
the putative samplers, showing none at all. For example, after leaving
colonies in eastern Canada, Brünnich’s Guillemots head into the Labra-
dor Sea for the winter. Most birds spent successive winters fairly close
to where they spent the previous winter, but a few clearly did not.^11 (See
Map 11.) This was manifest in the distance between the birds’ centre of
activity in successive winters; it ranged from 22 km, implying these
birds were bobbing on precisely the same seas from one winter to the
next, to 1,200 km, implying use of a sea area half an ocean away. Plausi-
bly the same sampling argument might apply to the Cory’s Shearwaters
encountered in Chapter 4. Most are faithful to a wintering area, but a
handful switch to an alternative, for no very obvious reason.
Although the idea of the ideal free distribution posits that the birds
join one or other group at random, birds could join groups according to
their gender. This would achieve the same outcome, dividing the species’
feeding activities between two (or more) areas. We have already seen
how, in the period before laying, male gadfly petrels tend to travel fur-
ther from the colony than females. Partly because of their greater weight
and ability to cope with stronger winds, male Wandering Albatrosses
routinely feed to the south of females. And there are consistent, gender-
based differences in the species that has been prominent in this chapter,
the Northern Gannet.
Adult gannets weigh about three kilograms, and females are larger
than males by some 200 g. Not only do the birds leaving the Bass Rock