162 | Chapter 8
similar weight, around 150– 180 g, and similar unexciting plumage, grey
above, white below. The most obvious difference is in the bill, broad in
the Antarctic Prion, thinner in the Thin- billed Prion and narrowest in
the Blue Petrel. On the basis of those differences, it would be difficult
to predict a priori where the species might spend the winter. Yet Quill-
feldt’s results were startlingly clearcut. In contravention of their name,
the Antarctic Prions spent their time almost entirely north of the Ant-
arctic Polar Front. (See Map 12.) The Thin- billed Prions had a distribution
that straddled the Front. The Blue Petrels predominantly passed the
winter south of the Front. Statistical analysis showed conclusively that
Antarctic Prions wintered in cool waters, Thin- billed Prions in cold wa-
ters, and Blue Petrels in the coldest. Since the diet of the three species is
broadly similar, surface- caught crustacea, the reason why the crustacea
caught in waters of different temperatures should be associated with
different bill sizes is a lingering conundrum. When I asked Petra Quill-
feldt about this puzzle, she wondered whether more northern crusta-
cea were smaller, more appropriate for sieving by the Antarctic Prions’
broader bill while perhaps the crustacea south of the Polar Front were
larger, ripe to be picked up one- by- one by the Blue Petrels’ narrow bill.
While seabirds focus their feeding activities on the most productive
seas, some species, especially those with lower flight costs, exploit less
productive areas. These differences confirm a tenet of classical ecology,
that different species adopt different ways of making a living. However
the differences between species may not only be in the sea areas they
use. They may also apply to how they capture food, the topic of the
next chapter.