152 | Chapter 8
renewal of nutrients in the sunlit surface waters. Thanks to that renewal,
planktonic growth flourishes. Small crustacea and their predators are
nourished, and so the very animals ultimately needed to sustain sea-
birds thrive.
Frontal systems occur over the continental shelves and in the deep
ocean. A small number of examples will illustrate the general principle
of their importance to seabirds, but I will not attempt any comprehen-
sive listing of the numerous frontal systems that help sustain seabirds
across the world’s oceans.
In the relatively shallow waters of the Irish Sea, the Irish Sea Front
forms in late summer south- west of the Isle of Man, at the interface
between an area of water stratified by weak tides and an area mixed by
strong tides. Productivity remains high through the summer along the
Front, and tracking has revealed how attractive is the Front to chick-
rearing Manx Shearwaters. From far and wide they come. In fact, al-
though they also make local trips close to their home colony, birds from
all four British and Irish colonies studied by the OxNav group visit the
Front.^11 These colonies are on Rum in the Hebrides roughly 375 km from
the Front, Copeland off Northern Ireland (120 km), Skomer off south-
west Wales (215 km) and Lundy in the Bristol Channel (280 km).
The major fronts of the world’s oceans have an even greater impact
on seabird feeding. None are more influential than the circum- global
fronts that divide the Southern Ocean into roughly parallel bands of
water of different temperature, just as a knitted sweater might be di-
vided into horizontal bands of different colour. Travelling southward,
the voyager will successively encounter the subtropical, the sub-Antarctic,
and the Antarctic polar fronts. Each represents a major temperature
discontinuity. For example, at the polar front the sea surface tempera-
ture drops from about 6°C to 2°C within a mere 40km. It is where rela-
tively fresh cool water flowing north from Antarctica sinks below the
warmer saline water of the sub- Antarctic. This sinking water generates
turbulence and hence prime seabird feeding, sometimes signalled by
blizzards of birds. At the maximum, densities of prions, mainly Antarc-
tic Prions, can reach hundreds of birds per square kilometre at the Front
where their crustacean prey, copepods, are concentrated. It is a zone so
enticing, so nourishing, that it is worthwhile for Sooty Shearwaters