where seabirds find food | 161some examples, beginning with kittiwakes. The familiar Black- legged
Kittiwake breeds around the Arctic Ocean, and further colonies dot the
coasts of the North Pacific and North Atlantic. Around the Bering Sea,
there is another species, the Red- legged Kittiwake, very similar but for
the difference in leg colour (for which there is no ready explanation).
The two species even breed alongside each other, in the Pribilof Islands
for example. But there are clear differences in where they feed and what
they eat. During breeding, Red- legged Kittiwakes rely heavily on lan-
ternfish from the Bering Sea shelf break, while Black- legged Kittiwake
prey on more varied fare caught above the shelf, including a larger pro-
portion of invertebrates. The spatial differences persist through the
winter. Geolocator- tracking of Pribilof birds has revealed how the Red-
legged Kittiwakes remain in the Bering Sea where, in the cold and dark
of winter, they use continental shelf waters and sea- ice edges. In con-
trast their Black- legged cousins head south and disperse widely across
the sub- Arctic North Pacific.^22
The four southern hemisphere diving petrel species are famous for
their ability to fly through wave tops on whirring wings as if the water
was not there. They are also notorious for looking remarkably similar, to
the extent that even identifying specimens arrayed on a museum bench
is far from straightforward. Two species breed on Bird Island, off South
Georgia, the Common and South Georgia Diving Petrels. When tracked
by geolocators through the winter, both species spent this non- breeding
period in waters either around the South Georgia archipelago (mostly
Common Diving Petrel) or around 3,000 km to the east- northeast (both
species).^23 There was therefore some but not great overlap in the areas
visited. Stable isotope data from tiny feather samples indicated that the
South Georgia Diving Petrels were catching their main prey, planktonic
copepods, at a slightly greater depth than the Common Diving Petrels.
Yet no petrel enthusiast could confidently assert why it is the South
Georgia Diving Petrels that tend to travel further afield in winter and
dive deeper.
We can even extend the thought to a three- way comparison. Petra
Quillfeldt used geolocators to document the winter whereabouts of the
Antarctic Prions, Thin- billed Prions and Blue Petrels breeding on the
Iles Kerguelen in the Southern Ocean.^24 The three species are of roughly