adUlt migrations | 63which birds from different colonies, on the one hand, mingle outside
the breeding season or, on the other hand, utilise distinct regions.
Minglers include Thin- billed Prions, relatively small grey- and- white
petrels whose specialised bill serves to sieve a diet dominated by plank-
ton from the ocean surface. The two principal breeding populations of
the prions nest on the Falklands (60°W) in the South Atlantic and, at a
similar latitude but one- third of the way around the world to the east,
in the French sub- Antarctic archipelago of Iles Kerguelen (70°E). These
two populations are both large, perhaps two million and one million
pairs respectively, so food in the breeding season is evidently abundant.
Yet geolocators attached to birds from the two places showed that the
prions that breed 8,000 km apart come together after the breeding sea-
son at the Antarctic Polar Front close to the Greenwich meridian (0°W)
about halfway between the two colonies. After moulting in this region,
the Falklands prions head homewards well ahead of the Kerguelen birds.^10
Another species where birds from distant breeding populations may
encounter one another outside the breeding season is the Ivory Gull
Pagophila eburnea. The clue is in the names. In plumage, it is as white as
ivory, in habits it is pagophilic, which is to say ice- loving. Where there
is sea ice, the Ivory Gull can make a living from fish and crustacea,
supplemented by more questionable fare like Polar Bear faeces and seal
placentae.
These birds have now been satellite tracked from several high Arctic
colonies, in Canada,^11 north- east Greenland, Svalbard, and Franz Josef
Land east of Svalbard.^12 Although the studies encountered predictable
problems arising from the fact that solar- powered satellite transmitters
fade into silence aboard an Ivory Gull living in the continuous darkness
of the Arctic winter, a fair picture emerged. By mid- winter, Canadian
and Greenland birds were both at the ice edge in the southern Davis
Strait bordering on the Labrador Sea, while other Greenland gulls, plus
those from Svalbard and Franz Josef Land, were all found in south- east
Greenland.
Slightly further south in the Atlantic, Danish researcher Morten
Frederiksen co- ordinated a geolocator study of Black- legged Kittiwakes
on a near- industrial scale.^13 Thanks to the joint efforts of 31 researchers,
439 devices were deployed at colonies spread far and wide; above the