64 | Chapter 4
Arctic Circle in Canada, Greenland, Svalbard, and Norway, as well as
further south in Iceland, Faeroes, the North Sea, and the western United
Kingdom. This spans almost the entirety of the species’ North Atlantic
range. No fewer than 236 devices were retrieved and provided data. The
picture was very clear. Kittiwakes from far and wide generally chose to
spend the winter off Newfoundland with an estimated 80% of the
4.5 million adult Kittiwakes in the Atlantic wintering west of the Mid-
Atlantic Ridge. Only birds from Ireland and western Britain stayed
mainly east of the Ridge towards the European side.
While there was clearly much winter mixing of Kittiwakes from differ-
ent breeding populations, there was a measure of segregation. I suspect
this may be a common pattern. Aevar Petersen, formerly of the Icelan-
dic Museum of Natural History, found that Icelandic Puffins largely
overwinter west of Iceland but, broadly, the birds from colonies further
north in Iceland spend the winter further north than those from south-
ern colonies.
If birds from separate colonies demonstrably remain apart outside
the breeding season, the chance of genetic differentiation and, maybe,
eventual speciation increases. An exquisite example comes from the
Antipodes. Ignoring a tiny cluster on Great Barrier Island, the world
population of Cook’s Petrels today nests on two islands at the north and
south ends of New Zealand, on Little Barrier Island in the Hauraki Gulf
off Auckland and on Codfish Island off Stewart Island respectively.
As is true of several other petrel species nesting around New Zealand,
Cook’s Petrels make a west- to- east migration after breeding, crossing
the Pacific to spend the non- breeding period off the western seaboard
of the Americas. The story developed when Matt Rayner, a New Zea-
land ornithologist with a tattoo count that is normal for a building site
and above average for a scientific conference, obtained geolocator data
from 11 petrels on each of the two principal stations.^14 The southern
Codfish birds spent about six months (April–October) in Peruvian wa-
ters while the northern Little Barrier birds flew initially to seas off Baja
California. Later they headed north- west to the central Pacific before
returning to New Zealand.
What makes the work so fascinating is that the geolocator tracking
was combined with an investigation of stable isotope signatures of the