70 | Chapter 4
2– 4 weeks. From there, they continued to the next point, off Ecuador
and northern Peru. Another stopover followed, off the coast of central
Chile. At this point, about 40°S, the Terns turned east and headed
across Chile and Argentinian Patagonia. This route of course entailed a
traverse of the Andes that took the terns, familiarly known as sea swal-
lows, at least 1,500 m above sea level – but it did avoid the tempestuous
seas of Cape Horn.^22 Once reunited with salt water off Argentina, the
terns enjoyed a final stop- over on the Patagonian Shelf before proceed-
ing to the Weddell Sea where they encountered their cousins from
Greenland!
Most simply, stopover points on migration can be detected from geo-
locators because the bird remains in a restricted geographical area for
several days or weeks. The immersion data allow a more sophisticated,
if oblique, approach to identifying stopovers in more aerial species, for
example terns and shearwaters. The legs of an actively- migrating flying
bird will remain dry. A bird resting in mid- ocean is likely to have wet
feet. A feeding bird may quickly alternate between quite short wet and
dry spells.
Tim Guilford of the OxNav group is a pioneer of this approach. He
studied Manx Shearwaters breeding off the Pembrokeshire coast, west
Wales, thereby ensuring that I was fascinated by his study since I had
studied these same shearwaters for my doctorate. The migratory route
that Guilford reported broadly matched that which had been deduced
from ring recoveries. In autumn, the shearwaters first head south as far
as west Africa. Then they veer south- west and, aided by the north- east
trade winds, approach the coast of Brazil. Onward they fly, barely rest-
ing, until reaching the wintering area, rich seas off Argentina about 40°S,
comfortably south of Buenos Aires. The northward journey through the
North Atlantic back to Wales sees the shearwaters taking a more west-
ern route, swinging towards the eastern seaboard of North America and
finally approaching Wales from the west. (See Map 4.)
In two respects, the study broke new ground. Firstly, the shearwaters
appeared to be spending the winter about 10– 15 degrees of latitude fur-
ther south than ringing recoveries, mostly obtained several decades
earlier, had indicated.^23 It is very difficult to know whether this reflects
a change in the shearwaters’ migratory habits, or whether the ringing