Bulletin - United States National Museum - si-pddr - Smithsonian ...

20 BULLETIN 133^ UNITED STATES NATIONAL MUSEUM
America. Their itinerary thus takes advantage of the shifting sea-
sons in both continents.
Because some of these species now or formerly occurred in the
Argentine in great abundance, it has been held by some that there
are in these species two groups of individuals, a northern body that
breeds in North America and migrates south to clement regions in
Mexico or Central America, and a southern group, that occupies a
breeding ground in Patagonia, the islands of Antarctic seas, or even
the great Antarctic Continent, that comes north to winter in the
Argentine. This belief was based in part upon the seemingly irreg-
ular occurrence of some of these migratory birds, with records
(scattered and few) of certain species that were found on the pampas
during the northern breeding season, and in part upon disbelief in
the powers of flight in creatures apparently small and weak. There
are certain species, such as the pied-billed grebe, cinnamon teal, fulvous
tree duck, and others that have a breeding range in both North
and South America. In some of these individuals from the two
colonies appear indistinguishable; in others the two groups may
differ slightly in minor characters. There has never been any cer-
tain indication, however, of the breeding south of the Equator of
such species as the golden plover, Hudsonian godwit, the yellow-
legs, and other species considered as migrants from the north.
The scattered individuals that remain in Argentina during the
northern summer are wounded, sterile, or otherwise diseased indi-
viduals that have been unable to perform the long flight northward,
or that have lacked the physiological incentive to do so. The few
supposed occurrences of their nesting have, on investigation, proven
erroneous, and the migration and seasonal movements of these spe-
cies is so well understood that there is no question that they nest
in the north and pass south of the Equator only in migration. Data
from birds banded in the north eventually will authenticate these
facts.
In their movements after reaching the northern coast of South
America these northern species have three main routes, one north
and south along the Atlantic coast, one that passes along the Pacific
coast line, and a third that follows the great interior north and
south river system of the Paraguay and Parana. Some of the birds
that follow this last route on their southern journey apparently drive
straight south across the pampas until they strike the southern coast
of Buenos Aires, and then swing around to follow up to some wintering
ground in the eastern pampas, or near the mouth of the Rio
de la Plata. In November, on the eastern coast of Buenos Aires, I
witnessed a curious phenomenon where one line of northern migrants
came driving south down the coast, and a second, traveling in the
opposite direction, came sweeping up from the south. My only sup-