By the kind permission of Mr Cooke I am able to reproduce three of his maps, illustrating the longest known distance travelled by any bird in a single flight, and the probable evolution of this extraordinary oversea voyage [(21)]. This long journey, roughly 2500 miles at a flight, is used in autumn by several species of American shore birds, and the particular species most easily recognised, is the American golden plover, Charadrius dominicus, which differs but little from our C. pluvialis. An important point to notice is that the route followed in the fall is not the one used by the bird in spring, an undoubted proof that all routes are not identical with the original line of dispersal of the species. Nor is the route directly from the north to the south, though there is plenty of evidence to show the fallacy of the notion that all birds move in this one direction.
The golden plover nests along the Arctic coasts of North America from Alaska to Hudson Bay. So soon as the young are able to take care of themselves the birds migrate south-east to Labrador, where for some weeks they fatten on the autumn harvest of fruits. A short journey across the Gulf of St Lawrence brings them to Nova Scotia, where they gather before starting on their oversea flight. The eastward trip to the food-supplying districts is support of the idea that a route is originated by passage from food-base to food-base, rather than by any hasty rush from the dangers of approaching winter. The birds start south from Nova Scotia for South America!
During this long oversea journey, which Mr G. H. Mackay thinks, with reason, may be undertaken under favourable conditions at a speed of from 150 to 200 miles an hour by birds with such magnificent power of flight, the plovers may meet with many different winds. The Cape Cod sportsmen look for them if the wind is strong from the north-east; the Barbados gunners expect them when there is squally weather from the south-east, but when westerly breezes are blowing they will pass so far as 400 miles east of the Bermudas. Only when the wind is adverse and strong do the plovers visit the Bermudas or even stop at any of the northern Lesser Antilles, 600 miles from the coast of South America. In favourable weather they neglect any of these "emergency stop-overs" and hasten on. In the Guianas the birds rest and feed, but they soon move on. Across the Brazils their actual route is uncertain, but they have been met with in Amazonia, and are known to winter in Argentina, and, it is suspected, in eastern Patagonia.
Map showing the evolution of the migration route of the American Golden Plover.
(From The National Geographic Magazine.)
The return migration is, so far as it is known, in a steady northerly direction, rather north-west across Bolivia towards Central America. From Yucatan they cross the Gulf to Texas, then slowly travel up the great Mississippi highway and across Canada to their northern breeding grounds. "Its round trip has taken the form of an enormous ellipse with a minor axis of 2000 miles and a major axis stretching 8000 miles from Arctic America to Argentina."
The following is Mr Cooke's suggestion of the origin of this great ellipse. Towards the close of the glacial era, when the ice began to recede, the Florida peninsula was submerged and only a small area in the south-east of the States was free from ice. Plover attempting to follow up the retreating ice were confined to an all-land route from Central America through Mexico to the western part of the Mississippi Valley. As the east gradually became uncovered the route would be extended to the north-east, until the area stretching to the Great Lakes was fit for bird-habitation. As the route lengthened and the power of flight developed, there would be a tendency to shorten the line by cutting off some of the great curve (No. 1) through Mexico and Texas, and a short flight across the Gulf (No. 2) would be gradually lengthened, until the present spring route, then also the autumn route (No. 3), was attained. As Canada opened out, the routes in spring and autumn diverged; in autumn the fruits of Labrador were an attraction, but the Chinook winds made the country east of the Rockies more suitable for spring migration; the fall route tended eastward (No. 4), the spring route remained unchanged. When the fall route had worked eastward to the Gulf of St Lawrence (No. 5), shortening took place in the same way from the great westward curve, culminating in an ocean flight, short at first (No. 6) and later extended, the total distance shortened, until the present route was attained (No. 7).
This reasoning, sound enough, helps to a more difficult problem—how the Pacific golden plover, Charadrius fulvus, found its way to the Hawaiian Islands, where numbers of the birds winter annually. Roughly the islands are 2000 miles from California, 2400 from Alaska, whence the birds fly, and 3700 miles from Japan. Mr Cooke scouts the idea that any bird flies aimlessly out to sea to find a new winter home, and the chance colonisation by a storm-swept party is as improbable; if this did occur it is hardly likely that they would at once depart, in a single season, from ancient habits and carve out an entirely new migration route. Probably the origin of the route is as follows. The bird breeds on the northern shores of eastern Siberia from the Liakof Islands to Behring Strait, and on the Alaskan side south to the northern base of the Alaska peninsula. It winters on the mainland of south-eastern Asia, in eastern Australia, and throughout the Oceanic Islands from Formosa and the Liu Kiu Islands on the north-west to the Low Archipelago in the south-east.