28 BULEETIN OFS 
to the eastward of these islands, when the light easterly winds of the horse 
latitudes, and later the increasing northeast trades, bring them to the South 
American coast. There is a gap in our knowledge of the route from Brazil 
to the Pampas, but as the prevailing winds in the southern tropics are from 
the east and southeast, and in the south temperate zone from the west, the 
southerly direction may be safely assumed. In late winter and early spring 
the prevailing winds of the Argentine Pampas are from the southwest, 
which would shape the course of the Plover northwest to the eastern slopes 
of the Andes, along which they would pass in a generally northward course 
(still across the southeast trades and easterly equatorial winds), crossing the 
Isthmus of Panama, traversing Central America and ascending the Missis- 
sipp1 Valley, always moving approximately at right angles to the prevailing 
wind; and their course from Manitoba to their breeding grounds in Alaska 
is again nearly at right angles to the prevailing westerly winds. 
This theory is one of the most attractive and suggestive which has been 
put forward in recent years, and it explains almost completely the hitherto 
rather mysterious course of plover, curlew and related shore birds which are 
known to take an easterly path in going south and a much more westerly 
one during the northward migration in spring. It remains to be seen how 
far this factor of migration on a “beam wind” may enter into the migration 
courses of species other than shore birds. One naturally thinks at once of 
the great southeastward movement of the Bobolinks and Blackbirds of the 
Mississippi Valley across the lower Alleghenies to the rice-fields and other 
feeding grounds of the South Atlantic and Gulf coasts. The whole fabric 
rests on a foundation of observed facts, which as yet, however, is not very 
strongly established. Is it a fact that migrating birds prefer to fly across 
the direction of the wind? If the answer to this question is unanimously af- 
firmative a new and vastly important factor in bird migration will have been 
established. But whatever the result may be, Mr. Clark is to be commended 
for having called attention to a very important matter which has been gen- 
erally overlooked heretofore. W258. 2k 
