THE MIGRATION OF NORTH AMERICAN BIRDS 61 
the bird under normal weather conditions on that date, and the 
average rate of migratory flight, are data basic to any reasonably 
accurate prediction of the time arrival may be expected in northern 
areas. 
Head winds are as unfavorable to migration as is rain or snow, as 
they greatly increase the labor of flight and cut down the speed of 
cross-country travel. If such winds have a particularly high velocity 
they may force down the weaker travelers, and should this happen 
over water areas, large numbers of birds would be lost. Even strong 
winds that blow in the direction of aerial travel are unfavorable for 
the birds, as they interfere with their balance and disarrange their 
feathers. Moderate tail winds and cross or quartering breezes appear 
to offer the best conditions for the passage of the migrants. 
B37612 
FIGURE 28,—A banded belted kingfisher, one of about 2,000,000 birds that have been marked with Biological 
Survey bands 
PROBLEMS OF MIGRATION 
BANDING STUDIES 
The study of living birds by the banding method, whereby great 
numbers of individuals are marked (fig. 28) with numbered aluminum 
leg rings, has come to be recognized as a most accurate means of 
ornithological research. Since 1920, banding work in North America 
has been under the direction of the Bureau of Biological Survey, in 
cooperation with the National Parks Branch of Canada. Every year 
volunteer cooperators, working under permit, place bands on thou- 
sands of birds, game and nongame, large and small, migratory and 
nonmigratory, each band carrying a serial number and the legend, 
NOTIFY BIOLOGICAL SURVEY, WASHINGTON, D. C., or on the smaller 
sizes, NOTIFY BIOL. SURV., WASH., D. C. When a banded bird is 
