THE MIGRATION OF NORTH AMERICAN BIRDS O99 
EXHAUSTION 
Although it would seem that the exertion incident to the long 
flights of many species of migratory birds would result in their arrival 
at their destination in a state bordering on exhaustion, this is contrary 
to the truth. Both the soaring and the sailing of birds show them to 
be proficient in the use of factors employed in aerial transportation 
that only recently have become understood and imitated by aero- 
nautical engineers. The use of ascending currents of air, employed 
by all soaring birds, and easily demonstrated by observing the gulls 
that glide hour after hour along the windward side of a ship, are now 
utilized by man in his operation of gliders. Moreover, the whole 
structure of a bird renders it the most perfect machine for extensive 
flight that the world has ever known. Hollow, air-filled bones, mak- 
ing an ideal combination of strength and lightness, and the lig htest 
and toughest material possible for flight in the form of feathers, com- 
bine to produce a perfect flying machine. Mere consideration of a 
bird’s economy of fuel or energy also is enlightening. The golden 
plover, traveling over the oceanic route, makes the entire distance 
of 2,400 miles from Nova Scotia to South America without stop, 
probably requiring about 48 hours of continuous flight. This is 
accomplished with the consumption of less than 2 ounces of fuel in the 
form of body fat. To be as economical in operation, a 1,000-pound 
airplane would consume in a 20-mile flight not the oallon of fuel 
usually required, but only a single pint. 
The sora, or Carolina rail (Porzana carolina), which is such a 
notoriously weak flyer that at least one writer was led to infer that 
most of its migration was made on foot, has one of the longest migra- 
tion routes of any member of its family, and easily crosses the wide 
reaches of the Caribbean Sea. The tiny ruby-throated humming- 
bird (Archilochus colubris) crosses the Gulf of Mexico in a single flight 
of more than 500 miles. 
While birds that have recently arrived from a protracted flight over 
land or sea sometimes show evidences of being tired, their condition 
is far from a state of exhaustion, unless unusual conditions have been 
encountered, and with a few hours’ rest and a crop well filled with 
proper food they exhibit eagerness to resume their journey. The 
popular notion that birds find the long ocean flights excessively 
wearisome and that they sink exhausted when terra firma is reached 
does not agree with the facts. The truth lies in the opposite direc- 
tion, as even small land birds are so little averse to ocean voyages 
that they not only cross the Gulf of Mexico at its widest point, but 
may even pass without pause over the low, swampy coastal plain 
to the higher regions beyond. Under favorable conditions birds can 
fly when, where, and how they please. Consequently the distance 
covered in a single flight is governed chiefly by the food supply. 
Exhaustion, except as the result of unusual factors, cannot be said to 
be an important peril of migration. 
INFLUENCE OF THE WEATHER ON MIGRATION 
_ The state of the weather at any point has little if anything to do 
with the time of arrival of migratory birds. This is contrary to the 
belief of observers who have thought that they could foretell the 
appearance of various species by a study of the weather conditions. 
