When the torch on the Statue of Liberty in New York Harbor was kept lighted, it caused an enormous destruction of bird life, tabulations showing as many as 700 birds killed in a single month.
In September 1948, bird students were startled by news of the wholesale destruction of Maryland yellowthroats, redstarts, ovenbirds, and others that were dashed against the 1,250-foot high Empire State Building in New York City, the 491-foot high Philadelphia Saving Fund Society Building in Philadelphia, and the 450-foot high WBAL radio tower in Baltimore. In New York the birds continued to crash into the Empire State Building over a 6-hour period and their bodies were scattered over a four-block area. The mortality was so heavy in Philadelphia that it was impossible to use the sidewalk below the sky-scraper until the birds had been gathered. A study of the weather conditions prevailing at this time in the Atlantic coastal region suggests the probable cause of this catastrophe. By early morning on September 11 a mass of cold, southward-flowing air had just reached New York City where it was forcing upward and was being overridden by a mass of warm, northward-flowing air. Presumably the migrants were riding the upper levels of the southbound current which, in the contact zone with the northbound current, was being deflected earthward, thus causing the birds to fly lower and lower until they were below the tower of the Empire State Building. Clouds and gusty winds in the zone of contact between the two air masses reduced visibility and disrupted avian navigation with the result that the confused travelers crashed into the stone and steel obstruction. As the cold air mass continued to move southward, the situation was repeated at Philadelphia and at Baltimore.
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 aeronautical 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, making an ideal combination of strength and lightness, and the lightest and toughest material possible for flight in the form of feathers, combine 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 gallon of fuel usually required, but only a single pint.
The sora, or Carolina rail, 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 migration routes of any member of the family, and easily crosses the wide reaches of the Caribbean Sea. The tiny ruby-throated hummingbird 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—as, for example, pintail ducks that have flown from the North American mainland to the Hawaiian Islands—their condition is far from being a state of exhaustion. With a few hours' rest and a crop well filled with proper food, most birds 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, generally does not agree with the facts. The truth lies in the opposite direction, 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. Though the insistent crescendo note of the ovenbird is ordinarily associated with the full verdure of May woods, this bird has been known to reach its breeding grounds in a snowstorm and the records of its arrival in southern Minnesota show a temperature variation from near freezing to full summer warmth. Temperatures at arrival of several other common birds vary from 14 degrees between highest and lowest temperatures to 37 degrees, the average variation being about 24 degrees.
It should be remembered that North American species spending the winter months in tropical latitudes experience no marked changes in climatic conditions from November to March or April, yet frequently they will start the northward movement in January or February. This is in obedience to physiological promptings and has no relation to the prevailing weather conditions. For migratory birds the winter season is a period of rest, a time when they have no cares other than those associated with the daily search for food or escape from their natural enemies. Their migrations, however, are a vital part of their life cycles, which have become so well adjusted that the seasons of travel correspond in general with the major seasonal changes on their breeding grounds. With the approach of spring, therefore, the reproductive impulse awakens, and each individual bird is irresistibly impelled to start the journey that ends in its summer home.
In other words, the evidence indicates that the urge to migrate is so ingrained that each species moves north in spring when the average weather that will be encountered is not unendurable. The word "average" must be emphasized since it appears obvious that the migrations of birds have so evolved that in general they synchronize with average climatic conditions. The hardy species travel early, fearless of the blasts of retreating winter, while the more delicate kinds come later when there is less danger of encountering prolonged periods of inclement weather. Some of the hardy birds pause in favorable areas and allow the spring season to advance. Then, by rapid travel they again overtake it, or, as sometimes happens, they actually outstrip it. Occasionally this results in some hardship, and rarely in the destruction of large numbers of individuals. Cases are known where early migrating bluebirds have been overwhelmed by late winter storms. Nevertheless, unless such climatic conditions are prolonged, no serious effect on the species is noted. The soundness of the bird's instincts is evidenced by the fact that natural catastrophes, great though they may be, do not permanently diminish the avian populations.