The spring migrations of the blackpoll warbler (Dendroica striata) and the cliff swallow (Petrochelidon albifrons) afford an interesting comparison of the flights of day and night migrants. Both spend the winter in South America, at which season they are neighbors. But when the impulse comes to start northward toward their respective breeding grounds, the warblers strike straight across the Caribbean Sea to Florida, while the swallows begin their journey by a westward flight of several hundred miles to Panama ([fig. 5]). Thence they move leisurely along the western shore of the Caribbean Sea to Mexico, and continuing to avoid a long trip over water, they go completely around the western end of the Gulf of Mexico. This circuitous route adds more than 2,000 miles to the journey of the swallows that nest in Nova Scotia. The question may be asked, Why should the swallow select a route so much longer and more roundabout than that taken by the blackpoll warbler? The simple explanation is that the swallow is a day migrant while the warbler travels at night. The migration of the warbler is made up of a series of long, nocturnal flights, alternated with days of rest and feeding in favorable localities. The swallow, on the other hand, starts its migration several weeks earlier and catches each day's ration of flying insects during a few hours of aerial evolutions, which at the same time carry it slowly in the proper direction. Flying along the insect-teeming shores of the Gulf of Mexico, the 2,000 extra miles that are added to the migration route are but a fraction of the distance that these birds cover in pursuit of their food.
Although most of our smaller birds make their longest flights at night, close observation shows that travel is continued to some extent by day. This is particularly the case during the latter half of a migratory season, when the birds manifest a desire to hasten to their breeding grounds. At this time flocks of birds while feeding maintain a movement in the general direction of the seasonal journey. Sometimes they travel hurriedly, and while their flights may be short, they cover considerable distances in the course of a day.
HOW BIRDS MIGRATE
SPEED OF FLIGHT AND SPEED OF MIGRATION
There is wide-spread misconception concerning the speed at which birds normally fly, and even regarding the speed they can attain when occasion demands, as when closely pursued by an enemy. It is not unusual to hear accounts of birds flying "a mile a minute." While undoubtedly some birds can and do attain a speed even greater than this, such cases are exceptional, and it is safe to say that even when pressed, few can develop an air speed of 60 miles an hour. They do, however, have two speeds, one being the normal rate for everyday purposes and also for migration, and an accelerated speed for escape or pursuit; this in some cases may be nearly double the normal rate of movement. Nevertheless, the effort required for the high speeds could not be long sustained, certainly not for the long-distance migratory journeys that are regularly made by most birds.
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Figure 5.—Migration of the cliff swallow; a day migrant that, instead of flying across the Caribbean Sea as does the blackpoll warbler ([fig. 11]), follows around the coast of Central America, where food is readily obtained.
The theory that migrating birds attain high speeds received encouragement from the German ornithologist Gätke, who for many years, made observations on birds at the island of Heligoland and published thereon in 1891 ([21]). He postulated that the blue-throat (Cyanosylvia suecica), a species of thrush smaller than the American hermit thrush, would leave African winter quarters at dusk and reach Heligoland at dawn, which would mean a sustained speed of 200 miles an hour; and that the American golden plover (Pluvialis dominion) flew from the coast of Labrador to Brazil in 15 hours, or at the tremendous speed of 250 miles per hour. These conclusions are now considered unwarranted by most ornithologists.
Sportsmen also often greatly overestimate the speed at which ducks and geese fly and sometimes attempt to substantiate their estimates by mathematical calculations, based upon the known velocity of a charge of shot, the estimated distance, and the estimated "lead" that was necessary to hit the bird. If all three elements of the equation were known with certainty, the speed of the bird could be determined with a fair degree of accuracy. The majority of the ducks that are reported as killed at 40, 50, or even 60 yards, however, actually are shot at distances much less than estimated. To sight along a gun barrel and estimate correctly the distance of a moving object against the sky is so nearly impossible for the average gunner as to make such calculations of little value.