Geese winging their way south in wrinkled V-shaped flocks is perhaps the classic picture of migration—the annual, large-scale movement of birds between their breeding (summer) homes and their nonbreeding (winter) grounds. But geese are far from our only migratory birds. Of the more than 650 species of North American breeding birds, more than half are migratory.
Swallow migration versus hibernation edit
Aristotle, however, suggested that swallows and other birds hibernated. Even in 1878, when Elliott Coues compiled a list of 182 papers discussing swallow hibernation, this belief was still held. In his posthumously published 1789 The Natural History of Selborne, even the “highly observant”[6] Gilbert White quoted a man who claimed to have seen swallows “while he was a schoolboy at Brighthelmstone” in a chalk cliff collapse, though the man denied being an eyewitness. [7] Nevertheless, he notes that he doubts that early swallows would “return for a week or two to warmer latitudes” and that if they “happen to find frost and snow they immediately withdraw for a time—a circumstance this much more in favor of hiding than migration.” [7] He also states that he has never heard of any swallows being found in a torpid state during the winter in the Isle of Wight or any part of this country. [8].
Migration was not recognized as a plausible reason for the wintertime disappearance of birds from northern regions until the late 1700s. [5] According to Thomas Bewicks A History of British Birds (Volume 1, 1797) there was a report from “a very intelligent master of a vessel” that “saw great numbers of Swallows flying northward” [9] between the islands of Menorca and Majorca.
Bewick then goes on to detail an experiment that was successful in preserving swallows for a number of years in Britain, where they were kept warm and dry during the winter. He concludes:
In waders edit
With waders, or what are known as shorebirds in North America, a similar situation arises. Numerous species, including the western sandpiper Calidris mauri[55] and the dunlin Calidris alpina[54], travel great distances from their Arctic breeding grounds to warmer regions within the same hemisphere, but others, like the semipalmated sandpiper C Pusilla traverse greater distances to reach the Southern Hemisphere’s tropical regions. [56].
The availability of specific essential food resources at rest stops along the migration route is critical to the success of migration for some species of waders. This allows the migrants to replenish their fuel supplies for the remainder of their journey. Key places to make stops include Delaware Bay and the Bay of Fundy. [57][58].
The longest known non-stop migration flight by any migrant is made by some bar-tailed godwits Limosa lapponica baueri, who travel 11,000 km from Alaska to their non-breeding areas in New Zealand. [59] Fifty-five percent of their body weight is stored as fat before migration to power this continuous migration.
Origins of long-distance migration
Long-distance migration patterns have far more complicated origins than short-distance migration, which most likely evolved from a fairly basic need for food. They have developed over countless years and are influenced, at least in part, by the genetic composition of the birds. They also take into account reactions to other elements such as the day’s duration, food sources, geography, and weather.
It seems strange to tropical birds that spend the winter months there to think about moving north and starting a migration. One theory for why these birds make such a difficult journey north in the spring is that their tropical ancestors dispersed from their tropical breeding sites northward over many generations. Compared to their stay-at-home tropical relatives (two to three on average), they were able to raise an average of four to six young due to the seasonal abundance of insect food and longer days. The birds continued to migrate back to their tropical homes during periods of glacial retreat, as the harshness of winter and diminishing food supplies forced them to do so. The majority of North American vireos, flycatchers, tanagers, warblers, orioles, and swallows have evolved from forms that originated in the tropics, which lends credence to this theory.
The mechanisms that trigger migration are diverse and sometimes poorly understood. A combination of shorter days, colder weather, altered food sources, and genetic predispositions can cause migration. Cage bird keepers have observed for centuries that migratory birds experience periods of restlessness every spring and fall, fluttering repeatedly toward one side of their cage. This behavior was named zugunruhe (migratory restlessness) by German behavioral scientists. Diverse bird species, and even subpopulations within a single species, may exhibit distinct migratory behaviors.
The BirdCast project is developing the capacity to forecast when, where, and how far birds will migrate; it’s almost like receiving a weather report.
It’s a treasure for bird watchers and important information for conservation efforts. Knowing the locations and times of birds’ migration allows for the prevention of millions of bird deaths by informing conservation decisions like the positioning of wind turbines and the dimming of building lights on particular nights.
More broadly, accurate migration models enable researchers to study the behavioral aspects of migration, the ways in which migration pathways and timing adapt to changing climatic conditions, and whether there are any relationships between variations in migration timing and ensuing shifts in population size.
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