Reading practice 4
It has been well known among even casual observers of the natural world that many bird species make seasonal trips between cooler breeding grounds in the spring and summer and warmer locations in the autumn and winter. Migration is a part of the annual cycle of over 50 billion individual birds worldwide. (This is not to say all birds migrate; there are many species, particularly in the tropics, that maintain a single residence year round.) However, the biological mechanisms that prompt birds to choose a particular date to begin migration are complex and seem to be influenced by many factors.
The most obvious pressure on the timing of seasonal bird migration is the weather. As the weather cools and precipitation increases, birds living in temperate climates move in the direction of warmer weather. However, seasonal weather patterns are notoriously unpredictable. Cloud cover can obscure even reliable measures of the seasons, such as the position of the sun and the stars. Therefore it is clear that birds must rely more heavily on an internal clock to time their seasonal movements.1 In fact, biologists have observed a phenomenon known as "migratory restlessness" in caged birds at the same time the wild members of their species set off on seasonal migrations.
For most migratory birds, food supply is a major factor that contributes to the need to move from one location to another. Birds must have ample nutrition to reproduce, and seasonal changes in weather affect the availability of berries nuts, insects, rodents, and other sources of nourishment2. Furthermore, birds must store a great deal of energy in the form of body fat to fuel their migration, so they have to leave while their reserves are high, before changes in weather cause the food supply to dwindle. Many species seem to anticipate changes in weather and food sources, and adjust their migration schedules to accommodate significant variations in weather and food supply that occur from year to year, causing scientists to speculate that unknown internal stimuli may have a strong effect on the timing of the migration decision.
In 1967, Russian ornithologists Viktor Dolnik and Tatiana Blyumental collected chaffinches as they migrated along the Baltic Coast. By examining the fat content and food in the gut of the carcasses of birds gathered at different stages of the waves of migration, and then comparing these findings to the number of birds out of the total population yet to migrate, the scientists determined the social influence fat healthy birds have on the remaining population that was not as physically fit for migration.3 Dolnik and Blyumental found that on the first day of each wave of migration, only very fat birds flew. These birds left at sunrise, on days when weather conditions were favorable. They did not feed before beginning their migratory flight, instead relving4 on stored fat for energy.
On the second day, the chaffinch migration began again in the morning with fat birds. By the afternoon the migration volume peaked, as more and more lean birds began to migrate, many with the morning's food still in their stomachs. By the third day, almost all the birds that began to migrate were very lean. These birds began their migration only after feeding in the morning. Often these leaner birds began their migration despite inclement weather. Dolnik and Blyumental suggested that the social pressure exerted by the large volume of healthier birds in the flock that had already begun to migrate was an influence strong enough to override the lean birds poor physical readiness and the adverse conditions as factors5 in their decision to migrate. The scientists conducted their experiment on only one species, and though chaffinches are typical of diurnal migratory land birds, they do not accurately represent sea birds, raptors, or nocturnal species. Nonetheless Dolnik and Blyumental's work suggests that social pressure could explain why many different species of birds choose to migrate at apparently unfavorable times.
Some migratory birds travel much further than chaffinches6. Most migrants spend a great deal of time storing energy to make trips that take a matter of days or weeks to complete and travel tens to hundreds of kilometers, spending most of their lives in residence at their breeding grounds and their winter habitats. A few species, however, can spend months of each year en route between residences continents apart, and expend little more energy flying than sitting still. Wandering albatrosses spend almost all their time in the air, either migrating or foraging over oceans, moving up to 2000 kilometers on a single foraging expedition, and flying over 250,000 kilometers in a year (a distance equivalent to 4.6 times around the earth's equator.) Such sea birds rely on their specialized wing structure to hold them aloft in air currents that transport them long distances, and some travel back and forth along the same paths on schedules dictated by the prevailing winds7.
