Migration – the incredible journey
Twice a year, certain species of birds make immense journeys, often in excess of ten thousand miles, spending the summer months in a temperate climate and the rest of the year in more tropical climates. This migration is a long journey which many birds do not complete, yet it is an essential part of their natural pattern.
Many reasons can be given for this migration. Some argue that it is a result of some birds being unable to withstand extremes of temperature, especially the cold weather, explaining why many birds migrate to Africa during the European winter. It has also been suggested that this migration is a result of travelling instincts of millennia ago, before the continents drifted apart. As the land mass spread, birds continued to travel the required distance back to the area they knew was a good source of food or protection. The primary cause, however, is considered to be the search for food, particularly for their chicks. Staying in one place, the food sources would become increasingly scarce.
In preparation for their long journey, migratory birds undergo a number of physical and chemical changes. These changes are triggered by the rising or falling daylight and result in a considerable increase in the birds’ appetite (up to 40% more than during other times of the year). This food is stored in fat deposits and in some long-distance migrants, this fat becomes 30-50% of their weight, ready to be released gradually to fuel their journey. In addition, they are also considerably more active at night, influenced by chemical changes preparing them for their long-distance haul. Most birds travel long distances overnight, partly as migratory flight generates considerable heat that needs night-time temperatures to dissipate.
The speed at which migratory birds travel varies depending on species. Some birds can reach speeds of nearly 90 kilometres an hour, while others travel at a more sedate pace. Ducks and geese can fly between 60 and 80 kilometres an hour where herons and hawks travel at less than 40 kilometres an hour. Many smaller birds are capable of travelling at considerably faster speeds for short periods in order to escape predators, but cannot maintain these speeds for the distances required for migration. Another difference between species is that for some, the males migrate first, arriving at the breeding grounds early in order to establish territories before the female arrives to choose a suitable site for a nest.
One of the most impressive aspects of this migration is how birds can maintain a direction. Most migrating birds return to the same nesting areas year after year. Theories to explain precision of travelling such distances without getting lost have considered landmarks on the Earth’s surface, the sun and the stars, even wind direction and an acute sense of smell. Yet these theories do not explain how some birds can travel thousands of miles on windy and wet nights and still arrive in the same area year after year.
Naturally, given the dangers involved, migration is a dangerous journey. Untold thousands of smaller migrants die each year from storms and attacks by predators. Mortality during migratory flight, of course, is one of the several costs that are covered by the increased production of offspring that migrants obtain by nesting in locations where food is more abundant and competition for most resources is lower. Flying at night, lighthouses, tall buildings, monuments, television towers, and other aerial obstructions have been responsible for destruction of migratory birds who simply do not detect the problem before it is too late. There is also the simple matter of exhaustion, particularly for smaller birds with less energy reserves. Birds flying for up to 48 hours straight can run into adverse winds and find the last of their energy depleted before they reach land. Flying lower and lower as fatigue sets in, these birds fall into the sea.
The most recent challenge to migration is, however, man. Slow climatic and environmental changes have always occurred, but not on the grand scale people have been responsible for. Extensive forests have been burned or cut away, and open fields have been claimed for agricultural purposes. Urban expansion has further encroached on the birds’ natural habitats, and pollution, particularly in the form of acid rain, has damaged many of the remaining fields, mountains, lakes and forests. Wetlands that were once home to many species of birds are increasingly drained or filled as land is needed for new housing developments. After overcoming all the trials and risks involved in migration, many birds find that their homes of the year before are now unrecognisable and they are forced to find new grounds on which to breed, often straying into areas defended by larger and aggressively territorial birds.