A mention of the word ‘migration’ conjures up many images; Wildebeest, Caribou, Monarch Butterflies... even Indians. Rarely birds, though. But (aside from the daily movement of plankton from the ocean depths to the shallows and back), birds migrate on the largest scale of all organisms, and perhaps the least known.
Countless millions of individuals comprised of over 4,000 species (40 per cent of those on the planet) move vast distances twice each year. Birds in the northern hemisphere spend their winters in the southern, and move back each summer to breed. There are various reasons for this: summer in the temperate north is easy – food is plentiful and predators are few. Longer daylight hours mean they can devote more of the day caring for their young. Winter prevents feeding as most of the ground is covered with frost or snow, and a lot of food (particularly for raptors) hibernates to avoid the chill.
Larger birds, such as raptors and storks, must fly using thermals, and so are confined to a land migration. The African Rift Valley is a major flyway, as these birds use landmarks to navigate. Smaller birds fly directly over sea and land, often using the stars or magnetism to find their way. The mechanics are still relatively poorly understood. Although migrating to avoid these extremes is a strategy that has evolved and perfected over millions of years, it has its costs, but the instinct that compels a bird weighing less than a cricket ball to travel thousands of miles is extremely powerful and several unbelievable feats are achieved. Smaller birds must double their body mass before embarking on the journey (like the Blackpoll Warbler, that travels 2,300 miles in 86 hours), young birds travelling the 6,000 miles or for the first time face huge mortality rates, and there are massive obstacles to overcome.
Yet it happens. Bar-Headed Geese fly at over 29,000 feet over the Himalayas, Barn Swallows cross the Sahara desert in one go and some waders, such as the Bar-Tailed Godwit, cross the Pacific in 9 days of non-stop flying. The Arctic Tern takes the prize though – this graceful and elegant
seabird travels up to 49,700 miles a year from Alaska to Antarctica and back.
Recent advances in technology, like the ability to attach minute GPS tags onto birds as small as a lark, have provided a huge amount of data, and revealed how and why this happens. It also shows how difficult man has made this ordeal for certain birds. Every year, thousands upon thousands of birds are trapped in the Mediterranean, shot in Egypt, or caught in fishing nets in Myanmar. Species that return to the same patch of forest or swamp may find a shopping mall or business park the next year instead.
As the seasons change at the end of the month, and you find yourself in Turkey, Egypt, or in the Rift Valley, look up, and you may see clouds of birds, heading home, to breed.