Hundreds of species of birds all around the world migrate both modest and magnificent distances each year. In Europe and Asia, bird migrations are observed in around 5 billion birds from 187 different species travelling to Africa every fall. Likewise, in North America a similar number of birds travel annually to Central and South America annually and then back again with the subsequent change of season. For migration to be such a common feature in the lives of so many bird species, it undoubtedly provides significant advantages to the birds that invest their energy into finding a new home that is often thousands of miles away.
A migration may be local or long distance. For example, a bird population may simply migrate down to the lower parts of a mountain over winter while other species migrate thousands of miles across continents and oceans. Migrating, unlike hibernation and dormancy, allows birds to be active year round but inflicts extreme physical demands on the birds during their travels. Many birds completely exhaust their fat and energy reserves during their migration and begin breaking down their body’s protein for energy in order to see the trip through.
Why do birds migrate?
The question of why has long been a difficult question for biologists (more specifically ornithologists) to answer. In order for migration to be worthwhile, the benefits must outweigh the risks and the benefits come in the form of increased survival and breeding success.
Many species migrate in order to take advantage of better food supplies in different environments at various times of the year. Others migrate to areas that are better for nesting. Certain tropical rainforests in particular can be densely populated and good nesting sites or food can be hard to find.
Most bird migrations occur during fall when days are becoming noticeably shorter. Their new homes are usually graced with longer days which gives birds more time to find food. Other environmental differences, such as warmer temperatures, less wind and more sun, can make life much more energy efficient than if they were to spend winter in their summer location.
Bird migrations generally follow a predictable annual cycle, leaving in the fall and returning in spring. The decision of when to begin a migration is guided by changes in day length, hormones and gonad cycles. Weather and food availability influence daily decisions of when to move and when to stop. Birds attempt to migrate during the safest, fastest and most efficient times to fly. Many birds fly at night in order to avoid predators such as hawks.
Flight paths are often guided by geographical features such as mountains, rivers and coastline. In North America, birds typically migrate from north to south following coastline and rivers such as the Mississippi. In Europe and Asia, migrations more often follow an east-west orientation following the Himalayas, Alpes and Mediterranean Sea. Many birds migrate to Africa in fall and fly over 1000 miles over the Saharan Desert.
Birds that complete long distance migrations between the Northern and Southern Hemispheres often follow different routes on the north- and south-bound trips. From the southern hemisphere, Pacific birds may travel north along Asian coasts and then south the following season along the west coast of North America. Southern Hemisphere birds do not generally travel as far north as Northern Hemisphere birds travel south.
The timing of bird migrations depend a lot on their lifestyle and flying technique. For example, hawks, which fly predominantly by gliding, migrate during the day to make use of rising air which helps them maintain altitude without having to flap their wings too often. Smaller birds, such as thrushes and flycatchers, migrate during the night to reduce the risk of being eaten by hawks. Night time is also commonly calmer which helps to make flying more efficient.
Altitude and distance
The altitude at which birds fly and the distance they cover varies considerably between species and populations. Most night time migrating birds fly around 700-800 m (2300-2600 ft.) but will often rise to above 3000 m (1000 ft.) to avoid turbulent air. The highest recorded bird flight was a vulture that was sucked into a jet engine up at a massively impressive altitude of 12,000 m (40,000 ft.).
Many birds travel over 10,000 km each year between the upper Northern Hemisphere and the lower Southern Hemisphere. The bar-tailed godwit, for example, travels over 11,000 km (6,800 miles) from Alaska to New Zealand and Australia every fall, often flying many thousands of kilometres without stopping, and then returns six months later. The Arctic tern flies over 12,000 km (7,400 miles) from the Arctic and northern Atlantic to the waters of the Antarctic. The longest migrations of all birds is over 13,000 km (8,000 miles) completed by Arctic shorebirds who travel between the Arctic to southern parts of South America.
Last edited: 15 December 2015
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