Hundreds of species of birds all around the world migrate both modest and magnificent distances each year.
Around 5 billion birds from 187 different species in Europe and Asia migrate to Africa every fall. Likewise across the Americas, a similar number of birds travel annually from North America to Central and South America and then back again with the subsequent change of season.
Migrating requires an investment of energy for birds. For migration to be such a regular part of the lives of birds, it clearly provides significant advantages, otherwise, why would birds invest their energy into finding new homes, 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 – benefits coming in the form of improved 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 making good nesting sites and/or food difficult 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.
The annual cycle of bird migrations
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.
Bird migration routes
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 the 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 depends 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 nighttime migrating birds fly around 2300-2600 ft. (700-800 m) but will often rise to above 10,000 ft. (3000 m) 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 40,000 ft. (12,000 m).
Many birds travel over 6,000 miles (10,000 km) each year between the upper Northern Hemisphere and the lower Southern Hemisphere.
The bar-tailed godwit, for example, travels over 6,800 miles (11,000 km) from Alaska to New Zealand and Australia every fall and then returns six months later. Even more incredible, godwits often fly several thousand miles without stopping.
The Arctic tern flies over 7,400 miles (12,000 km) from the Arctic and northern Atlantic to the waters of the Antarctic. The longest migration of all birds is over 8,000 miles (13,000 km) completed by Arctic shorebirds who travel between the Arctic to southern parts of South America.
- Interactive Migration Map of American birds – National Geographic
- The Migration of Birds – articles by the Royal Society for the Protection of Birds
- Earthflight – stunning BBC Earth documentary series
- The Genius of Birds – novel by Jennifer Ackerman
Last edited: 4 April 2019