Many of the birds that are common sightings around the UK are actually migrants that spend some of the year in other parts of the world. Other birds that aren’t seen so often can appear during winter because they are migrating here from colder places further north. So what is bird migration, why do they do it and who does the most impressive routes?
The main reason that birds migrate is the availability of food. It is a seasonal movement that usually happens with birds moving south of their summer breeding place and happens mainly in the Northern Hemisphere. Birds that do not migrate are called resident or sedentary.
As well as food availability, the other reason birds migrate can be the suitability of places to breed. The longer days found in the northern summers give them a longer period of time to feed their young than it would if they were further south but it means as the weather turns cold, they need to move somewhere warmer.
Migration has a huge effect on the birds who undertake it in terms of stress, physical exertion and risks from predators. In fact, some predators have learned to use the migration period and routes as a time to breed because there is more food available. One such example is the Eleonora’s Falcon that lives on the islands in the Mediterranean and breeds late in the year when the autumn migrating birds pass by.
Some birds don’t migrate huge distances are this is called a partial migration. For example, around 44% of non-passerine birds (perching birds such as sparrows and finches) and 32% of passerines in Australia are partially migratory.
When you see flocks of geese flying in a V-shape, this is a migration technique designed to reduce the cost in energy the flight needs. They conserve around 12-20% of the energy needed to fly alone when flying in a group formation. Some birds such as Red Knots have been found to actually fly around 5km an hour quicker in a flock.
Flying high is another technique used by some birds, though not always successfully. An expedition on Mount Everest found the skeletons of northern pintails and black-tailed godwits at 5000 metres while Bar-headed Geese have been recorded on GPS tracking crossing the Himalayas ay 6540 metres. This is the highest altitude of any recorded bird.
One of the most recognisable British birds that makes a long distance migration is the swallow. Properly known as the barn swallow, this little bird with its beautiful blue feathers and deep red face is found across Europe, Asia, Africa and even the Americas. The subspecies found in the UK has had its migration documented since 1912 when a ringed bird from Staffordshire was found in Natal, South Africa.
Long distance migrants learn their routes as young birds when they undertake their first migration with their parents. Routes are often dictated by bodies of water for land birds and bodies of land for water birds.
The longest known non-stop flight undertaken by any migrant is by the Bar-tailed Godwit, a wading bird that flies from Alaska to New Zealand. It covers 11,000km and stored up 55% of its bodyweight in fat to get them through the journey.
Meanwhile the record for the longest migration of all goes to the Arctic Tern. This seabird is found in cooler temperate areas of North American and Eurasia in the northern summer then at sea along the northern edge of Antarctica during the southern summer. This means it flies from the Arctic breeding grounds to the Antarctic and back again every year: the minimum distance to this flight is 19,000km. One ringed bird left the Farne Islands, in Northumberland, in 1982 and three months later, reached Melbourne, Australia. This was a distance of 22,000km and only half the annual travel that the birds will do.
Raptors, referring to broad winged birds of prey, often migrate using thermal columns of air. Storks also use a similar technique to move from one area to another during the day but the limitation to this technique is that they cannot cross large bodies of water. This is because thermals are only created over land. Therefore, they are formed to find the narrowest point of a body of water to get across it – for example, lots of raptors and storks pass over Gibraltar and the Bosporus during migration as these are the best places to cross the Mediterranean.
Meanwhile, mountain ranges can also cause a problem for these soaring birds. The Batumi bottleneck is a spot in the Caucasus is one of the heaviest used bottlenecks in the world where large birds pass through the area of Batumi, Georgia. By using thermals, these birds lose only 10-20% of body weight during migration.
Many of the smallest birds that migrate cover large distances and do this at night. Hummingbirds, flycatchers and warblers all use this technique where they land somewhere in the morning, feed for a few days then continue their way. By flying at night, they minimalize the risk of overheating, being caught by predators and also they can forage for food during the day.
The main downside with this is that the birds don’t have a chance to sleep. To compensate for this they adjust the quality of their sleep so that they can continue on their way without falling into a deep sleep.
So how do birds know when to migrate? It seems with a lot of species this is dependent on the length of the day or the hours of light. Others may be triggered to migrate by weather conditions and temperature. Some species simply move to the southern end of their range when the seasons change, such as the Merlin or the Skylark. This is known as short-distance migration.
Finally, there are other birds, such as the Common Chaffinch that never go further than 5km in their whole life. They really are local birds!