Spring is upon us, and with that comes - hopefully - sun, warmth and an increased desire to be out in nature. Another thing that happens in spring is the migration of birds from their wintering grounds to their breeding grounds. Bird migration is a phenomenon humans have known of for a very long time. It has been recorded as far back as 3000 years ago, and has been mentioned by both Homer, Aristotle and in the Bible. Originally, however, many people thought birds hibernated, however, it was confirmed in the 18th century that the birds did migrate.
Why do birds migrate?
There are two main reasons why birds migrate: to find food or a place to nest - or both. The migration happens when the place the birds are at has low or decreasing amounts of those ressources, and they therefore move somewhere better. On the other hand, if there are a lot of extra ressources one winter, some birds who usually migrate might choose not to that year.
A lot of birds only migrate short-distance to find better food options, but some species travel huge distances and thousands of miles every year: some species literally fly across half the earth - and twice a year! The species traveling the farthest is the Arctic Tern, as it spends the summers in Arctic and winter in Antarctic!
In total there are about 1800 different bird species that are long-distance migrants. Most of these spend winter in the south due to both the temperatures and lack of food in the northern hemisphere during winter, but when spring comes, they migrate north to take advantage of the longer day times, food sources and nesting places.
How does the migration work?
Migrating birds usually travel in large groups. These groups typically fly together along a specific route called a 'flyway'. The route usually isn't a straight line between breeding and wintering grounds, but often follow mountain ranges, coastlines or rivers. Avoiding long stretches of open ocean is common - unless it is for sea birds, then it is the opposite - as well as the route taking advantage of weather and wind patterns to make the flight easier. Detours to a nice stopover habitat are also common. The route almost always stay the same from year to year, although some birds have different routes for spring and autumn which take the seasonal food options into account. Often even the layover stops are the same from year to year, due to the stops being ideal in terms of food. It is typical for different age or gender groups of a specific bird breed to have different patterns in their migration, with some migrating earlier than others. To help with navigation, the birds use experience, innate biological senses, and the electromagnetism of the earth.
There are certain risks to migration: the route is long and very physically taxing, and some birds of prey have developed late breeding seasons so they correlate with the time smaller birds migrate past their living area.
This is why flying in a large group is safer than a bird migrating alone; spotting predators is easier and the chance of them catching one specific bird is lower when there is a whole lot of them, and especially for larger birds, flying in a flock can reduce energy costs a lot.
A different risk is that birds might lose their way, thus appearing outside their normal ranges. This can be by flying past the destination, if the genetic programming of the route in young birds fail, or if the birds are blown out of their way by heavy wind. Some birds might also join similar birds from another breeding region in the common winter grounds, and then migrate with that new group in the spring.
To finish with a bit of a fun fact: It is not only birds who can fly that migrate! Penguins do it by swimming, often more than 1000 km, while certain other bird species migrate by walking.
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