Bird migrations are one of the great wonders of the natural world and something many people don't really think about until they see unusual species in their gardens or nearby countryside.
They have fascinated scientists for decades too - and it's only fairly recently that ornithologists have been able to glean a greater understanding about where birds go. Indeed, at one time, people used to think that swallows spent the winter at the bottom of lakes and ponds.
If you're interested in finding out more about bird migrations and how you can help the species that travel, then we'll provide some basic information and top tips here.
Which birds are migratory?
According to the RSPB, at least 4,000 species of bird migrate regularly, which is a truly astonishing figure. The most famous are those that travel hundreds of miles, but lots of them take shorter trips too.
Summer visitors are those that come to the UK to breed in spring and stay to make the most of our warmer weather. These include swallows, house martins, warblers, nightingales, cuckoos, nightjars and many more.
Winter visitors travel to Britain from colder countries in the northern hemisphere and include redwings, fieldfares, whooper swans, and many types of geese.
Meanwhile, passage migrants are the birds that are on their way to somewhere else north or south, but use the UK as a type of stopover point. Species including green sandpipers and black terns do this.
The final category is partial migrants, which sometimes migrate but not always. For instance, starlings from eastern Europe might find it too cold there in winter and so will come to Britain, whereas those from elsewhere may not need to and so can stay in the same place all year.
Why do birds migrate?
Simply put, birds migrate to survive, even though it seems to be a very arduous task to us as we contemplate a journey by wing from Britain to Africa.
The phenomenon started at the end of the last ice age and has evolved because the birds that flew fared better than the ones that stayed put.
For example, the insects a willow warbler depends on for nourishment disappear in the winter in the UK, but are abundant in the warmer climates of Africa. If the warbler goes there, it can enjoy a ready supply of food while Britain is experiencing a cold winter.
On the flip side, Africa has more competition for nest sites and a greater number of predators than the UK, so it is in a bird's interest to return in spring for breeding. In addition, this is the period when our days will be lengthening and so parent birds can spend more time feeding their chicks on the insects that are abundant once more.
For species that breed in colder climates like Scandinavia, food can get hidden under thick layers of snow and ice during the winter. In this case, birds come to Britain to enjoy a bit of warmer weather and more food than would be available at home.
When do birds migrate?
Birds use their body clocks to know when to migrate. At specific times of year, mostly down to changing hours of daylight, hormones will be released that alert the creature to the fact that it's nearly time to get on the wing again.
At this point, they feed as much as they can for energy and then await a spell of warm weather and clear skies, which can be pinpointed thanks to changes in air pressure.
It's then thought that birds use a combination of skills, including gauging the Earth's magnetic field and the position of the sun, to guide their journeys.
How can I help migratory birds?
Unfortunately, the human race is making migration much harder for birds by eliminating habitats and feeding opportunities.
There is also evidence to suggest that man-made climate change is making weather more extreme and therefore not only reducing opportunities for them to travel, but also altering populations and appearance times of prey species such as caterpillars.
In developing countries too, economic progress is causing harm by damaging habitats and draining ecosystems like wetlands.
For these reasons, conservationists are warning that migratory birds are in decline - in some cases, drastically.
However, there are ways you can help birds that spend at least part of their year in Britain. The best way is by providing food and suitable habitats. For instance, plant bushes that produce berries and will give birds somewhere to nest and shelter, as well as trees in which you can add nest boxes if you have space.
A pond is a good way of boosting insect populations, while compost heaps will ensure a ready supply of worms and grubs.
When insects are scarce or you know certain species will need to fatten up before (or after) a long journey, supplement birds' diets with specially formulated seed, such as our Hi-Energy No Mess or All Seasons Seed Mix.
You can also help to carry out population studies and follow migration news via the British Ornithology Trust's migration blog, which includes regular updates from around the country.
These birds complete incredible journeys every year, so it's well worth doing all you can to give them a helping hand.