It’s the bi-annual #WorldMigratoryBirdDay! Why Bi-annual you ask? Spring and Autumn are two huge times for our avian friends, as it is when they embark on their lengthy migrations to and from their breeding grounds. In Spring, we say goodbye to a number of our waders, ducks and geese, but say hello to Swifts, Swallows, Martins and more. And then vise-versa in Autumn. And so twice a year we celebrate the birds that embark on their thousand mile journeys.
Did you know, some birds have been born just months before they embark on these journeys without the guidance of their parents? How do they know which way to go? There are numerous components which go into a bird being able to find their way.
Information is passed to generations through genetics. Meaning they follow instincts that tell them which direction to go. But if a first year bird is blown off course then it may find it difficult to find its way, and so there is a high mortality rate for first year migrants. More experienced migrants will use a number of different methods in order to find their way. These include Piloting, Sun Compass, Star Compass and Magnetic Compass.
Piloting means that the bird is noting landmarks, such as coastlines, rivers and islands, but also they use their sense of smell in order to remember their way home.
Birds will track the movement of the Sun in order to find their way, as well as the movement of stars during night time. Birds will often track the movement of the stars to find true North which can help them navigate. And most impressively, birds have a Magnetic Compass meaning that they can detect the Earth’s magnetic fields. It is still up for discussion on exactly how they manage to do this, but most popular theories are that they have a metal compound in their beaks which help them detect the fields, but also that they have receptors at the back of their eyes which pulse when passing through a magnetic field.
Birds have an internal clock that is not purely dependent on the rise and fall of the sun. Experiments were conducted where birds were kept under continuous light for 24 hours showed the same cycle of resting, roosting and activity.
We get Nocturnal and Diurnal migrants, some birds may prefer to fly at night due to the fact that following the position of the setting sun and the fixed positions of the stars may be easier to navigate than the moving sun. Plus, night air is more favourable, as it is cooler and more humid. Humidity makes the air denser, and so flight becomes easier. Some birds may also choose to fly at night to avoid predators when flying overland.
Finding their way isn’t the only struggle these birds face. How does a tiny songbird or warbler manage to fly thousands of miles non-stop? There are many different evolutionary factors that go into play here. Fat is the best fuel to supply the energy for flight. And it is not unusual for a small bird to double its body weight, having 50% fat before embarking on migration. To acquire a store of fat, migrants must have an abundant supply of nutritious food so they can fatten up quickly and efficiently before their time to leave. This is why it is so important for birds to be left undisturbed as much as possible to give them the best chance at survival. Birds have a unique breathing system, in which the lungs are linked to three pairs of thin-walled sacs. These cause air to pass through the lungs in a single direction instead of in and out making it much more efficient at extracting oxygen and flushing carbon dioxide. This also makes it easier for the birds when flying at great heights, when oxygen is scarce.
And finally, why do birds migrate? Bird Migration is usually taken to mean the seasonal movements of birds at predictable times of year between breeding and non-breeding areas. The key to migration is food, and food supplies are usually linked with climate. In Arctic regions, flying insects largely disappear, lakes freeze over and permafrost stops plants from being able to grow, and so the birds that breed here must move South in order to find food. But for birds that breed in warmer climates, some might ask why they bother to move if the food is readily available year round. This is usually to avoid overcrowding, where there may not be enough food to go around, and so some sort of migration is required to acquire a substantial amount of food year round.
As you can tell, there is a LOT that goes into a bird’s migration, and there are many things that could risk a bird’s life during migration. Climate change, severe weather, droughts, exceptional weather conditions, lack of food, migration failure. And so it is important to reduce any other factors which could reduce these birds chances of survival wherever possible. And so that’s why it is so important to give birds as much space as possible when walking along the coastline, so as to not interfere with their important routine.