Birds fly to get to their food source, whether that means foraging locally or migrating hundreds of miles to get to feeding grounds. Where birds nest is a by-product of getting to their food source. Flight is also very useful for evading predators. Birds also fly to attract mates. How they fly is also a very interesting story. Birds are able to fly because of a variety of specialized adaptations. They have high metabolisms to supply their body with energy. They have lightweight bones. They have feathers, some of which are “flight feathers” that are long, strong, and able to produce lift and act as control surfaces. They also have a bone called the furcula, more commonly known as the “wishbone,” in their chest, which is very important for being able to produce the strength and skeletal support needed to flap their wings.
Birds also have very powerful muscles to power their wings, which means they can flap their wings so fast that they can actually take off from the ground and move forward through the air, as well as glide “on” the air. A very important fact is that a bird’s wings are made of feathers. The feathers are not just ornaments! A bird can make its set of feathers close tightly together, which, when gliding or flapping, forces the air to flow around their top and bottom wing surfaces to give “lift”, or it can make its set of feathers “open up”, which allows the air to pass straight through them and, therefore, the wing, which allows the bird to fall under the force of gravity.
So, because a bird can set its wing feathers to be in any of a vast number of intermediate positions between “feathers fully open” and “feathers fully closed” – not just feathers fully closed for the start of a “flap” going to fully open before the end of a “flap”, as would be necessary whilst flapping to take off – a bird can take off, glide, swoop down, glide again and then rise up again or drop, just as much as it wants to do!
Amazing how the ways birds have evolved to be able to change their wing feathers – see KIrsty’s last two paragraphs – is a kind of model for the ways wing flaps are used on airplanes. Watch a plane landing, and then watch a duck coming in to land on a lake, and spot the similarities.
One of the things that’s changed in the years since I was in school learning about dinosaurs, and then about birds, is that it now seems close to settled that birds are directly descended from the dinosaurs. You can read more about this here. Nowadays, whenever I see a bird like a magpie close up I find myself thinking of a Velociraptor!