大象传媒

Swifts spend most of their life on the wing and all because of insects. They fly from South Africa each year for an insect feast in Europe's summer, a return trip of around 14,000 miles. A long history of aerial pursuit has given swifts a streamlined body with long narrow wings to slice through the sky. Swifts are so specialised for living on the wing that they can not even land on the ground. Their tiny legs and long wings would make it impossible for them to take off again. That is why they always nest up high. One mouthful of food passed from mother to young can contain up to 500 insects, and when food is plentiful the chick may get 10 meals in a day. That is 5,000 insects. But in bad weather most insects are grounded and the parents will have a hard job finding food. Sometimes the adults have no choice but to desert their chicks and fly ahead of the storm to feed. A British swift will even cross the sea to Europe, no big deal when you can cover more than 180 miles in a day. Back at the nest, the helpless chick can only wait whilst the adults feed in fair continental skies. As the chick waits it relies on its fat reserves. But swift chicks are tough. They can lose half their bodyweight and survive for days without a meal. But there is a limit. After a round-trip of perhaps a thousand miles or more the parent is just in time to deliver a life-saving meal. Millions of years spent chasing tiny winged insects have shaped swifts into true aerial aces.