In September, in Britain, you may see a lot of birds【C1】______ on roofs and telegraph wires. These birds are swallows. They are【C2】______ together because, very soon, they will be flying【C3】______ to much warmer lands, where they will find【C4】______ small flying insects on which they【C5】______ There are no such insects【C6】______ in Britain during the winter, as it is【C7】______ cold for them. The swallows settle, fly off, swoop, and【C8】______ again. This they do many times, for they are making short【C9】______ flights in order to be fit for the long journey【C10】______ them. 【C11】______ of these migrating birds leave Britain in the autumn. They fly【C12】______ for hundreds of miles【C13】______ they reach the warm lands of Africa. But not all the birds get there, for many of them perish in the stormy weather they meet with【C14】______ . In the spring of the following year they【C15】______ the long and tiring journey back to Britain. They return to the identical barn or tree in the【C16】______ district which they had left the【C17】______ autumn. How do these birds find their【C18】______ there and back over such vast distances? Nobody knows exactly【C19】______ , but it has something to do【C20】______ winds and air currents.