I have wanted to be asked this question for years! Are you ready?
There was an older song featuring the same tune - "Beach Baby" by (I think) a one-hit-wonder group called First Class. But the classical music on which both songs are based is the last movement of the 5th Symphony by Sibelius.
Allegedly this movement was inspired by a group of swans he saw flying. The movement starts with fast, busy music (representing the dots of birds in the far distance). But then, as the swans come closer, the horns play that wonderful majestic tune, representing the slow graceful flapping of the swan's wings.
Do buy the whole symphony - it's all lovely!