* The birds whistled cheerfully in the morning sun.
* He whistled a merry tune as he walked down the street.
* The wind whistled through the trees, creating a haunting sound.
* She whistled for a taxi, but none came.
* The referee whistled the game to a halt.
* The boy whistled with amazement at the magician's trick.
* The train whistled as it pulled into the station.
As you can see, "whistle" can be used to describe the sound of birds, people, wind, or even machines. It can also be used to describe the action of calling attention, as in the example with the taxi.