This birdcall has many possibilities.
The variations can be achieved by closing the different holes, (end and metal tubes - when the end of a tube is closed the sound is deeper).
For the Redstart's most common song, close the end of the birdcall with one finger. Blow in short calls so as to obtain the first high pitched notes, then, opening the end continue with a warble. By rolling the tongue it is possible to finish with a trill. So as to follow him well you must listen carefully to your redstart.