Hi l have a very bad singing voice soooooo l whistle so my daughters cockatiels copy me its great ! But if you whistle bad they will sound bad .
Also if u would like them to talk l have been told not to teach them to copy u while u whistle as l guess they will take the easy way out !
Good luck just remember to stick with the same song until they get it ,
P.s. teach them a fast one ( a woof whistle they seem to pick this one up very quick ! catch u later *******