Tuning a stringed instrument works by tightening or loosening individual strings to  achieve exact pitches on every note. Pianos, guitars, harps etc. all work on this principal and you can use static turners and tuning forks to set the individual strings.

Drums are tuned by tightening or loosening the head of the drum.

Whistles do not have a true tuning slide.

Each whistle is tuned to an individual key. The slide on a whistle exists to allow the mouthpiece and tone body to be separated for ease of carrying the whistle and for the maker to correct any length measurement error between the head and the holes in the tone body when assembling the instrument.

You also may want to read out whistle tips and tricks here.

This slide also sets the exact distance from the bottom of the sound hole in the mouthpiece to all the other holes in the whistle including the overall whistle length at the butt of the whistle. When this distance is set, the whistle plays exactly to the measurements of the original design and is tuned as closely to the notes in the frequency scale as is possible.

You cannot tune the whistle one note at a time like a stringed instrument, so the whistle is not tuneable to another instrument by moving the slide up or down. Adjusting the slide up or down will only make the whistle play out of tune by being sharp or flat on all the notes at the same time.

Once the whistle is set to specification on the slide, then all the notes may only be PITCHED Up or Pitched Down as a unit. The whistle has very close tolerances and adjusting  the slide can move the whistle tuning completely out of tune. You should find a setting mark on the slide to show where the whistle smith set the instrument when it was made. You can tune to a guitar, mandolin etc. by using the whistle as the pitch pipe for the other instrument to tune to.

Whistles are tuned to get an even run between notes in both octaves to eliminate any small variance  in individual notes. Whistles made of hard machinable materials like aluminum, brass, silver and polymer plastics do not lose their tuning and play well as long as the instrument is kept clean and buffed inside the bore and played in a warm place.

Polymer whistles play “warm” because the polymer material has an insulating factor and retains heat.  This is the best material to make a whistle that plays well in cold temperatures and an instrument that drys out quickly when playing in a damp location. Whistles made from wood that has been stabilized is in effect, made from polymer and works well in a range of temperatures.