I just started playing the whistle (I got mine yesterday, in fact), and I’ve gotten down a few simple songs and such. Anyways, I came across something with music written for Bb whistles, and it says something along the lines of “If the tablature is played on a D whistle the tune will be played in the key of D.”
My question is… what does it mean by that? Do I just play as it reads in the music, or do I play with slightly different keys because I’m using a high D?
The rule of thumb is: tell your brain that the lowest note on your whistle is a D, and go from there.
The D whistle will play in D, G, A, Em, Am. If the tune is going to be keyed in Bb, just play a Bb whistle, and play the tune using the same fingerings you used on the D whistle. If it’s in C, use a C whistle, etc…
With practice and compromises, you can play in other keys on your whistle, but it’s generally not worth the time and effort. That’s why we buy many, many whistles, and go on about it at such length! 
Get a C whistle and play the tune using the same fingering you’d use on your D whistle if the tune was in G and you’ll get the tune in F, which has a Bb.
Bwa-ha-ha-ha-ha!
If you play from sheet music, learning whistle tunes in different keys can be challenging. Whistles are designed to play in one key (although they can be easily played on a couple related keys). However, once you’ve memorized a tune in D on a D whistle, you can play that tune in C by simply picking up a C whistle and using the same fingering as you used on the D whistle while playing in D. If you try to play the C whistle from sheet music written in C, you’ll find yourself learning a new fingering pattern because playing a C on a C whistle takes a different fingering than playing a C on a D whistle. That’s why recorders on come in only two keys, C and F; that’s as many fingering patterns as we recorder players can handle! If you get in the habit of learning whistle music by ear, or at least memorizing the tune so you can play without sheet music, then you can play that tune in any key simply by picking up a whistle in the desired key. Thus begins the long, journey into the World of WhOA.
Thus begins the long, journey into the World of WhOA.
You will find multiple posts on this dreaded condition.