I hear a lot of songs - especially on the acoustic guitar - that I would love to play along with. But I have a problem figuring out which key to use (I currently have A, Bb, C, D - all high whistles). This may be way over my head, getting into things like half-holing, half-halving, and playing the whistle upside down. In which case, feel free to send me packing back to Mr. Duns to complete his tutorial 
er… is there a question somewhere in that? 
Over-simplification, but here’s something:
Most tunes resolve to (end on) the tonic (key) note. Listen until you can hear that note and hum it, then find it on your whistle.
You’ll want to use a whistle that matches that note when you have all holes covered, or when you have the top three covered.
Your jabbing is starting to hurt 
My question is how to discern which key to use while playing with other instruments like the acoustic guitar.
That is very helpful. Thanks for the input.
You’ll also have to deal with key changes and melodies which go beyond the comfortable playing range of the whistle, but don’t let these stop you from trying.
explanations should be as simple as needed but not simpler
(badly paraphrasing Einstein here).
You could also ask the guitar player what key he’s in. Unless it’s a recording, in which case he probably won’t answer.
Earned me a “very”. Had one lately?
he is like, ah, so far beyond very
- A good rule of thumb is that a lot of songs are performed in the key of the sheet music. This isn’t always true but “find the sheet music, find the key.” This rule of thumb is more true with the original artists rather than cover bands and even less true with diva vocalists.
- I used to not have an ear for detecting the key of a song but after years of playing irish whistles and flutes I am getting pretty good at recognizing songs in the Key of D and somewhat good about the Key of G. Not so much with other keys. One of my favorite things to do when I hear a song on TV and recognize it’s in D or G is to hurry up and pick up my flute and play along. My family thinks this is incredibly funny. I do too.
yes, you may also hear something in B minor and you pick up your D flute
It’s handy, when you’re trying to figure something out and you don’t know what key it’s in, to have a load of whistles in various keys so that you can noodle along until you find the “spot”.
Though I have every key, you only need half the keys to get all the Major keys, because every whistle plays two Major keys, one based on the “six finger note”, the other based on the “three finger note”, or in musical terms, one Major scale based on the tonic, another based on the 4th. So a D whistle gives you D Major and G major, a B whistle gives you B Major and E Major, an A flat whistle gives you A flat Major and D flat Major. (Yes I’ve had to play in all these keys.)
Likewise each whistle gives you two minor keys, one based on the five finger note, one based on the one finger note.
This is all because it’s standard to use two fingerings for the whistle’s 7th degree, the note that’s C on a D whistle, one for a sharp 7th and one for a flat 7th.
Though what I usually do is grab my Low D and try matching the note that seems to be the “tonic” of the tune. You have to be adept at halfholing for this to work, because the tune might end up being in F or E flat or some other note that’s not part of the D scale.
There are many folk songs and pop tunes, by the way, that don’t start or end of the tonic at all, but maybe dwell on the 3rd, or on the 5th, or whatever. In these cases I’m not trying to match the melody notes but rather the roots of the chords, to find out the key.
One tune was particularly difficult, Paul McCartney’s “Mull of Kintyre”, because as soon as I was getting locked into it, it would modulate to a different key. The version I was trying to play along with seemed to change keys for each verse, chorus, and bridge. It was far easier to sing along till I had the tune in my head, then play it in whatever key I wanted to.