Somewhere Over the Rainbow
The last verse is:
If happy little bluebirds fly
Up in the sky
Why oh why can't I?
The first line and the last line pose very different problems in terms of glides.
Why of Why Can't I?
This is a musical cliche -- the notes run up the scale a note at a time, ending on the dominant (the eighth). Yes, there can be beauty, charm, and innocence in simplicity. But the fact is, if you don't put in glides, but instead just sing the notes, it will sound like a cliche.
That leaves open the choice of what glides to do. One theory of glides is that you glide from the note you are singing. That doesn't tell you what glide to do on the first note, but it suggests a glide from fourth to fifth on the second note, from fifth to sixth on the third note, and so on.
This actually works well for the second and third notes. If you do it for all of the notes, I think it becomes a cliche. But one way to sing the song is to continue this pattern for the next-to-last note, then put no glide on the last note. This gives the ending a very simple, innocent, ending. (I perhaps am assuming a plaintive ending, which is how I like to sing the song and the way Judy Garland sings it.)
When I deviated from this, I usually started the deviation on the next-to-last note. If there is no glide on this note, then the last note needs a glide. Another interpretation I played with was starting the glide into the seventh from an octave below the sixth; the glide into the last note (the eighth) can also be from an octave below the seventh.
Why of Why Can't I? Harmony?
There is another theory of glides, which is that they add harmony. Gliding from the fourth to the fifth (on the second note) seems to fit the harmony perfectly -- the chord there seems to be fifth, fourth, and second.
I am not sure what the chord is for the next note. It could be sixth, fifth, and third, in which case the glide from fifth to sixth fits the harmony. But I would choose a different chord for that note (and I don't know what the real chord is). However, when I play just two notes at a time, playing the sixth and the fifth (as harmony) sounds fine.
Things break down for the next-to-last note. The chord, whatever it is, does not include the sixth, and playing just two notes, again the sixth is not acceptable harmony for the last note (the seventh).
Okay, I went on the problem backwards -- playing two notes on the piano, what is a good harmony. Perhaps because I like harmonies that go up or down the scale, I chose a fourth to go with the seventh and a third to go with the eighth. I then sing the song with these glides, and it sounds good.
And the Winner is...
I actually prefer the nonharmonic glides to the harmonic glide I constructed. Perhaps it is just what I am used to, but I like the simplicity of the nonharmonic ending.
However, I think the haromonic glides would have worked better in many other situations, and the two-note technique seems to be a valid way of constructing glides.
Also, another good way to sing the song is to combine the two -- the next-to-last note is a glide from the fourth to the seventh, then the last note is a glide from the seventh to the eighth.
I am pretty sure that Judy Garland sings it the simple, nonharmonic way.
Diane Schuur is not trying to sound plaintive, but she follows an intrepretation pretty close to my recommended interpretation: Two whole note glides, a difficult-to-understand glide where I think the glide is optional, then no glide at the end.
Patty LaBelle puts one glide -- or more -- on every note she sings. I assume she would have put at least 3 notes in the last word if she sang it. But she does not sing the last note of the song -- she comes to it twice and does not sing it either time. Provocatively speculating, maybe she knew the last note had to be glideless and couldn't fit that into the rest of her intrepretation.
If Happy Little Bluebirds Fly....
This phrase, except for the first note, is an alternation between two pitches. In my opinion, there should be no glides, and any attempt at glides sounds very bad.
Why? I would guess that this is a Bach-like melody, where the notes jump up and down the scale, and these simply are not suitable for glides.