From the session website https://thesession.org/tunes/2618

X: 1
T: If I Was A Blackbird
R: waltz
M: 3/4
L: 1/8
K: Dmaj
z D |:B4 ^AB |D2 G2 A2 |B e3 d2 |A4 Dd |
d2 c2 d2 |e2 =c2 A2 |D =c3 B2 |A4 DD |
B3 ^A B2 |D2 G2 A2 |B e3 d2 |A4 DD |
d3 c d2 |e2 =c2 A2 |D2 B2 A2 |G4 D2 :|

comment from Guernsey Pete https://thesession.org/members/6734

This song is TRAD, ie a folk-song, collected originally in Dorset, amongst other places. I haven't tried to play the tune yet to check, but, with slight variations in melody, it was a big hit in GB in the '50's for Ronnie Ronalde, a music-hall entertainer who specialised in whistling as well as singing, and who ended his days running his own hotel on my native Guernsey.