Celebrate Babe Ruth's birthday with this photo of him singing 'Happy Birthday' to himself

Saturday would have marked Babe Ruth's 121st birthday. Eighty-one years ago, as he turned 40 years old, Ruth and his family celebrated by gathering around the family piano and singing "Happy Birthday." You can see the lovely moment in the photo above.
But here's my question: Is Ruth singing "Happy Birthday" to himself?
Look, I'm not going to begrudge BABE RUTH from singing whatever song he wants to sing, and certainly not on his birthday. All I'm saying is that you don't typically see a person singing "Happy Birthday" to himself. Yet there he is, with his mouth open while his wife Claire and daughters Julia and Dorothy sing to him, and unless it was just an inopportune yawn at the precise moment the photograph was taken, I have to assume Ruth was singing along with them. Perhaps he just wanted to harmonize with the bass part? Like I said, it was his birthday and he's the most famous baseball player to have ever lived, so if he wants to sing "Happy Birthday" to himself, by all means. 
And all that said, Ruth was a pretty good singer: