Even if I think is possible that the reason why paul sang lead on Day Tripper verses was because john coulnt reach the notes, my theory is that Paul wrote the melody of the verses. ¿Why? First we have to take in consideration that McCartney considered Day Tripper very much a co-written song, and only gave more credit to Lennon because “he sung the lead” ( what McCartney did´nt rememeber (or least , at the time of his many years from now book, its that he is the lead singer in the verses of the song not John ( maybe Paul later remembered it, and that´s why is he is playing the song his present gigs). Now in Lennon´s Rolling Stone 1970 interview (you have to hear the audio version not the transcripted one), John used Day Tripper as another example, besides A Day In the Life, where one of them had “the good bit “( he even sings a tiny bit of the chorus) and then the other took up the cause and completed the song. Then in 1980, Lennon rewrote the history of the song in his 1980 playboy interview, but that´s typical of him, he changed his stories as much as his clothes. In 1972 to the Hit Parader, Lennon said:· Me, but Paul helped me on the verses”. Notice that when John talked about lyrical help from him to Paul, or viceversa, on a song, he used to said things like “ I helped with a couple of words” “ or he helped me on the lyric”. But when he talked about musical and lyrical, or only musical, contributions on a particular song, from one composer to another, he used said things like “ He helped me on the middle” or “ I did the middle with him”, naming entire sections of the song in discusion. That´s why I think when Lennon said McCartney helped on the verses, he meant (even if he kind of didnt what to talk much about it) that Paul came up with the melody for that particular section, and probably with a good part of the lyrics too. I know, I could be wrong, its simply a personal theory. But I also could be right. Dave, keep up with the good work, Like always, forgive me for my poor english. You can download the torrent of that audio version of much of Lennon´s 1970 rolling interview, where you could hear the comment about Day Tripper I´m talking about in: http://torrentz.eu/9389e9e2f674c4b276e1d017441bc8bbecc7109f