Scarecrow is singing Fancy Girl, the opening credits theme song of Ozu no Mahōtsukai, the Wizard of Oz anime series. I'm pretty sure I've mentioned it in the news posts before, but I'm not going to go back and check.

It adapted the first 3 books and also the 6th book. The only official translation is a dub which butchered the first few episodes, so I'd recommend finding a fan sub if you can. Apparently, as of 2019, there's an official subtitled blu-ray release of the entire series which I was not aware of!