Can Robins and Chaffinches interbreed?

Hi. We have a young male robin in our garden with a very black flash across the top of its beak and white flashes on its wings. Other, young, possibly sibling, robins in our garden have the white flashes on their wings. Am I being fanciful, or can chaffinches and robins interbreed? We've a "birdily" busy garden because we feed birds all year. It it possible that these unnatural circumstances have led to a close encounter of a highly unusual kind? I refer to a post from 2010 where a robin and another species of bird (forget which) were sharing a nest. The assumption, there, was that the robin had been duped into rearing a ready made clutch of younsters of the other breed. I wonder?????