'Dog' is very similar to 'bird' in the relationship between English and Swedish—the common Swedish word (hund) has an English equivalent (hound) which is rare, old-fashioned and/or context-specific, while the common English word (dog) is unrelated—how interesting that it also appeared mysteriously! I think the fowl/fågel words are of common Germanic descent—as liriaen points out below, modern German has the same word—although I have no idea what happened as far back as Indo-European.