I don't think you can single out corvids from the rest of the avians and at the same time bunch all mammals together. I don't think there's evidence that their smarts or brain architecture are qualitatively different from other birds, some of which are plenty smart. If intelligence - for some definition of intelligence - evolved separately in mammals and birds, it's not implausible to me that the basic architecture developed before avian dinosaurs branched from other theropods, and some theropods might have been quite smart indeed.