Has anybody ever asked, what species of bird Mr. Birdies are? It's six years since their first appearance! If not, I do! I was pretty annoyed by the result of my google-search on "Birds Japan".
They are some form of passerine. Though given the lack of the "ugly as sin" stage of their hatchling lives, and their varied diet of everything from fish to seeds, they're probably not anything in specific. I think they really are nothing more than "Haruna and Wo's Birds". A cursory glance shows a number of songbirds from Japan with a yellow breast, but none fit the pattern of these birds. If I had to guess, they're a mix of a few different passerines to create a simple, yet effective, design.