Ave is the birdkind, two wings is an ave (like penguins, ostriches, pigeons). Meanwhile, pájaro is just the birds we imagine when we hear the word, those who fly and chirp (Pigeons, toucans, crows, cannaries). Hope this helps.

There is a difference. Ave is any kind of bird whereas a pájaro is a member of the Passeriformes order. So all pájaros are birds but not all birds are pájaros.

My understanding is that the Spanish tend to use the word ave more for biggish birds like parrots or hawks and pájaro more for smallish birds like hummingbirds or sparrows. YMMV.

Ave Maria.