A friend I was with last Sunday did pay for a purchase with a Birds $20. The Birds $20s seem to be the most commonly found of all the Birds series notes in existence, IMO.
As for the other denominations, I don't see very much in terms of Birds series anymore, though I did receive a Birds $10 in my change in the summer of 2006. And in November that same year (just days before the upgraded Journey $5s were released), I did pick up some Birds $5s from a very busy Scotiabank branch, and they were in favourable condition, though not crisp.