Confidence: It's likely to be this, but I can't be certain.
Notes: Presuming this is a native bird, not someone's escaped pet (which is not unlikely in C/S America!), the closest fit I can find is this. Fairly plain green, horn-coloured bill and looks as though there's a tiny bit of orange under the bill too. See http://en.wikipedia.org/wiki/Orange-chinned_Parakeet and http://www.birdlife.org/datazone/speciesfactsheet.php?id=1625