thanks, Janice and Max!
Max, the green on their heads is quite visible if you're close enough and the light is right... needs to be lit from a pretty low angle - you can't really see it in typical mid-afternoon sun or on an overcast day. i have a number of photos of these guys where you can clearly see the green iridescence, and just as many - if not more - where their heads just look almost black. in these shots, like you said, the bird was close enough, and the light was just right for the green to show up quite nicely.