Funny, I only now realised not all of them have that red spot on the back of the head.
Well, I just feed them; I'm not an ornithologist.
Anyway, thank you, Google. It seems it's only the males that have it - and the ones with a bigger red patch on top of their heads are the fledglings.
Live and learn.
They are all greedy little so-and-sos though. They're regulars but not quite daily visitors - and when they do visit those fatballs disappear mighty quickly.
