Sexing Robins

It is virtually impossible to ascertain the sex of a robin just by looking at it. The only sure way is by observing behaviour. I'm guessing that my number thirteen glasshouse robins are a male and a female. A male wouldn't tolerate another male in his territory. If one of these robins starts feeding the other he is a male. If one of these robins starts building a nest she is a female. I'm hoping to see these behaviours soon. 

However, the robin in my image looks smaller, rounder and fluffier than the one I posted the other day. I've a hunch that it's the female. :) 

I got two shots of the birds in flight but was disappointed to see on screen that in both of them it was the feet rather than the eyes that were sharp. :(