Great idea for a thread, Kiwi. Good chance to see the differences in birds that we don't always recognize because of their gender. I can't help but wonder if there'd be even more interest in this thread if you had of called it "Bird Sex", or if that title would have just disappointed everyone once they discovered the real theme of the thread. 
Here's a pair of Northern Cardinals, female on the left, male on the right.
My unorganized pictures can be viewed at
http://picasaweb.google.com/lonestranger102
