How To Identify a Bob

I was telling Susan dear Susan about Isabell and the Bob's, and she asked if we had sexed them yet.  I said we had visual confirmation that one was a female, but the jury was still out on most of the rest of them, as i hadn't yet worked up the nerve to go in search of dangly bits.  Susan laughed at me and said that the drakes have a curly tail feather.  I knew that the curly tail feather was an indicator, but was unaware that it was definitive.  She said it was.  I said that in that case we had a whole bunch of Bob's.  What are the odds.

Here you see the definitive evidence that this Indian Runner Duckie is a Bob.

 I am going to assume that THIS is definitive evidence of a particularly virile Bob.