Below is a repost of Firefly's blog about why we think Millie is really Miles:
Is Millie really Miles?: "I think one of our Silkies is rooster. :( Millie is actually Miles. I started thinking about how different Millie and Petri looked and the wheels started turning... something is very different. So, I googled how to tell the difference between male and female Silkies and discovered that the males will have wilder feathers on the top of their head, the feathers will be different lengths and point more towards the tail, they will also develop bubbly 'brain' looking skin on top of their beak, and have wattles under their chin. The females have more of a puffball for feathers on top of their head, they don't have the bubbly looking skin, and no wattles. So, I went out and took some pictures of the birds to confirm with others (hopefully someone who knows more than I do) as to the gender of our feathered friends.
This is Millie, notice the bubbly skin and wattle.
This is Petri, notice the neater feathers, lack of bubbly skin and wattle.
And, just to be sure, I took some of them further away:
And because I didn't want the other two to feel left out... Here are Gibbardanna and Thelma: