They are mixed breeds, so your guess is as good as mine. I believe the darker one is mallard/pekin. He's growing so fast, and has white tipped wings, but strong mallard coloring. I think the gray is from my mallard and my fawn and white runner, she has mallard pattern, but gray feathers instead of black, and she is very light and slender, and stands more upright than her brother. That's my best guess, but they could also have Cayuga or welsh harlequin in them.
