I have noticed that my male muscovy duckling (about 15 weeks old) which has got bigger-taller legs than the girl and looks like he doesn't know what to do with them, gets tired more easily when he walks, than the female who is more active and agile and trying to fly and dosn't lie down on the grass that often. Also when they run together he looks to be so clumsy with those tall legs and when he stops, his legs start trembling and lies down almost imediately after. He looks to be perfectly healthy apart from that, he has a huge apetite and everything looks normal about him.