Need help

Please help!!
Hey I'm new to owning little ducklings! I have two little ducklings. One of my pekin ducklings is loosing his feather, I think. I'm not sure. First i noticed it on his neck then his wings now its on his back he's pink around those areas. He's about 3 to 4 weeks old and I was wondering if its normal to start loosing them at his age. The other one is fine is that normal? And he has a big yellow think on his tail where I read online that that means he loosing his feathers but I just wanted to make sure.