1 week old duckling

[TawnberryVII]

I have three 1 week old Cayuga ducklings, 2 of them are very hyper and upright. Meaning they are stretching their necks up high and curious about everything. I have one that isn’t doing that, she still moves around a lot but isn’t as erect as the other 2. She’s also a little smaller than them. Any ideas on why this could be? I’m new to the duck world, used to chickens.