I took a video of it, but it's taking a while to transfer over...will post it up later. I don't think it's much to worry about, because the pullet in question (5 week old Wyandotte) is acting otherwise completely normally. Doesn't have trouble standing, walking or roosting, eats and drinks all regularly and her poops all look completely normal and she even runs around, as if she's playing with her "sister". I'm more curious than anything about this behaviour, really, because I've never seen my other three chickens do it. She'll be pecking at the grass and occasionally, she'll stop and kind of...lift herself further away from the ground than usual and her wings will drop slightly - as if she's trying to stretch her chest out. Has anybody else noticed their chickens doing something similar? Does this have a specific purpose?