Crows are known to pluck sprouting corn out of the ground to get to the
seed. But you said they were leaving the
seed behind so I don't think it's crows. I have a big problem every
spring with birds plucking young
seedlings up and using them as nesting material. I've caught them in the act several times. They sometimes pluck just a few leaves, or just the bud, and sometimes the whole plant. The parts they pluck usually disappear but sometimes they abandon the piece they plucked or give up before they tear the piece completely off. They wiped my first tomatoes out, damaged many of the cold crop
seedlings, and got some corn. The problem seems to go away when they finish their nest building cycle.