most likley a ground squirl or chipmunk a cat is the best cure

Squirrels or chipmonks. you need a tunnel of chicken wire made in a hoop and stuck in the ground--or individual bowl shaped ones. The sunflower will grow up through the chicken wire and The squirrels can't get the seed. Once it's sprouted you can remove the cage

I use old window screens, set on something to lift them off the ground -high enough until the plants are strong enough to be unprotected