I'd bet on bunnies or squirrels along with the big caterpillars. Are the plants in pots or elevated anyway? Sounds like what ever is eating them can't reach any higher than the lower branches.
Good luck.
Hmmm... well, there are lots of squirrels. And by lots, I mean there are tons of them around here. They can constantly be seen roaming tree branches and telephone lines, and they can often be heard communicating (fighting?) with other squirrels. There's a massive, thick bush right beside the primary garden, but I don't often see much there except for the cats laying down in the shade (it attracts birds and bugs more than anything else, I think). The plants are in the ground, and not really elevated (though the sidewalk just in front of the bush is slightly lower ground, but I wouldn't really consider that elevated), and they're not in a raised bed.
There are plenty of groundhogs around here, which seem to inhabit the local rundown and poorly maintained buildings, including garages, holes in porches, etc.; in fact, I caught the bastards using a hole near our back door under the house for cover. Yes, this is a pretty pathetic, rundown, poorly-maintained town with lots of hiding spots/shelter for the rodents... probably every house and garage within the block has good sheltered locations where these things can enter, but shouldn't (like broken windows), and keep them safe. Something demolished my sunflower last year, and I'm pretty sure it was a groundhog. The reason? It was planted right near that hole I mentioned beside the back door, and the stem was completely snapped--twice; the second time while staked. I doubt a little rabbit or squirrel would do that to a 3 or 4 foot, relatively strong-stemmed sunflower plant (especially with a bamboo for support).
