It’s squirrels Bob. Pix 3 and 4 clearly show the tooth marks. Squirrels seem to love them more than nuts for some reason. I either sell Plastic tower, or all copper collars.
It is definitely squirrels. We run into this all the time here in KS. They seem to think its a delicacy!! I have actually seen them gnawing away at one and the other reply is right the teeth makes show clearly in a couple of the pics. You can treat them with a squirrel repellant or paint them but copper is an easier fix.