Sunflower seed heads

I grew some sunflowers ' evening sun' this year and they have done really well. They are mostly finished now and the seed heads are developing, or at least they were developing until this evening when I noticed that some of them had gone. It looks like they were chewed off, by squirrels I am guessing as there are plenty of them around here. There is no sign of any seed or heads on the ground. I am quite disappointed that the birds will miss out. I don't know if this is a common occurance  ..... or maybe it wasn't squirrels?