An alternative to a special feeder is to add chilli powder to the feed. Birds are completely unaffected by Capsaicin, but the squirrels, being mammals, get the burning sensation we get from chillies :eek!:
Put the peanuts in a bowl and add a teaspoonful of chilli powder and mix up well - a bowl with a lid is great for this. Pop the peanuts back into the feeder and after one fiery bite the squirrel should be deterred for quite a while.
This one got a taste for sunflower hearts in late April -
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One taste of chilli powder and we didn't see a squirrel again until early August, when I used another tsp of chilli powder and so far we're two months squirrel free this time.
PS - Use gloves when handling chilli powder!