Browsing online, learned that squirrels, in common with all other warm bloodied mammals suffer the familiar adverse reaction if they contact capsaicin, the hot component of chilli peppers. Birds, on the other hand, are immune.
I have a nyjer seed feeder which attracts lots of beautiful goldfinches but suffered unacceptable depradation by agile squirrels. I tried sprinkling very hot chilli powder over the seeds lying on the feeding ledge and, touch wood, this seems to be working.