Is your garden full of birds?  Mine is, and I have a cat.  I don't want to confine her to the house so I put a bell on her collar.  I found that one bell on her collar isn't enough to alert the birds as she is a clever minx and walks carefully so as not to tinkle.   Now she wears three bells and she can't  figure out how to stop them all from tinkling.  She hasn't been able to catch a single bird since wearing three bells.