of course, you have to take it off at night if you expect to get any sleep.  

I am seriously surprised by the reaction of the people here - like talking to people 20 years ago about paper recycling or something.  really, really backwards!

I have a wildlife-friendly garden.  It's taken years of neglect work to get it that way.  I've got hedges, trees, nestboxes, a wildlife pond and buy local organic birdseed in 30kg sacks.  

I like the birds in my garden.  Really.

But bells don't work, because cats don't hunt by running after birds. I used to have two bells on the collar of one of my cats, and, as ceebs said, it was an exercise in assuaging guilt.  A tabby, he was invisible when lurking under a bush, and all he had to do was wait for a bird to come within range and pounce.

What the bells did do was irritate him.  And with hindsight, I think it may have been rather cruel as well as pointless to attach an annoying noisy thing to the neck of a creature with far more sensitive ears than ours.