Trouble in the nest box!

There I was, peacefully eating my muesli and occasionally glancing at the nest box outside the window to watch the very busy little great tits bringing beaks full of moss to line their nest - they've been at it for two or three days now.

Imagine my shock when I looked out and saw next door's cat squished into the bird table gap and peering up at the nest box entrance hole!

In spite of impending doom, photography mode kicked in and I took a quick snap before knocking on the window to frighten away the evil bird killing cat!

After a bit of thinking (I needed a quick and quiet solution so I wouldn't disturb the nesters too much) I came up with this.

I hope it works!