Two years ago I had a blue tit nest with nine eggs in my birdbox. Incubation had just started. Unfortunately a cat climbed into the tree where my birdbox is located and put his paw through the opening. At the time the birds were not in the box but they witnessed this from a nearby tree and abandoned the nest. I saw it all happening.
While I was not able to save this nest, I wanted to take measures to prevent it from happening again. Unfortunately I'm quite limited in locations to place the box so I looked for a way to prevent cats from climbing the tree. Somebody gave me the idea to use a large sized surgical collar for dogs that can be bought at any vet for about £10. See picture:
I mounted it high enough such that the cat cannot jump past it.
Just to be sure, I also mounted anti-cat spikes on the roof of the birdbox. Because when the cat attacked the birdbox he climbed on the roof and from there he was fishing with his paw through the opening. That's not possible anymore.
The owner of the cat told me after I took these measures that the birdbox looks like a fortress and that she believed the cat would never be able to reach the birdbox again. And indeed I have never had any problems with cats since.
Check my birdbox!
What a good idea FF, you certainly have ensured the safety of your precious brood this year!
2013 photos & vids here
eff37 on Flickr