Bluebird Houses/Cats

I put up 2 bluebird houses last year and my cat "Sis" loves them! She thinks they were put up just for her and her tummy! She's killed two of the bluebirds and eaten some eggs too! Well, this year the birds have started making nests in the houses again. What would you suggest to put around them to stop her from getting to them. One house is on a swing frame and the other house is on our little garage/vehicle shelter thingy. Here is pictures of what they look like. I had thought of putting the tack things that you put your carpet down with along the top of the swing. You can see from the picture where she jumps up there. http://www.photobucket.com/albums/y191/sjjlyles/Bird%20houses/ Let me know!