I've just watched a 2011 episode of Dragons' Den with Julian Lipton pitching for money for a squirrel-proof birdfeeder.
I normally expect to see new products or services on the show, but the particular variation involved had been invented nearly twenty years ago by his father. I was delighted to see the featured product, as it is a concept which I use a lot when I run workshops on searching for patents, as the concepts are easy enough for everyone to understand, and why it's a good idea.
The main drawing from the patent application is shown below. The original World patent application by Leslie Lipton, Food dispensers for birds, gives details of how it works.
The basic idea is that the squirrels are kept away from the food by the big bars on the exterior, while small birds can slip through the bars.
The official entry states that the British patent, GB2275408B, is no longer in force as it has run its term of 20 years from the filing date and so anyone can use the invention.
The concept is a good example of how patent classification works. The invention was classified as A01K39/0113 in the Cooperative Patent Classification (CPC) and 372 patent specifications are listed for that class on the free Espacenet database. It can be amusing to look through them -- some have great drawings -- and to contemplate the various strategies used. Most involve using the squirrels' weight, or their lack of flying ability to try to outwit them.
So what happened ? Julian failed to secure any finance as the patent was shortly going to expire, and the dragons like monopolies. Also, the company was making losses.
The company website is The Nuttery.