Have you tried using a upside-down funnel to put on the top? It's round, but the angle is too steep for most birds to perch. I'd recommend looking at the auto parts stores for one, and getting a black or muted color that they won't pick too much. Another suggestion which may be more difficult depending on how close you are to water is called a rat guard. They're placed around the mooring lines leading to ships and keep rodents from scurrying aboard. Same idea as the funnel since it's conical in shape and they come in metal and plastic. These would be larger than the funnel, and because of it may be more appropriate for outdoor feeders. Just an idea...