Rats do vary in intelligence but they are basically the same size and weight. A feeder needs three things to stop them and I know, I have sold close to ten thousand of the contraptions since 2012.
First is reach difference, the treadle needs to be distant and narrow to prevent rats from piling up on the treadle and operating the feeder. Wide treadles are more comfortable for the birds but birds have grasping talons, not feet, they naturally grasp a narrow treadle and as long as that treadle is bottoming out they will learn to grasp the treadle with one foot, stand on the other foot, and have no problems eating.
Second is weight difference. Around three pounds difference between the critter you are trying to keep out and the one you are trying to feed. This is past the balance weight, that amount of weight of the door and treadle assembly that has been designed to balance out. If it is easy for small birds to open the feeder then the mice will get in because they can just push the door open while standing on the treadle or on the ground in front of the feeder. That is why I went with a heavy concrete counterweight instead of attempting to balance the door/treadle. There is mass, which is different that weight, even in orbit there is mass (no such thing as zero gravity BTW), you need that mass for the mouse to push against to keep his furry behind out.
Third, the door HAS to be spring loaded. Without the spring loaded door a good sized rat can just push the door open, remember mass is different than weight. If you put aside friction, a smaller person can get a much larger mass moving if given enough time and distance, i.e., as in pushing a car on a flat surface. The spring provides weight that increases as the spring stretches out, very little mass in a spring but they provide excellent "weight" which is equivalent to gravity, AKA "weight".
One caveat, massive populations of actual rats can overwhelm any treadle feeder but one of the reasons why I pushed the door back instead of up is that it allowed rats to be trapped inside if one or two managed to get inside during a massive rat pack attack. This has happened to one of the commercial free range flocks that bought our feeders but they had thousands of rats. One by one the feeders were choked with dead rats till all the feeders had killed a gallon of rats and the surviving rats would refuse to return to a feeder again after that happened. If I recall correctly they had twenty four of our feeders and either ran out of rats or the rats departed to a better feeding grounds. Even the Grandpa feeder has reports of rats lifting the lid because it is a balanced door/treadle constructed feeder. The large rats can push the lid up to get in and push the lid back up to get out. My feeder is impossible to push open from the inside resulting in smothered rats. One tip that a customer gave us was moving a feeder out into the summer sun if they catch a rat or squirrel. A few hours later you have one dead varmint. If that would work with politicians I would be a wealthy man for sure.
