The sensor window is just under 0.8" so probably 20mm. I agree that the spikes are larger than necessary unless cockatoos wouldn’t care about smaller ones. It may be that the rubber like material holds a larger spike better. If that is the case, I would think that a smaller spike with a 2mm base would work but then cost would likely be higher. Another reason they may be as large as they are is to reduce the likelihood of vibrations causing false rain.