What is the science of a bird feeder?

I already have gravity and newton's 2nd law are there any more you can think of? i need a total of 5 diferent types so please help me!What is the science of a bird feeder?
In general, there just isn't very much going on in a bird feeder. That makes your question difficult.


1. Friction -There is friction between seeds and between the seeds and the feeder.


2. Biology? The birds need to recognize that the feeder contains food.


3. Mechanics - the perch needs to be constructed so that it can hold the weight of the bird.

Fish