Why do you have to have a lot of geese in the control group and the experiment group instead of only one webbed goose and one-free goose?

Because when we conducting a research, we need to ensure that the sample at lest represent the whole population in general.

If we only use two type of them, the conclusion that derived from the research may not applicable to the real population. Because of this, we include large variety of the animal in the control group.