Observe their be'avior. Take for a start the sheeps' tendency to 'op about the field on their 'ind legs. Now witness their attempts to fly from tree to tree. One thing is for sure, the sheep is not a creature of the air. They have enormous difficulty in the comparatively simple act of perchin'.
Trouble is, sheep are very dim.
Once they get an idea in their 'eads, there's no shiftin' it. E's realized that a sheep's life consists of standin' around for a few months and then bein' eaten. And that's a depressing prospect for an ambitious sheep.