Regarding chicken breeding: individual selection would do better with a larger breeding population. In a population of size N, destroying a rival's egg reduces the average per-chicken egg output by 1/N, but laying an egg increases your own output by 1. Sabotaging rivals becomes less important as population size increases.

Group selection probably still produces better outcomes, because then chickens will actually cooperate instead of being (at best) indifferent to each other.

You can notice this same effect in all sorts of zero-sum games. If there are only two players, then sabotaging your opponent is exactly as important as helping yourself, but if there are many players, then sabotaging your opponents becomes less attractive.