On hatching, the chicks scramble to the surface and head for bush on their own.

I would write: On hatching, the chicks scramble to the surface and head for the bush on their own. The prepositional phrase for the bush fills the adverbial adjunct slot in this sentence for me. Pullum is not saying that bush has become preposition in standard English, he was just enthusing over a preposition in the birthing process. For those who like their grammar and language fixed, like a butterfly pinned to a placard, this can be annoying.

Ceci n'est pas un seing.