"lit" is apparently the past tense of "to alight", which means to either perch on something or to jump off the perch. "lit" is also the past tense of "to light", but that's not relative in this context. I never use "lit" in this way, but it seems to be right. The flock of birds jumped off the branches of trees in the orchard.

It's a funny use of the verb, because "light" in the sense of flying or travel usually means to settle. However, the text clearly says that the birds began to fly. There is a phrasal verb "to light out" (usually seen in the past form, "we lit out"), which means to leave quickly. That is probably what the writer wanted us to think. Also, the sense of "not heavy" is probably suggested here. It seems the reason we have seemingly opposite meanings for the one word is that the meanings originate from different words which used to have separate spellings.

"Lit" can mean quite a few things, but in short; in that sentence, it means "took flight/flew." "When we neared the orchard, a flock of birds lit from its outer rows" could also mean "when we neared the orchard, a flock of birds took flight from its outer rows." It is also slang for "to get dunk/high off of drugs". I hope this helps.