[welly]

I'm not sure I'd refer to crows, a murder of or a flock of, in a beginner's textbook on the English language. I don't think crows are commonplace enough in every day conversation to justify their inclusion.

If I was needing to refer to a group of crows in this textbook then I'd refer to them by their proper collective noun, a murder of, because that would be correct.

[Symbiote]

Flock, pack and herd are found in "beginner" English books all the time. Ask the nearest three year old to fetch the animal/farm/zoo book.

Those three words also stand on their own. "That herd looks unsettled" "Can you hear the pack howling?" etc.

I've yet to hear "murder" without "of crows" following it, so I reject it as the normal word.

(Native English speaker.)