What would be the plural form of "I saw a pair of birds, one flying to the north, the other - to the west"? "
My guess is:
"I saw a flock of birds, some flying to the north, the others - to the west"
But I am not sure enough
While technically the various animal groupings (flock, school, herd, etc.) can mean any number, in practice they're used to mean either "more than I can easily count" or "a related group".
A "flock of birds" implies the birds are all going in the same direction or doing the same thing, so that they seem like one entity. It's unusual to then say, "some was going one way and the rest going a different direction," since it's confusing and sounds like you meant two flocks.
However, your meaning still comes across so I wouldn't say it's wrong. It depends on context -- maybe it's your intention to describe something that ought to be one group but split into two.