Herd of cows but not of sheep:. a coven of witches: a school of fish;
you would not call these contrived would you?

Collective nouns for various birds contrived? I do not think so. More
likely to be various names used in different parts of the country and
brought together by some lexicographer.

I have a list of birds' collective nouns, if anybody would like a copy
contact me direct.

Geoff