There must be several threads on the various collective nouns (is that the correct name for them?) in the English language. A pride of lions, a gaggle of geese, a school of dolphins, a flock of sheep etc. "crowd" sounds out of place with sheep.

down of sheep
drove of sheep
flock of sheep
fold of sheep
hurtle of sheep
mob of sheep
trip of sheep

Just dont ask me where they all originate from or who invents these terms. :)

If the expression "crowd of sheep" were used by a literate person, it would not be the same as a flock. A "crowd" here suggests something disorganized and disorderly.

Then how would you 'classify' a mob of sheep *ggg*

http://en.wikipedia.org/wiki/Collective_noun

FWIW, "crowd" isn't mentioned here at all:
http://en.wikipedia.org/wiki/List_of_collecti...
http://en.wikipedia.org/wiki/List_of_collecti...

http://en.wikipedia.org/wiki/List_of_animal_names