Answer :

iGreen
Abstract nouns are nouns you cannot see, hear, smell, taste, or touch.

So yes, I believe 'flock' is an abstract noun.
'flock' is a singular, common, concrete noun; a word for a group of sheep, birds, or followers.

The noun 'flock' is also used as a collective noun, for example, a flock of pigeons.                                      
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