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Can I say

(a) A herd of cows is / are grazing.

(b) A flock of birds is / are flying over the houses.
Comments  
'Is' for both the cases because the reference is to "a herd" and "a flock" that are singular.

As to the first sentence, I think cows are being grazed because somebody is grazing them.
Can I say,

All colletive nouns take singular / plural verb ?
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All collective nouns take a singular verb.
Vincent Teo
Can I say,

All colletive nouns take singular / plural verb ?

Either singular or plural verb can be used. If you view them as a group, use the singular verb. On the other hand, if you view them as cows/birds, use the plural verb.

a) A herd of cows is grazing. (group)

a) A herd of cows are grazing. (cows)

(b) A flock of birds is flying over the houses. (group)

(b) A flock of birds are flying over the houses. (birds)
Well, I'm confused to some extent now, I must confess.

I used to think collective nouns took a singular verb. I've just made a search and read that collective nouns may take either a plural or a singular verb. In a while I found a collective nouns of Mister Micawber where he said that both are possible in modern English, while IS is the rule. Complicated!
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Collective nouns can be seen as notionally singular or plural, depending on whether one focusses on the group as a unit, or on the individual members. Americans are more likely to see "collective" nouns as single groups, and therefore use singular verbs, and British people are more likely to use plural verbs referring to the individuals within the group. However, in some cases, Americans and British people would agree that there is no question whether the individuals or the unit is being referred to.

This they call tension between principles. The principles are proximity and non intervention.

http://www.iei.uiuc.edu/structure/structure1/svagree.html

This site will help you know the principles and the tension
A herd of cows were grazing by the river.