collective

Can I say

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

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

Can I say,

All colletive nouns take singular / plural  verb ?

Vincent Teo wrote:

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)

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