In US English, because you are referring to the flock, which is singular, it would be : The flock of birds IS... In British English you would normally say: The flock of bird
…s ARE ... Group words such as flock tend to be used with both singular and plural verbs. Or, put slightly differently: 'A flock of birds is...' is correct in all styles of English. Usage, however, differs between styles of English. For example, in UK and Australian English it is equally correct - and considered more common - to say, 'A flock of birds are...' In any form of English usage should be consistent; so you'd avoid saying, 'A flock of birds are flying past; look, now the flock is turning,'. Stick with the singular or the plural within sentences. The consistency rule applies within all kinds of grammatical applications. Usage in any language depends on the term itself, or on the context, as well as on the personal preference of the person using the term. Consistency avoids confusion. Where group or collective words in particular are concerned, some are never referred to in the plural in any style of English; for example, 'The art collection is...' wouldn't become 'The art collection are...' no matter whether the collection happened to be spoken of in the US, the UK, or anywhere else. Art collections, on the other hand, would always be plural, everywhere. If what you're saying or writing feels or looks right, it very likely is, but it's always wise to check to see what's considered preferred usage among your target audience. A dictionary will frequently specify (US) or (UK), and so on, with the preferred usage in its country of publication listed first. (MORE) 
