I wondered that "a flock of birds" is always followed by a singular verb and "flocks of birds" is always followed by a plural verb. Please help me make this confusion crystal clear. Thanks so much!
It depends largely on context being American or British English. From this link, third bullet under the "But!" section:
American English versus British English differ. In British English, most collective nouns can be treated as singular or plural depending on context (e.g., one could say “the whole family was at the table” or “the family were opposed to the idea.”) American English tends to construe collective nouns as singular.
Several times I've heard British speakers say things like, "The crowd are loving it!" or "The family go to the beach every summer," although in America we would normally say "The crowd is loving it!" or "The family goes to the beach every summer."
The same seems to apply to any collective noun, including "flock."