From my hotel, I could see that a flock of birds _________ flying over the lake.
From my hotel, I could see that a flock of birds _________ flying over the lake.
Explanation
Flock is the main subject here.
Birds is a qualifier.
Note: Ensure the verb is according to main subject (and not qualifier).
Collective nouns such as a herd of, a collection of, a group of, a flock of, etc. are singular, so ‘is’ will be the verb in use.