Subject: A large flock of seagulls
Verb: was feeding
Prepositional Phrase: on the other side of the estuary.
In that sentence, feed is an intransitive verb, so it doesn't take an object. What looks like its object, the prepositional phrase on the other side of the estuary, isn't. That phrase tells us where the flock was feeding, which makes it adverbial in function. Verbs don't take adverbs or adverbial phrases as their objects, so it can't be part of the verb feed. To test this, move it around, away from the verb:On the other side of the estuary, a large flock of seagulls was feeding.