Referent in the sentence is the clause "Five fledgling sea eagles left their nests in wester scotland this summer". The occurrence of this phenomenon did something and it is the consequence of the phenomenon that is discussed.

yes i understand that part. I don't understand what tense to apply to it. i.e bringing, brought, etc. how do i know which one to use?

bringing is progressive, and implies an activity that is continuing.

Get a good book on grammer, and learn the difference between simple, perfect and progressive tense forms.

I can see that choice C, D and E are all wrong because of IT. I specifically noted that the IT can refer to the summer or scotland but it needs to refer to the seagulls migration. Am I on the right track?

I checked OG explanation and saw that option E is grammatical but not a logical one. Is option E grammattical?

Excerpt from OG
'Five eagles' can be a grammatical subject of the verb 'brought', but not a logical one.

IMO, comma followed by a connecting word joins two independent clauses. Here if we replace option E, the second independent clause lacks a subject or a referrant.

Initially, I thought it may be a list of items and hence grammatical...Five Fledgling sea eagles left...brought...But comma before 'and' creates two independent clauses.

May be I am missing something. Pls explain.

Five Fledgling sea eagles left their nests in western Scotland this summer, and brought.....