Sentence:I remember one day many years ago sitting on the top of Braich-y-Pwll on the Llyn Peninsula watching a [e1]chattering[/e1] of [e2]choughs[/e2] performing an amazing set of aerodynamics in the updrafts of a winter storm.
 e1: chattering. 
 e2 : choughs.
Answer: Member-Collection(e2,e1)