A flock or group of Terns is termed as cotillion.
Different collective nouns can apply to all bird species, such as flock, colony, fleet, parcel, and dissimulation. However, when a flock or group of birds consists of just one type of bird or closely related species of birds, certain specialized terms are often used to describe the group of birds.
So, cotillion is /are used to describe a group of Terns.
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