A flock or group of Frigatebirds is termed as fleet, flotilla.
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, fleet, flotilla is /are used to describe a group of Frigatebirds.
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