A group of crows is called a 'murder'.
Why it is true
This creepy collective noun dates back to the 15th century, inspired by old folklore and the birds' association with death and battlefields.
Source: Linguistics
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.