Thank you ! As a non-native English speaker, I absolutely did not get it. I thought it was linked somehow to "blackmailer" as in French, we also use the term "corbeau" (crow) to describe it. Instead of being a Crime on the Orient-Express, it would have been a "Blackmailer on the Orient-Express"...less gruesome.

It seems to be an Emglish linguistics in-joke or something, to come up with the most ridiculous words for groups of animals...