In English idiom, when flocks is plural, for some words the object that they are flocks of remains singular. "Flocks of duck", for example, is attested in this Smithsonian magazine article:
https://www.smithsonianmag.com/trave...ast-180962887/
However, I am not certain of the precise usage rule here to tell which of those words are eligible for this idiom beyond duck itself.
Bookmarks