“Nest” is the most common collective noun for vipers, a large family of venomous snakes that includes adders, rattlesnakes, and pit vipers. Most vipers don’t build nests in the bird sense; the term instead points to shared shelters, communal winter dens, birthing sites, or simply a cluster of individuals sheltering in the same crevice or burrow.
Many viper species are ovoviviparous, meaning they give birth to live young rather than laying eggs, and pregnant females will sometimes gather in the same sheltered, warm location as their due date approaches, a rough equivalent of a nesting site even without any actual nest-building. Outside of birthing season, “nest” more often refers to communal hibernacula, the same kind of shared winter dens seen in adders and rattlesnakes, where vipers rely on shelter and body heat pooled together to survive cold months.
Given how broad the viper family is, exactly what a “nest” looks like varies enormously between an alpine adder den and a tropical pit viper’s shelter.
Example: “The old stone terrace turned out to be a nest of vipers, undisturbed for years.”
Related: Collective Nouns for Reptiles · What Is a Group of Vipers Called?