On each level of the
Dungeons of Doom past level 16, there is a 1/8 chance of a
cockatrice nest existing on the level (provided
cockatrices have not been
genocided or made
extinct). The
nest is simply a large
room filled with
sleeping
cockatrices and
statues of their
unfortunate victims. Cockatrice nests do not generate any
ambient messages, so be careful and use your
telepathy early and often.
In my experience it is unwise to invade a cockatrice nest unless you can do so very quietly. If you accidentally wake the cockatrices, you'll have some problems, because although the chances of being petrified while fighting a cockatrice are fairly small, the overall chance of being petrified gets much higher if you're fighting twenty of them. Don't invade a cockatrice nest when there's a new moon. Just don't.