All the rabbits that escape the warren before the humans gas it are male, and Hazel quickly realizes that without any female rabbits -- does -- they will not be able to start a new warren. It is explicitly stated that the does handle much of the manual labor of digging and maintaining a warren, so the necessity of young rabbits to keep the warren going is almost an afterthought. Hazel gets information about two possible sources of does; a farm with tame rabbits, and a large warren. When they try to steal the tame rabbits, they are attacked by a cat, which Bigwig fights off, and then they are menaced by a few more cats. Then the humans return in their car:
At that moment the humming vibration grew louder, quenching the rushing of the night breeze in the elms. Then all four rabbits sat as though transfixed by the sudden, blinding light that poured over them like a cloudburst. Their very instinct was numbed in this terrible glare. The dog barked and then became silent once more. Dandelion tried to move, but could not. The awful brightness seemed to cut into his brain.
(Adams, Watership Down, Google Books)
The humans, thinking only that the tame rabbits have escaped, try to catch them, and the wild rabbits lose one of the does. Then Hazel, trying to distract the humans while the others escape, is shot; he is not killed, just wounded, and he hides in a drain. The other rabbits leave him, assuming he has been killed; only Fiver's premonitions lead them to rescue the wounded Hazel.