Brood Mothers are the mobile respawns of the zombie horde. Depending on mission conditions, objectives, and general rules, Brood Mothers are capable of respawing zombies in extremely short time intervals. They have little to no defensive capability themselves, and thus rely on the rest of the zombie horde to defend them.
Statics are essentially Brood Mothers, but are not mobile (thus, "static").
Depending on the mission, zombies can have anywhere from 2 to 5 Brood Mothers roaming the battlefield.
There have also been rumors of a special Brood Mother that is activated whenever Bro-tential's casualties reach critical numbers. This Brood Mother is known as the Bro-Mom.
While Brood Mothers are generally female, there are also male Brood Mothers. These are colloquially known as Brood Fathers, or Brood-vestites.
Popular Respawn Types
These are some of the possible respawn types. They are sometimes able to be combined, so listen to the nightly Brood Mother rules.
Rolling Respawn: The most common respawn type. Brood Mothers have a timer that counts down continuously; at the end of that timer, every zombie that is stunned in the Brood Mother's radius is respawned. After that wave is respawned, the Brood Mother starts the timer again.
Touch-To-Start: The Brood Mother requires a stunned zombie in their radius in order to start the timer. As soon as every zombie in their radius is respawned at the end of that timer, the timer doesn't start again until another stunned zombie comes up.
Critical Mass: The Brood Mother does not respawn until a certain percentage of stunned zombies are around them; can be an immediate respawn, or on a timer. Critical Mass can also influence the other Brood Mother types-- sometimes it is possible to shorten timers based on Critical Mass.