They have the ability to live off the land (hosts, like human bodies and PCs) once they are unleashed.
NOTE: In many cases, they can also make perfect copies of themselves (copies in the trillions).
But what about drones?
Aren't they limited by quantity of energy in their batteries?
Yes, drones do have the capacity to self provision too. One of the more elegent ways is for a drone to use power lines to "induct" the energy it needs.
A drone that can recharge itself from a power line has the potential to operate for years -- monitoring, relaying, etc. -- without returning to base.
If the decision making software is good enough it could source its energy and target data for years without referencing any command system.
In fact, with wireless access to the Internet (including RSS feeds), GPS, and other easily accessible data sources... it decision making can be very dynamic.
Here's a video showing some US DoD contractors working on making that a reality, right now: