you take them
The real questions is, How do zombies communicate and tell the difference between each other?
You never see a zombie attack or fight another zombie, it's like the other zombies don't exist. All though in DotD the zombies fought over a victim in a cage? Does this suggest that zombies can starve, and when hungry enough will fight another zombie for food.
And why does not acting like a zombie work(except in Shaun of the dead)?
The Land of the Dead zombies were approaching intelligence of a rudimentary form. Apparently, once a Romero Z has been shuffling around for 10 or 15 years they begin to orient themselves. This speaks ill for the remaining population, as the basic drives remain intact. Zombies that have problem solving skills, form social groups like pack hunters, and communicate complex ideas like "hold this, pull here, point at that" could well end the remains of the human race. The fortified city in that film had lost contact with all other outposts in the last year. This may well indicate that all others had been overrun. Their inexplicable retreat after converting more than half the population into the newly dead may indicate that the desire to feed has mutated into a desire to repopulate their numbers. In this way, fortified cities may survive for a time as farms for the undead untill they are "ripe" enough to harvest.
Shaun of the dead Z's seem to be yet another mutation in the viral genome. They barely use their senses, being primary vision hunters, and poor at that. Being able to "blend in" with a zombie pack for short periods would be impossible if they used scent, hearing, or tactile feedback. I suspect it's a British thing. Even when cornered on the street, the Z's were polite enough to wait for a cell phone conversation to end before begining the attack.
Romero and 2004 Z's, even chemical versions all seem to readily identify members of their own group, even at a distance. It's thought to be a combination of body language, scent, and auditory feedback. Without the sound of blood rushing through your veins, your hearing may become quite acute. And with no thought in your head to distract you from your senses, those senses drive a Z with unerring instinct towards live meat. The only time "communication" takes place, it's on a level of "I heard the noise I make when I see food. Shuffle toward it untill I see food". Single celled animals exhibit as much intelligence as is required for this kind of communication. When a large pack hears a moan, and all orient on a target, it might seem like they have communicated, but it's really a large number of individuals reacting in exactly the same way to the same stimulus.
