From The Urban Dead Wiki
Feral zombies generally have little access to metagame information, so they have to rely on in-game clues to locate viable survivor targets, or turn to eating their fellow zombies for xp. The introduction of the Feeding Groan skill has been a great help to them and most organized zombie attacks now attract a significant number of ferals to their frenzied groans. Scent Death has been a limited help in attracting ferals to a feeding ground, mainly due to its passive nature (unlike feeding groan, the feral has to actively smell for death), and limited range.
There do exist, however, loose coalitions of ferals. The cohesion demonstrated is both mild and optional, as the groups' primary goal is more or less to herd loners in a given direction, often towards a major assault by more established clans. Such groups include the Feral Undead and the Eastonwood Ferals. Feral zombies that follow an organised zombie horde or event around are collectively called feral cloud.