When you've restored the portal network by helping Styx, you can get to Alwyn's Forest. Once there follow the main road, killing all chicken groups you find. Once the chickens are dead, open your menu and click "Respawn" to reinitialise the map. This will cause the chicken groups to respawn so you can kill them again.
Chickens are the easiest enemy to kill, so any horde/weapon should be able to handle them - however anything which does AOE (such as a fire staff) will make things quicker.