Feathers and eggs are awesome for Thaumcraft, in which case you'll want a steady source of both cows and chickens. I do support violence against people who build poorly-designed T5 spawners and then leave them running 24/7 though. That reminds me, I need to build some sort of system that allows me to naga the site from orbit.
Chickens do indeed need grass at lower tiers. If you need just a few chickens then place just a few grass blocks.