A while ago, I wrote about the importance of animal intelligence. While we shouldn't base our decision to kill a sentient being on how smart s/he is, it is important to know that animals that many of us had previously thought were just plain "stupid", are, in fact, more intelligent than we had imagined. Studies such as the one below, concerning the intelligence of chickens, proves this.
http://freefromharm.org/chicken-behavior-an-overview-of-recent-science/