I don't think a reason is needed for not causing unnecessary pain to a creature.
However, here is some info about how great chickens are.
I learned a few things :-)
http://www.goveg.com/f-hiddenliveschickens.asp
The Hidden Lives of Chickens
Brainy Birds
Several research teams have recently published findings on chicken
intelligence that have challenged old notions about avian cognitive
abilities. For instance, scientists have found that chickens clearly
understand cause-and-effect relationships, an advanced comprehension skill
that puts their intellect beyond that of dogs. In the book The Development
of Brain and Behaviour in the Chicken, Dr. Lesley Rogers, a professor of
neuroscience and animal behavior, concludes, "[I]t is now clear that birds
have cognitive capacities equivalent to those of mammals, even primates."7
In one experiment that explored chickens' understanding of causal
relationships, researchers found that when injured chickens were offered the
choice between regular food and food that contained a painkiller, the birds
soon understood that the medicated food made them feel better, and they
learned to seek it out it over the other choices. "The chickens will take
the analgesic every time," says Dr. Joy Mench, a professor in the Department
of Animal Sciences at the University of California at Davis. They understood
cause and effect and learned how to make the best decision.8
Chickens can also grasp other complex mental concepts. For instance,
according to Evans, chickens are able to understand that objects still exist
even after they are hidden or removed from view. This level of cognition is
actually beyond the capacity of small human children.9 Researchers also
recently reported that chickens "can anticipate the future and demonstrate
self-control, something previously attributed only to humans and other
primates."10 Scientists made this discovery after they observed that when
given the option between pecking a button and receiving a small food reward
instantly or holding out for 22 seconds in order to receive a larger food
reward, chickens in the study demonstrated self-control by holding out for
the larger reward over 90 percent of the time.11