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
Chickens are social animals who form complex social hierarchies and interact
in complex ways that are indicative of what anthropologists call "culture."
For example, researchers have shown that chickens learn from observing the
success and failure of others in their community. One experiment that
demonstrated this finding involved teaching one group of chickens to peck
red and green buttons a certain number of times to obtain a food reward.
Researchers were surprised to find that when a new group of chickens watched
those who had learned how to push the buttons for food, the new chickens
quickly caught on by watching the others. At a scientific conference, Dr.
Christine Nicol, who worked on the on the study, told her colleagues, "They
may be ‘bird brains, but we need to redefine what we mean by ‘bird brains.
Chickens have shown us they can do things people didnt think they could do.
There are hidden depths to chickens, definitely."12
Researchers have also found that chickens have a cultural knowledge that
they pass down from generation to generation. John Webster, a professor at
Bristol University in the U.K., set up a study in which he gave chickens a
mixture of yellow and blue kernels of corn. The blue kernels were tainted
with chemicals that made the birds feel sick, and they quickly learned to
avoid the blue corn entirely (this is also another example of their
understanding of cause and effect).
When the chickens in Websters study had their young, he spread yellow and
blue corn around the farm, and even though he made it so that both types
were harmless, the mother hens remembered that the blue corn had previously
made them sick, and they steered their young away from it. In an article in
the London Times, Webster explains, "What this tells us is that the mother
hen has learnt what food is good and what is bad for her, that she cares so
much for her chicks she will not let them eat the bad food, and she is
passing on to her young what she has learnt. To me, that is pretty close to
culture - and an advanced one at that. Chickens are sentient creatures and
have feelings of their own."13
Scientists have been so impressed with the cognitive capabilities of birds
that a group of international experts recently called for a new naming
system to reflect the advanced nature of birds brains. According to an
article that appeared in The Washington Post, "The new system, which draws
upon many of the words used to describe the human brain and has broad
support among scientists, acknowledges the now overwhelming evidence that
avian and mammalian brains are remarkably similar - a fact that explains why
many kinds of bird are not just twitchily resourceful but able to design and
manufacture tools, solve mathematical problems and, in many cases, use
language in ways that even chimpanzees and other primates cannot."14
7 Lesley Rogers, The Development of Brain and Behaviour in the Chicken, CABI
Publishing: Oxfordshire, U.K., 1995: 217.
8 Specter.
9 Grimes.
10 Jennifer Viegas, "Study: Chickens Think About Future," Discovery News 14
Jul. 2005.
11 Viegas.
12 Ananova.
13 Valerie Elliott, "Why Did the Chicken Cross the Road?" London Times
Online 18 Mar. 2005.
14 Weiss.
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The Hidden Lives of Chickens
Social Smarts
People who have spent time with chickens know that they have complex social
structures, adept communication skills, and distinct personalities, just as
we do. Colorado State University Distinguished Professor Dr. Bernard Rollins
notes,
"[C]ontrary to what one may hear from the industry, chickens are complex
behaviorally, do quite well in learning, show a rich social organization,
and have a diverse repertoire of calls. Anyone who has kept barnyard
chickens recognizes their significant differences in personality." 15
Like people, chickens each have a place or rank within their
group - some birds are dominant, and others are expected to be more
submissive because they are on a lower social rung. Chickens know their
places within the hierarchy, and they act accordingly - for instance, when
learning how to perform a new task, they often follow the lead of the
dominant members in their group.16 Mench explains, "Chickens show
sophisticated social behavior. - Thats what a pecking order is all about."17
Chickens also remember the faces of those in their social group; Mench
continues, "They can recognize more than a hundred other chickens and
remember them."18 Scientists agree that chickens complex social structures
and good memories are undeniable signs of advanced intelligence comparable
to that of mammals.
Talkin Chicken
Chickens communicate with each other through their "clucks" - Mench
explains, "They have more than thirty types of vocalizations."19 They have
different calls to distinguish between threats that are approaching by land
and those that are approaching over water, and a mother hen begins to teach
these calls to her chicks before they even hatch - she clucks softly to them
while sitting on the eggs, and they chirp back to her and to each other from
inside their shells.20,21
15 Bernard Rollin, Farm Animal Welfare: Social, Bioethical, and Research
Issues, Iowa State University Press: Ames, Iowa, 1995: 118.
16 Ananova.
17 Specter.
18 Specter.
19 Specter.
20 Grimes.
21 The Humane Society of the United States, "Chickens," 2005.
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The Hidden Lives of Chickens
Small Birds, Big Personalities
Like all animals, chickens love their families and value their own lives.
The social nature of chickens means that they are always looking out for
their families and for other chickens in their group. In the wild, chickens
spend most of their time in groups - they enjoy foraging for food, taking
dustbaths, and roosting in trees together at night. After he toured United
Poultry Concerns in 1998, Ira Glass, the host of National Public Radios
This American Life, was so impressed with the personalities of the chickens
he met that he hasnt eaten chicken or any other animal flesh since.
Mother hens care deeply for their babies - Jesus even refers to the
loving protectiveness of a hen toward her chicks in the Gospels, which were
written almost 2,000 years ago.22 Indeed, a mother hen will turn her eggs as
many as five times an hour and cluck soothingly to her unborn chicks.23 Hens
prefer to have private nests for their eggs in protected areas far away from
predators. According to The Humane Society of the United States, "The desire
[for a private nest] is so strong, in fact, that a hen will often go without
food and water, if necessary, to use a nest."24 This demonstrates the fact
that hens will sacrifice their own comfort if it means protecting their
chicks.
Besides bonding to their young, chickens also form strong friendships and
enjoy spending time with their companions, just like we do. Kim Sturla, the
manager of Animal Place, a sanctuary for farmed animals near Sacramento,
recounts a touching story of two chickens. "We rescued an elderly hen, Mary,
from a city dump and later an elderly rooster, Notorious Boy. They bonded,
and they would roost on the picnic table. One stormy night with the rain
really pelting down, I went to put them in the barn and I saw the rooster
had his wing extended over the hen, protecting her."25
22 The Bible, Matthew 23:37-38 (New King James version), BibleGateway.com
2005.
23 The Humane Society of the United States.
24 The Humane Society of the United States.
25 Alex Cukan, "Chickens More Than Just Dumb Clucks," United Press
International, 20 Sep. 2002.