There are more chickens in the world than any other bird thanks to the chicken's role as a food source for humans. Humans are thought to have domesticated the chicken from the wild red junglefowl which is found in Asia. The exact date of domestication of the chicken is a subject of debate.  Some paleontologists originally fixed the date at 4000 years ago at a site in Pakistan. However, later discoveries of chicken bones at Neolithic sites at the mouth of the Yellow River in China push the date back to about 7,500 years ago. The wild red junglefowl was not native to that arid region of China and the latest research suggests that domestication may have taken place even earlier in what is now Vietnam and Thailand where wild junglefowl can be found.

There is tremendous variety in the shape, size, color and even feather structure of domestic chickens. There are over 60 recognized breeds of chicken in the world.  Breeds range from the familiar white leghorn chicken found in large scale agricultural production to the rare and unusual breeds like the Transylvanian naked neck. Some breeds are strictly ornamental such as the sultan breed with a decorative feather crest on its head or the phoenix breed where roosters have extravagantly long tails. There are also miniature varieties called bantams. Two popular breeds of bantam chicken are the Chinese silkie chicken which has feathers that are fluffy and look more like fur or the frizzle breed which has curly feathering.  Some of these more unusual breeds are rare and threatened with extinction just like endangered wild species. Some breeds of chicken are rare due to the spread of large scale animal agriculture where production is focused on one or two breeds. However, genetic diversity in domestic breeds is important for the long term health of the species as it is in wild animals. Without this diversity, a disease might affect some breeds while others might be resistant. If those genes are lost due to breed extinction, the human food supply could be adversely effected.
 

Chickens on average live 7-8 years but some birds have been documented to live as long as 14-15 years of age.
 

Chickens live in social groups which usually consist of a dominant rooster and hens with their offspring. Some juvenile roosters will be tolerated until they reach maturity and begin to compete with the resident rooster. Within their social group there is a distinct dominance hierarchy or pecking order.  In the peck order, the dominant birds get first access to food, nesting locations, and roosting spots. The subordinate birds wait until the dominant birds leave the area before they will approach.

The phrase "bird brain" is proving not to be accurate. Many researchers have been surprised at the intelligence demonstrated by the domestic chicken. They recognize other members of their flock from facial features, have over 20 distinct vocalizations and have demonstrated that they understand that when an object is hidden, it still exists. This is something that is beyond the capacity of young children! Here at the Zoo, we have trained a group of chickens to complete an agility course; something that is normally only done with dogs. Our chickens mastered the course faster than some dogs have been able to.

Chickens will breed almost year round but activity increases in months with milder weather. Male and female chickens can begin to reproduce at an early age (roosters as young as five months, hens slightly later). The males are polygamous, which means they will mate with as many females as they are able to. A dominant rooster will often have a group of hens, or harem, that he will guard and prevent other roosters from mating with. Roosters perform a mating "dance" where they drag their wing across the ground as they walk around the hen.

Hens will lay eggs regardless of whether there is a rooster around or not. A rooster is necessary to obtain fertilized eggs that will develop into chicks. The majority of eggs that are purchased at the market are unfertilized. Once the hen had laid several eggs, she will sit on them for incubation. This behavior, called broodiness, has been reduced with artificial selection in some breeds. Egg incubation takes 18-21 days. Chicks develop an "egg tooth" with which they use to help break the shell while going through the hatching process. This sharp projection falls off within a day or two of hatching. At hatching, chicks are covered with down feathers and by three days of age can walk, run, jump and hop. Since chicks can move about very soon after hatching and feed themselves, parental responsibilities are confined to brooding chicks, guiding them towards food sources and watching for predators. Still, hens are attentive mothers, who will care for their chicks by sitting on them or by using their wings to shield chicks from harsh weather or danger.