Chicken

Chickens (sometimes called Ducks ) are passive mobs that supply feathers, raw chicken, and eggs. If killed by fire, they can drop cooked chicken instead of raw chicken.

Appearance
Chickens have white feathers and wings, yellow beaks, yellow feet and red wattles. Their texture is somewhat checkered, in a similar fashion to wool. They are small enough to fit entirely within a block: 0.875 blocks tall, 0.5 blocks wide and 0.8125 blocks long. Like most other mobs with baby forms, their baby form has a smaller body, but the head is the same size as adults, and so appears disproportionately large.

Behavior

 * Chickens appear to wander around aimlessly, without attempting to stay out of water or avoid falls.
 * They are immune to fall damage. When falling they will flap their wings quickly and fall slowly.
 * While in a loaded chunk, a chicken lays one egg approximately every 7.5 minutes (6000 to 12000 ticks).
 * They share some behaviors with other "farm animals":
 * They are drawn to light when in a dark environment.
 * The adults can swim, visibly flapping their wings as they stay on the surface, The babies, however, are prone to drowning; being so small, they can drown even in some partial blocks of water.
 * They can be led around by holding a seed, and are also bred with seeds.

Breeding
Chickens can be bred like other farm animals, by feeding any two adult chickens with wheat seeds, melon seeds, pumpkin seeds or nether wart. After breeding, chickens will not accept any more food for about 5 minutes.

Chicks can also be produced by breaking eggs. The eggs must be explicitly broken; eggs sitting around as items will not hatch, and will eventually despawn like other items.

When an egg is thrown (or shot from a dispenser), it will eventually hit a solid block or a mob, and break. It then has a 1/8 chance to spawn a baby chicken; if it does, there is a 1/32 chance of spawning 4 chicks instead of one. If the egg was thrown at a wall at certain angles, the chicken may spawn in the wall and either suffocate (opaque blocks) or pass through entirely (transparent blocks).

Hatching chickens by throwing eggs, and breeding them, can be done simultaneously to multiply them much more quickly than other mobs.

Chicks

 * Chicks move faster than adult chickens.
 * Chicks follow adult chickens until they grow into an adult. If produced by breeding, they will follow one of their parents; if that chicken dies or they were produced from an egg, they will choose a nearby chicken as their "parent".  Since they are faster, they are prone to pushing their parents around.
 * Chicks take exactly 20 minutes (24000 ticks) to fully mature.
 * The NBT Tag which causes this is called "Age", and the integer value is in ticks. From spawning the value is -24000. As soon as it reaches 0, it changes to an adult.
 * An example of a way to force this behavior is /summon Chicken ~ ~ ~ {Age:-100} which summons a chick that grows up in exactly 5 seconds / 100 ticks.
 * Chicks do not lay eggs, and like other baby animals, they do not drop anything (including experience) if killed.
 * Due to their small size, chicks can pass through openings even smaller than a full block.
 * Unlike adult chickens, chicks will drown if put in water.
 * A chick's height is somewhere in between 0.51 of a block to 0.80 of a block.

Farming
Farming chickens is very useful, as it provides a renewable source of chicken eggs, chicken meat, feathers, and optionally experience points. As with other animals, you will want to keep them in an enclosed or fenced area so they don't wander off. (Chicken farms can generate quite a bit of noise; you probably will want to build large farms at least sixteen blocks away from frequently used areas.)

However, there are various glitches which can allow animals to escape through fences, and chickens are particularly subject to these, so special measures can be required. The minimal Chicken farming technique would include a walled yard or large pit, at least 2 blocks deep with a ladder out. (A door or gate allows chickens to escape whenever you open it.)

Another glitch is if the chickens are overcrowded and put into a 1 block high space, they will suffocate.

Remember that ungathered eggs will vanish after 5 minutes—merging stacks does extend that, but even so, if you're trying to build a large population you'll want to gather them several times a day.

Automatic egg farms can be created by placing chickens in water arranged so as to support the chickens, while letting the eggs (and any drops from killed chickens) fall into a collection area. (However, as of 1.5.1 chicks can drown in water, which again requires more complex arrangements.)

The introduction of hoppers makes egg farming much more convenient, allowing eggs to be automatically collected into a chest, where they will not despawn. Further, a dispenser driven by a redstone clock can be used to turn the eggs back into chickens, which will eventually mature for a new crop. Note that careless use of automatic hatchers can produce enough chickens to cause serious lag on a single-player game or server!

The above tutorial link gives much more information on how to build egg farms, ranging from simple collectors to fully automatic systems.

Some useful numbers:
 * A single chest (or Minecart with chest) full of eggs will yield an average of approximately 59 chicks.
 * Assuming a farm with automatic hatching, then later automatic collection of eggs, a given large number of eggs (say 1 chest, or 2 chests, etc.) will take roughly 75 minutes to replace itself. This includes the 20 minutes for the chickens to grow to maturity, but not the time needed to actually hatch all those eggs.  (Even with a clock-driven dispenser, hatching a few hundred eggs takes significant time.)

Trivia

 * When a chicken is killed off a ledge, the "corpse" also falls more slowly than normal, and the wings still make a flapping animation.
 * If chicken eggs are thrown at a transparent block, the chick can spawn on the opposite side. This is more likely with thin transparent blocks such as glass panes or iron bars.
 * Chickens are the only animals that can be farmed from a single animal: The first chicken's eggs can be hatched to produce a second, and when that grows up, the pair can be bred as normal.
 * There is no texture for the underside of a chicken's head, allowing to see through it and see the inside.
 * Ocelots attack chickens.
 * If no adult chickens are present a chick can imprint on a player.
 * A baby chicken is small enough to fit through the gap of a stair block facing a full block.
 * Chickens are able to "hide" within hoppers due to their small size, protecting them from lava or water that is directly on top of the hopper.
 * Chickens become invisible at a certain distance, with the chicks disappearing at a shorter distance.
 * When first released, chickens took fall damage like other mobs, and would drop leather if they died from it (a bug).
 * A few weeks before the end of Beta, Notch tweeted that he changed the chickens to ducks, causing a turmoil on Twitter. A few days later, Jeb, still getting feedback on the idea, stated that it was just a joke.
 * In Pocket Edition there is a glitch that allows chickens to get their heads stuck in chests. This makes it easy to collect their eggs which are always in the same spot.
 * On the covers of Pocket Edition and the console editions, a chicken is shown carrying a Block of Diamond, even though chickens are not able to carry anything in any edition.
 * While you are loading/creating/exiting a world in the Xbox 360 Edition, the message that appears can say "Is it a Chicken, or a duck?". Also, the summary for the mobs avatars at the end says "... and a chicken... Or is it a duck?".
 * As of 1.7.4, baby Zombies and Baby Zombie Pigmen have a very low chance of spawning whilst riding chickens. They are just as fast as regular Baby Zombies but also take no fall damage.
 * As a result, chickens can now spawn in the Nether as well as the Overworld. This allows players to obtain raw chicken, eggs and feathers from the Nether without returning to the Overworld.
 * If a chicken is killed with a bow that has flame enchantment in it, it will automatically be a cooked chicken.
 * Despite chickens being birds, they give offspring immediately after being bred with seeds.