Chickens have been a part of the Muppet world since 1972, when the first group of Muppet chickens was created for the Tales From Muppetland special The Muppet Musicians of Bremen. They were precursors to the chickens from The Muppet Show, except they had slightly different eyes. T.R., one of the main characters, would later appear on The Muppet Show as well.
The Bremen chickens were used during the first and second seasons of The Muppet Show. In one episode, guest starring Rich Little, a bunch of real chickens were featured, auditioning for Gonzo's dancing chicken act. It was in this episode that Gonzo developed his obsession with chickens.
During the third season, some new chickens were built with more defined eyes. One of the chickens got the name Camilla and became Gonzo's girlfriend. (Although even Gonzo doesn't know which chicken is Camilla sometimes; they all look alike.)
A black rooster was added to the Muppet Show cast during the second season, and a little chick during the third season. In one of the first episodes, guest starring Harvey Korman, a chicken suit was made for the guest star; it would later be used occasionally whenever a huge full-body chicken character was needed.
The white Muppet chickens made a few rare appearances on Sesame Street; sometime during the 1980's, a group of brown chickens were added to the Sesame cast, as well as a brown/black rooster. The main difference, aside from their color, from their Muppet Show counterparts is their rounded eyes and sharper beaks.
Other notable Muppet chickens are the Giant Man-Eating Chicken from John Denver and the Muppets: A Rocky Mountain Holiday, and the Humongous Chicken from The Adventures of Elmo in Grouchland. The latter was a white chicken with a much smaller head and big, round eyes, but no eyelids. A slightly altered version (donning feminine eyelids) made several subsequent appearances on Sesame Street; one incarnation being Super Chicken. The puppet was later recycled as many more characters on Sesamstrasse.
The Muppet chickens usually communicate with squawks, although they are known to speak at times.
The Muppet chicken, owned by Telly, got a small part in a 4D movie. After this the chicken moved to The Netherlands and got his own part in Sesamstraat. The chicken listens to the name Stuntkip and is a daredevil. Another international chicken is actually a rooster named Kareem from the Israeli/Palestinian co-production.
The chickens are built without an eye mechanism. Instead, the Muppet builders found an alternative by giving some chickens one wide open eye and one 'relaxed' eye with eyelid; this way, the chicken could change emotions by quickly turning its head.
Although Camilla is the only chicken with a regular name and personality, there have been instances where other generic chickens have been referred to by name. In the audio commentary for "Bohemian Rhapsody", Gonzo mentions that the other two chickens in the video besides Camilla are named Ethel and Stephanie. In The Muppet Christmas Carol Gonzo introduces a live chicken to Rizzo named Louise. Gonzo also did a dancing chicken act with a live chicken named Lolita.
In a 2010 episode of Sesame Street, members of the Mostly Poultry t-ball team seek respite from a storm inside Hooper's Store; the names of the four brown chickens are revealed to be Henrietta, Patty, Teri(aki), and Attila the Hen (Tilly). Henrietta had also appeared earlier in episode 4127.
Gundel Gackerburg is a pop star chicken who appeared in Folge 2486 of Sesamstraße where Wolle was excited to meet her and hear her sing, but as soon as he hears her horrible singing voice he doesn't like her anymore.
On Are You Smarter than an Egg Layer? in Sesame Street episode 4179, the five chicken panelists are named Megan, Jennifer, Kyle, Brittany, and Tandoori.
Two chickens have simply been named "Mrs. Chicken". One of them appeared in the Mopatop's Shop episode "Flowers" to swap the flower she got for the one she wants. The other one appeared in Sesame Street episode 4153 where Zoe and Rosita think they have turned Maria into a chicken.