The phrase 'pecking order' originated in the traditional behaviour of which birds?

Pecking order or peck order is the colloquial term for the hierarchical system of social organisation. It was first described by Thorleif Schjelderup-Ebbe in 1921 under the German terms Hackordnung or Hackliste and introduced into English in 1927.

The original use of pecking order referred to the expression of dominance in chickens. Dominance in chickens is asserted by various behaviours, including pecking, which was used by Schjelderup-Ebbe as a measure of dominance and leadership order. In his 1924 German-language article, he noted that "defense and aggression in the hen is accomplished with the beak". This emphasis on pecking led many subsequent studies on fowl behaviour to use it as a primary observation, however, it has been noted that roosters tend to leap and use their claws in conflicts.

The term dominance hierarchy is often used for this type of social organisation in other animals.

Pecking order is a basic concept in social stratification and social hierarchy that has its counterparts in other animal species, including humans, although the term "pecking order" is often used synonymously.