It is not necessary for animals to be able to recognise individuals within the group for a pecking order to be maintained. Animal behaviours may be motivated by "rule of thumb". For example, if chickens can predict the fighting ability of others simply by assessing their body size, they will be able to maintain the hierarchy whilst avoiding a fight which could cause injury and will be energetically costly. Using this rule of thumb, if Chicken A sees that Chicken B is larger, Chicken A will defer; if Chicken B is smaller, it will defer to Chicken A. In this way, only chickens of similar sizes will fight and the pecking order of the group overall is maintained without requiring individual recognition.