The tits make a variety of calls and songs. They are amongst the most vocal of all birds, calling continuously and loudly. They are only ever silent for reasons such as avoiding predators or when intruding on a rival's territory. Quiet calls are made while feeding to keep contact with others in their social group. Other calls are used for signalling alarm—the most famous of which is the "Chic-a-dee-dee" of North American species in the genus "Poecile". The call also serves a rallying call to summon others to mob and harass the predator. It has been demonstrated through experiments that the number of "dee" syllables at the end of the call increases with the level of danger the predator poses.