Avian hissing sounds: occurrence, mechanism, ontogeny, function and phylogeny.

Sound production is widespread across the animal kingdom and can take many forms and serve various functions. A hissing sound is a non-vocal acoustic signal produced by forced air ventilation and is hypothesized to be a behavioural symplesiomorphy in Amniota. Yet, hissing sounds are typically associated with reptiles and have received comparably little research attention in birds. Here, we identified at least 86 families within 34 avian orders in which members produced hissing sounds. Despite being widespread, almost nothing is currently known about the mechanism(s) of avian hissing sound production. Functions of hissing signals were divided into broad categories based on the social/behavioural context in which they are typically expressed and most evidence pointed towards a main role in threat/defence displays towards predators. Yet, interesting examples exist where avian hissing signals are involved in agonistic interactions, parent-offspring communication, sibling competition, mating displays, pair communication and heterospecific eavesdropping. Our review also emphasizes that research on hissing sounds regarding individual variation, acoustic individuality, geographic variation and fitness consequences is still in its infancy. Overall, hissing sounds are widespread and can be involved in communication in many crucial aspects of avian life, thereby spanning various contemporary disciplines in behavioural and evolutionary ecology.