Besides Colours and Songs, Odour is the New Black of Avian Communication

Odours are broadly used for individual, sexual and species recognition in vertebrates and may be reliable signals of quality and compatibility. Yet, chemical signals in birds have rarely been investigated. In fact, birds exhibit a wide array of communication mechanisms (e.g. colours and calls) but rarely display obvious olfactory-driven behaviours. This is probably why, despite three decades of physiological and behavioural studies establishing the existence of avian olfactory functions, chemical communication has been essentially ignored. In spite of the fact that pheromones have never been highlighted in birds, several species produce characteristic scents that may have a social function. For example, odours seem to contribute to the courtship behaviours of ducks and chickens. In crested auklets, a characteristic citrus odour may act as a sexual olfactory ornament broadcasting resistance to ectoparasites. Eventually, it was shown that zebra finches (a passerine bird with a very small olfactory apparatus) display olfactory-driven behaviours. Petrel seabirds are probably the most striking case-study, since findings relating to many aspects of petrels' ecology including homing, recognition, mate choice and even interspecific competition for nesting sites, provide a comprehensive evidence for avian chemical communication. Some burrowing petrels use the odour of their mate to recognise their own nest and, interestingly, prefer the odour of a conspecific to their own. Using chemical analytical methods, it has been demonstrated that the preen secretions of these birds contain social information including species, sex and identity (i.e. a chemical signature). Results further show that some of this information is still present on the plumage and in the airborne volatiles emitted by birds. Recent results also show that detective mice (i.e. biological olfactometers) can identify similarities in individual kin-related petrel odours. Also, one species of storm petrel has been shown to recognise and avoid kin-related conspecifics in choice experiments. Together, these results, almost 50 years after the first works on avian olfaction, indicate that chemical signals can contribute, as well as colours, calls and songs, to avian social behaviours; a realisation that has important implications for behavioural processes such as individual recognition and/or mate choice.