Repertoire and vocal behaviour of the grey heron (Ardea cinerea) during the breeding season: a case study in the western Po Valley

Abstract

The grey heron is an extensively studied and widely distributed colonial bird species in the Old World; however, relevant information on the role played by vocalisations during the life cycle of individuals is still lacking. This research aimed to deepen the available knowledge on the repertoire and vocal behaviour of adults during the breeding season, through a bioacoustic approach and choosing as study areas two heronries in different environmental contexts. Colonies were monitored 2 days a week each, for 9 weeks, from 8:00 am to 6:00 pm (360 hr of sampling). Results showed that adults use a wide range of call types, mainly related to courtship and the reproductive phase. Vocalisations are emitted especially by males and are a key element during mate attraction, nest building, in breeding events and in strengthening bonds within pairs. Most vocalisations are expressed in the first weeks of the breeding cycle, concurrently with the maximum number of courtship males, which exhibit most of the detected sound categories and emit calls with lower frequencies than females. Moreover, these vocalisations are expressed to a greater extent in the heronry characterized by an important forest component, where an almost entirely exclusive sound category was detected. In conclusion, the results suggest a possible correlation between vocal behaviour and female choice. The wide variety of sound signals and the high rate of calls produced by males during attraction and courtship displays, together with the acoustic characteristics of the signals, could provide honest estimates of the quality of emitters.