vocal behaviour in grey heron
A case study in the Po Valley
Herons are of high naturalistic value to Europe’s riverine and agricultural ecosystems and are a major component of wetland biodiversity. For centuries they have become part of local culture and folklore, as evidenced by paintings, ancient stone carvings, and decorations on utensils and homes that often depict them while hunting or flying.
Like other waterbirds, vocal behaviour is still poorly investigated, and to date for some species there is a complete lack of relevant information about the function played by sound communication at different stages in the life cycle of individuals.
Despite the remarkable diffusion of the grey heron, many behaviours such as mate choice patterns and associated rituals, parent-offspring conflict and begging should be further investigated, especially from a functional and adaptive point of view. Sibling conflict, which often results in siblicide in this species, has only been partially documented in northern Europe, and the underlying causes of this phenomenon need to be further clarified. In addition, the intrinsic relationships between aggression and vocal communication, parental efficiency, nest size, direct or indirect feeding, prey type and size, and abiotic factors characterizing the developmental environment cannot be underestimated.
The aim of the project is to help fill these gaps by exploring aspects of vocal behaviour and species ecology involved in the dynamics of breeding, parental care, sibling rivalry, and nestling survival.
The first step to be achieved is a detailed knowledge of the vocal repertoire and associated behaviours, followed by data collection on diet, bioclimatic variables, demographic parameters (clutch/brood size and mortality at different stages of nestling development), parental abilities to provide for offspring needs, and mating events (frequency, duration, and extra-pair copulations). The information gathered will be subsequently correlated with an in-depth analysis of begging calls and sibling fights. This is a powerful non-invasive way to assess the carrying capacity of the ecosystem for the species because sound emissions and competition of nestlings approximate their nutritional and health status and provide information on suitable environmental conditions.
In addition, a bioacoustic approach could reveal fundamental to test the evolutionary models of honest signalling between adults and between nestlings (“good genes hypothesis”; songs of need, quality, hungry; scramble or negotiation begging).
Two heronries are surveyed in different environmental contexts to highlight possible differences between sites and/or local adaptations: heronries of LA TORBIERA Wildlife Park (Novara province, Piedmont) and Cascina Isola Nature Reserve (Pavia province, Lombardy).
Ardea cinerea Linnaeus, 1758
IUCN Red List status: Least Concern
The grey heron is widespread in most of the Old World, except for Arctic areas and deserts, and the world estimated population amounts between 500,000 and 2,500,000 adult individuals. In 2018, the Italian population ranged from 20,600 to 23,600 reproductive individuals and had a significant increase from 1970 until 2002; however, it appears to have stabilized around 2010. In Italy the species is sedentary or only partially migrant and nests mainly at altitudes under 500 meters along the entire Po Valley. Other nesting sites can be found in Tuscany, Latium, Abruzzo, Campania and Sicily.
Males are larger than females and can exceed 2 kg in weight and 1.8 meters in wingspan. They are efficient hunters, and preys are speared and grabbed with quick, well-aimed blows delivered by their long beak. Their diet may include fish, rodents, reptiles, amphibians, molluscs, arthropods, and chicks of other waterbirds; incidents of cannibalism are known. Adult individuals congregate between late January and April in heronries, high-stemmed lowland forests near wetlands or rice paddies in inland, forming pure or mixed breeding colonies that may consist of hundreds of birds. On the coast, the species also frequents deltas, estuaries, and tidal mudflats. They lay from 3 to 5 eggs in stick nests built in trees by both partners, at a height ranging from 5 to 30 meters, and laying is asynchronous. In most cases both parents will provide for the offspring, and the pair remains united and faithful throughout the breeding season until fledging, which occurs about 60 days after the eggs hatch.