Analysis of 20,000 recordings of crossbills has rewritten the understanding of variation in the species' calls, revealing changes in vocalisations over time.
Crossbill calls have long been a subject of fascination for ornithologists and birders alike, with the existence of specific populations, or 'call types' discovered in the 1980s. Different call types are identified by a unique combination of flight and excitement calls, learned by a fledgling from its parents and generally staying with the bird for life.
Crossbill call types are not as fixed as once thought, according to researchers in Germany (Tim Melling).
A number of ornithologists have categorised these call types since the 1990s, but discrepencies between the vocalisations for call types described by different researchers prompted a team from Germany to seek an explanation for the variation.
The team, led by Ralph Martin from the University of Freiburg, gathered in excess of 20,000 recordings of crossbill flight and excitement calls made between 1962 and 2019. Adopting the latest classification of crossbill call types, they looked at eight Crossbill call types, as well as incorporating Parrot Crossbill, Scottish Crossbill and Two-barred Crossbill into the analysis.
Any deviations from the defined call type were recorded, then all recordings were scoured for similiar calls to capture the most exhaustic range of call variations possible. The team also looked at whether call types were influenced by each other, including checking whether call type 'N06', a Crossbill call type found in northern Britain and Ireland, was influenced by the irruption of continental call types.
The researchers found that flight calls varied over time in all crossbill species included in the research, with the exception of Two-barred Crossbill. Excitement calls showed less temporal variation, this only being significant in 'N06' and Scottish Crossbill.
They said that their results suggested that several of the call types categorised in earlier works were in fact 'snapshots in time', but were unable to prove a reason for the temporal variations.
However, they suggested that the phenomenon might be due to different populations influencing each other during periods of contact driven by the irruptive movements crossbills are well-known for.
Noticeable call variations were found to have taken place over just 5-10 years across distances of more than 10,000 km, showing rapid cultural evolution in crossbill vocalisations and putting forward a new understanding of call types.
Martin, R, Rochefort, J, Mundry, R, & Segelbacher, G. 2023. Fast cultural evolution of Crossbill (Loxia spp.) calls in the Palaearctic. Ibis. DOI: https://doi.org/10.1111/ibi.13253