Decoupled Acoustic and Visual Components in the Multimodal Signals of the Common Cuckoo (Cuculus canorus)

Martina Esposito,Daniela Campobello,Giuseppa Buscaino,Maria Ceraulo,Matteo Dal Zotto,Luigi Sala,Salvatore Mazzola,Beniamino Tuliozi

doi:10.3389/fevo.2021.725858

Abstract

Because of its parasitic habits, reproduction costs of the common cuckoo (Cuculus canorus) are mostly spent in pre-laying activities. Female costs are limited to searching host nests and laying eggs, whereas, males spend time in performing intense vocal displays, possibly with territorial purpose. This last aspect, together with a sexual plumage dimorphism, points to both intra- and inter-sexual selections operating within this species. One element triggering sexual selection is a differential fitness accrued by different phenotypes. Before analyzing possible sexual selection mechanisms operating in cuckoos, it is therefore necessary to verify whether there is a variability among male secondary characters by describing and quantifying them. Here we aimed to quantitatively characterize the main two potential candidates of sexual selection traits, i.e., calls and displays, shown by males at perches. During the 2019 breeding season, in a site within the Po Plain, we both audio and video recorded cuckoo males at five different perches. We analyzed acoustic variables as well as display sequences searching for potential correlations. We found a significant variation among calls that could be clustered into four vocal types. We also found that no visual displays were associated with vocal displays; cuckoo males were either vocal and motionless or soundless and active. We discuss our results under the perspective of the potential value of sexual selection in brood parasites and its role in its parasitic habit.

Highlights

The common cuckoo (Cuculus canorus) is an obligatory brood parasite, as it lays its eggs in nests of other species who raise its young until fledging at the expenses of their own offspring (Chance, 1940; Wyllie, 1981)
Given the extended amount of time this species devotes to the acoustic displays, it might be possible that a visual display might be a less generic, more targeted message for a specific set of other individuals – for example, a female approaching or a challenging male –. This might point to a sequentiality in the two displays, with the visual component following the acoustic one. We studied both acoustic and visual displays in the common cuckoo, aiming: (i) to tentatively reveal how many individuals resided in the area by identifying vocal types (VTs, i.e., male individuals differing by their vocal features) and to determine VTs spatial preferences among their perches; (ii) to describe a cuckoo male ethogram and quantify consistent behavioral sequences; (iii) to verify the presence of multimodal visual/acoustic signals in male cuckoo displays and to characterize their relationship, and (iv) to verify whether there are VT-specific behavioral rituals
Calls and visual displays are perfect candidates of traits on which sexual selection might operate as they may serve to compete for a territory or attract a mate

Summary

Introduction

The common cuckoo (Cuculus canorus) is an obligatory brood parasite, as it lays its eggs in nests of other species who raise its young until fledging at the expenses of their own offspring (Chance, 1940; Wyllie, 1981). Multimodal signals for example might have evolved to convey redundant information, with multiple components having the same meaning and retaining it if emitted separately (Hebets and Papaj, 2005; Mitoyen et al, 2019) This is known as the back-up signal hypothesis and it is linked to environmental variability: the more channels are used in communications, the least probable it is that the signal might be missed (Johnstone et al, 1996)

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