Visual discrimination of polymorphic nestlings in a cuckoo-host system

Alfredo Attisano,Jörn Theuerkauf,Keisuke Ueda,Roman Gula,Ralph Kuehn,Yuji Okahisa,Nozomu J Sato,Keita D Tanaka

doi:10.1038/s41598-018-28710-5

Abstract

Mimicry by avian brood parasites favours uniformity over variation within a breeding attempt as host defence against parasitism. In a cuckoo-host system from New Caledonia, the arms race resulted in both host (Gerygone flavolateralis) and parasite (Chalcites lucidus) having nestlings of two discrete skin colour phenotypes, bright and dark. In our study sites, host nestlings occurred in monomorphic and polymorphic broods, whereas cuckoo nestlings only occurred in the bright morph. Irrespective of their brood colour, host parents recognised and ejected parasite nestlings but never ejected their own. We investigated whether host parents visually recognised their own nestlings by using colour, luminance and pattern of multiple body regions. We found that the parasite mimicked multiple visual features of both host morphs and that the visual difference between host morphs was larger than the difference between the parasite and the mimicked host morph. Visual discrimination alone may result in higher chances of recognition errors in polymorphic than in monomorphic host broods. Host parents may rely on additional sensorial cues, not only visual, to assess nestling identity. Nestling polymorphism may be a trace of evolutionary past and may only have a marginal role in true-recognition of nestlings in the arms race in New Caledonia.

Highlights

The interactions between brood parasites and their hosts are a classic example of a co-evolutionary process in which adaptation on one side leads to counter-adaptation on the other and so on[1]
The main objectives of our study were to (1) verify that ejection of the parasite occurs in any type of gerygone brood, (2) assess the visual mimicry of the host morph by cuckoo nestlings, (3) quantify the effect of down on the overall colour, luminance, pattern and contrast compared to bare skin areas, (4) identify which visual cues may convey “host” vs. “non-host” information and (5) measure changes in skin colouration of host nestlings over time
We found that visual mimicry by Shining Bronze-cuckoo nestlings from New Caledonia was based on the mimicking of multiple visual features of the host, similar to bronze-cuckoos from Australia[34,42]

Summary

Introduction

The interactions between brood parasites and their hosts are a classic example of a co-evolutionary process in which adaptation on one side leads to counter-adaptation on the other and so on[1]. The same mechanisms that determine egg polymorphism should apply to nestling polymorphism[2] and a recent study showed that polymorphism does occur at the nestling stage in a cuckoo-host system in New Caledonia[37] In this Pacific island, the local subspecies of the Fan-tailed Gerygone, Gerygone flavolateralis flavolateralis, is the exclusive host of the local subspecies of the Shining Bronze-cuckoo, Chalcites lucidus layardi. Two Australian Gerygone species[25,26] and the Fan-tailed Gerygone from New. Caledonia[37] are able to recognise and eject the cuckoo nestling from their nest before it can evict any host egg or nestling. In our former research[37], we were not able to confirm ejection of the parasite in polymorphic broods because none of the polymorphic broods in the previous study was parasitised

Objectives

Methods

Results

Conclusion