Decades of research have shown that the coevolutionary arms race between avian brood parasites and their hosts can promote phenotypic diversification in hosts and brood parasites. However, relatively little is known about the role of brood parasitism in promoting phenotypic diversification of nestlings. We review field data collected over four decades in Australia, New Caledonia and New Zealand to assess potential for coevolutionary interactions between the shining bronze-cuckoo (Chalcites lucidus) and its hosts, and how diversification at the nestling stage may be generating different subspecies. The shining bronze-cuckoo is a specialist parasite of a few hosts in the family Acanthizidae. It has diversified into subspecies, of which the nestlings closely mimic the respective host nestlings in each region. Additionally, some cuckoo subspecies have polymorphic nestlings.  The Acanthizidae hosts have similar breeding and nesting habits and only moderately effective frontline defences against parasitism at cuckoo egg laying or at the egg stages. However, some hosts have developed highly effective defences at the nestling stage by recognising and ejecting cuckoo nestlings from the nest. As with the cuckoo nestlings, some hosts have polymorphic nestlings. The coevolutionary interactions in each region suggest different evolutionary stages of the arms race in which either the parasite or the host is currently in the lead. The presence of moderately effective defences at the egg laying and egg stages might explain why some hosts do not have defences at the nestling stage. The south-Pacific cuckoo – host systems are excellent models to explore the evolutionary mechanisms driving the diversification at the nestling stage in the coevolutionary arms race between avian brood parasites and their hosts.

We measured spectral reflectance (300-700 nm) from the skin of 1-4 days old cuckoo and host nestlings in the field using portable spectrometers (AU and NZ; USB2000, Ocean Optics, Dunedin, FL, USA; NC: Jaz-EL-200, Ocean Optics, Dunedin, FL, USA) with illumination by pulsed xenon or deuterium-tungsten-halogen light sources. Before measurements, we calibrated the spectrometers with a diffuse reflectance standard (WS-1, Ocean Optics, Dunedin, FL, USA). We took 6 measurements from each nestling (2 from the hips, 2 from the shoulders and 2 from the head) and calculated an average spectrum for each individual. 

We used the package pavo to import, process and analyse the spectral data in R 3.6.3. The shining bronze-cuckoo and its Acanthizidae hosts are predicted to have a violet-sensitive (VS) visual system, therefore we run a visual modelling analysis using the peafowl as a VS avian vision model. We run an additional analysis using an ultraviolet-sensitive (UV) avian vision model (blue tit). We used the function vismodel to calculate the quantum catches of the VS model with a D65 (standard daylight) irradiance spectrum. We used a log form of the receptor-noise limited (RNL) model of Vorobyev and Osorio (1998) to estimate noise at low light levels.

File can be accessed using Microsoft Excel, OpenOffice.