Grey Gerygone hosts are not egg rejecters, but Shining Bronze-Cuckoos lay cryptic eggs

Many brood parasites rely on mimicry to prevent the detection of their eggs by hosts, yet most Australasian cuckoo species lay darkly colored eggs while the eggs of their hosts are pale and speckled. In the dimly lit nests of their hosts, these cuckoo eggs may appear cryptic; however, it is unclear if this disguise has evolved to fool hosts or other cuckoos. Recent work suggests that in at least one species of bronze-cuckoo, cuckoos are more likely to reject conspicuous eggs than are hosts, but it remains unclear whether this is common across the species group. Here, we present field experiments on the sole host of the Shining Bronze-Cuckoo ($\textit{Chalcites lucidus lucidus}$) in New Zealand, the Grey Gerygone ($\textit{Gerygone igata}$; known locally as the Grey Warbler), that explored whether this host ignores cuckoo eggs because they are cryptic. Using an avian vision model, we showed that Shining Bronze-Cuckoo eggs were variable in their conspicuousness, but were more cryptic in host nests than the host's eggs. We then experimentally parasitized all available clutches with model eggs that mimicked darkly or brightly colored cuckoo eggs, or were of maximum conspicuousness (white) as determined by visual modeling. Hosts never rejected our model eggs, nor cuckoo eggs when naturally parasitized. Instead, only cuckoos rejected model eggs: In 3 out of 4 experimental nests that were subsequently parasitized, the model egg was taken and replaced by a cuckoo's egg. Together, these data and previous experiments suggest that competition among cuckoos, rather than rejection by hosts, provides a stronger selection pressure for the evolution of cryptic eggs across the genus $\textit{Chalcites}$.A Phyllis and Eileen Gibbs Travelling Fellowship awarded to R.T. from Newnham College, Cambridge, UK, funded these experiments. R.T. was further supported by an Independent Research Fellowship from the Natural Environment Research Council (NERC UK), and J.L.R. by a Natural Sciences and Engineering Research Council of Canada (NSERC) Postgraduate Scholarship-Doctoral (PGS D) and an International Doctoral Scholarship from the University of Canterbury, New Zealand