Empirical evidence of different egg morphs that match host eggs in the brush cuckoo (Cacomantis variolosus)

One of the most efficient defences against obligate brood parasitism in birds is egg ejection, where a host recognises and removes the parasitic egg from the nest. This defence often selects for egg mimicry in parasitic species to reduce the likelihood of egg ejection. If a parasite uses multiple host species with distinctive egg types, this could lead to the evolution of egg gentes (host-specific egg types) in the parasite. There is observational evidence that the brood parasitic Brush Cuckoo (Cacomantis variolosus) might exhibit egg gentes, but there has been no objective study conducted to determine how closely eggs of this cuckoo species resemble those of its hosts from a bird's visual perspective. Using objective measurements to quantify egg appearance, we found that Brush Cuckoos exhibit at least two egg morphs that closely match the eggs of two of its primary hosts in colour, luminance and volume. While the determination of actual egg gentes in the Brush Cuckoo was beyond the scope of our study, our results are a first and necessary step in determining whether egg gentes might exist in this species. We suggest at least a third egg morph matching another primary host (or at least the genus of that host) might exist, but more data would be necessary to confirm this. Additionally, we provide a mechanism researchers can use to help distinguish between Brush Cuckoo eggs that are closely matched to their host eggs for future studies in this system.