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.