Visual complexity is ubiquitous in the natural world. Drivers of such complexity include selection in coevolutionary arms races between antagonistically-interacting species. However, in general the causes and consequences of biological complexity are understudied, in part because complexity is difficult to quantify in a biologically relevant manner. Here, we address this gap by studying egg pattern complexity and its perception in hosts (tawny-flanked prinias Prinia subflava) of brood parasites (cuckoo finches Anomalospiza imberbis). Using field data and an optimisation algorithm, we compute a complexity metric which predicts rejection of experimentally-placed conspecific eggs in prinia nests. Real eggs of the cuckoo finch exhibit significantly lower pattern complexity than prinia eggs, suggesting that hosts benefit from high complexity because it distinguishes host eggs from parasitic eggs. We show that prinias perceive complexity differences according to Weber’s Law of proportional processing, which states that relative, rather than absolute, differences between stimuli are processed in discrimination. We discuss how such proportional processing can influence coevolutionary trajectories of hosts and parasites.  The new methods that we present for quantifying complexity and its perception can help us to understand selection pressures driving the evolution of complexity, and its consequences on interactions between organisms.

The dataset provided includes raw data of complexity scores for eggs in the field, and egg rejection data from field experiments carried out in 2007-2009 and 2018-2020.