Visual complexity of egg patterns predicts egg rejection according to Weber's law

Visual complexity is ubiquitous in nature. Drivers of complexity include
selection in coevolutionary arms races between antagonists. However, the
causes and consequences of biological complexity and its perception are
largely understudied, partly because complexity is difficult to quantify.
Here, we address this by studying egg pattern complexity and its
perception in hosts (tawny-flanked prinia Prinia subflava ), which
visually recognize and reject mimetic eggs of their virulent brood
parasite (cuckoo finch Anomalospiza imberbis ). Using field data and an
optimization algorithm, we compute a complexity metric which predicts
rejection of experimentally placed conspecific eggs in prinia nests. Real
cuckoo finch eggs exhibit significantly lower pattern complexity than
prinia eggs, suggesting that high complexity benefits hosts because it
distinguishes host eggs from parasitic eggs. We show that prinias perceive
complexity differences according to Weber's law of proportional
processing (i.e. relative, rather than absolute, differences between
stimuli are processed in discrimination, such that two eggs with simple
patterns are more easily discriminable than two with complex patterns).
This may influence coevolutionary trajectories of hosts and parasites. The
new methods presented for quantifying complexity and its perception can
help us to understand selection pressures driving the evolution of
complexity and its consequences for species interactions.