Rapid evolution of a brood parasite’s egg pattern does not lead to large increases in mimetic fidelity

In coevolutionary mimicry systems, mimics evolve to resemble models. When model fitness is reduced by close mimicry (such as when models are hosts and mimics are brood parasites), selection should drive mimics to evolve greater similarity to models over time, and models to evolve away from mimics (‘chase-away evolution’), potentially resulting in mimetic fidelity remaining constant over time. Evidence for reciprocal evolution of models and mimics and its effect on mimetic fidelity is limited, however, likely because long-term data are required to observe such trends. Here, we test for these dynamics in an avian brood parasite system. The brood-parasitic cuckoo finch (Anomalospiza imberbis) lays eggs which mimic those of its host, the tawny-flanked prinia (Prinia subflava). In defence, prinias have evolved complex egg patterns which facilitate recognition of parasitic eggs. Prinia egg patterns are more complex than cuckoo finch egg patterns, and so selection should favour elevated complexity in parasite eggs (i.e. mimics evolving towards models) and even greater complexity in host eggs (i.e. models evolving away from mimics). Using a dataset spanning 50 years, we show that egg pattern complexity has indeed increased in both species over this time period, reflecting rapid adaptive evolution of both models and mimics. Both species have evolved at similar rates. Accordingly, we find no detectible increase in mimetic fidelity over time. Thus, host evolution can counteract even rapid parasite evolution, and result in the persistence of imperfect mimicry.