How to Spot a Stranger's Egg? A Mimicry-Specific Discordancy Effect in the Recognition of Parasitic Eggs

Egg discrimination by hosts is an antiparasitic defence to reject foreign eggs from the nest. Even when mimetic, the presence of brood parasitic egg(s) typically alters the overall similarity of all eggs in a clutch, producing a discordant clutch compared to more homogenous clutches of composed only of hosts' own eggs. In multiple parasitism, the more foreign eggs are laid in the nest, the more heterogeneous the overall clutch appears. Perceptual filters and recognition templates cannot explain the known pattern of lower rejection rates of foreign eggs in multiple vs. single parasitism. We therefore assessed the role of clutch homogeneity and manipulated the colour of one or more eggs in the clutches of great reed warbler (Acrocephalus arundinaceus) hosts of common cuckoos (Cuculus canorus). Varying the colours of both the majority and the minority eggs caused predictable shifts in the rejection of the focal egg(s), and ejection rates of the minority egg colour consistently increased but only when it belonged to a more mimetic egg colour, relative to the less mimetic colour of majority eggs. The results imply that in addition to sensory filters, and template-based cognitive decision rules, discordancy-based rejection is affected by the overall clutch appearance and interacts with specific colours varying in the extent of mimicry, to contribute to the recognition decisions of hosts to reject parasitic eggs.