Evidence of egg polymorphism in a host of Klaas’s cuckoo

Abstract Hosts may evolve more than one egg type within a population (egg polymorphism) to make mimicry of any one individual more difficult. This process of co-evolution between cuckoos and their hosts has been well studied in certain species but has been ignored in other, lesser-known species. Using avian visual modelling, reflectance spectrophotometry, digital image analysis and egg volume, we examined the degree of egg mimicry between Klaas’s cuckoo (Chrysococcyx klaas) and its host, the red-chested sunbird (Cinnyris erythrocercus), using museum collections. We found that the sunbird has evolved two distinct egg types based on luminance, colour and pattern, which we refer to as the light morph and the dark morph. Cuckoo eggs were significantly more similar to the dark morph eggs in luminance compared with the light morph eggs but were not more similar to dark morph eggs in colour, pattern or volume. To our knowledge, this is the first time that egg mimicry by Klaas’s cuckoo has been studied objectively, and egg polymorphism was described in the red-chested sunbird host. Future studies focused on living populations would help us to gain a better understanding of the co-evolutionary interactions between this cuckoo and its host species.