How to avoid being spotted? Pigments help cuckoo eggs to blend with host eggs
Why do animals tolerate parasites? Historically, the best studied example of deceiving host birds to accept parasitic eggs has been the egg color mimicry by the common cuckoo. Our HFSP Young Investigator Grant team has now discovered that to generate similar egg colors, cuckoos trick hosts to accept foreign eggs by depositing similar compositions and concentrations of colorful pigments into eggshells.
HFSP Young Investigator Grant holders Phill Cassey, Tomas Grim and Mark Hauber and colleaguesauthored on Thu, 29 September 2011
To avoid the costs of raising young other than their own, many hosts of brood parasitic birds have evolved to recognize and reject foreign eggs. In turn, parasites mimic the colors of host eggs to avoid discrimination. Mimicry, as a prime example of sensory trickery, received much attention from ancient times to modern scientific investigations. Surprisingly, the structural basis of egg color similarity between hosts and parasites has remained enigmatic. In our HFSP Young Investigator Grant team project, we studied the chemical basis of egg color mimicry in the best known avian brood parasite, the common cuckoo (Cuculus canorus) in Europe. Females of this parasite specialize on parasitizing particular host-species by laying an egg-morph whose color and size match closely the host eggs. The central aim of our project was to test the two alternative scenarios whether (1) cuckoo eggs replicate host pigment chemistry, or (2) cuckoo eggs use discreet mechanisms to produce a similar perceptual effect to hosts’ own eggs. We used an integrative approach, based on behavioral, comparative, biochemical, optical, and structural analyses of eggshell pigments to answer these fundamental issues, and discovered that cuckoos chemically replicate host egg pigmentation, i.e. parasites use the same pigment composition, and in similar concentrations, relative to their host species (Igic et al. 2011).
Figure: Representative color mimicry between different populations of the brood parasitic common cuckoo’s eggs (black arrows) in their respective host species’ nests from across Europe. Photo credits: C. Moskat, T. Grim, & M. Honza.
 A shared chemical basis of avian host – brood parasite egg colour mimicry. Igic B, Cassey P, Grim T, Greenwood DR, Moskat C, Rutila J, Hauber ME (2011). Proceedings of the Royal Society of London B (First Cite online). doi: 10.1098/rspb.2011.1718.
 Alternative mechanisms of increased eggshell hardness of avian brood parasites relative to host species. Igic B, Braganza K, Hyland MM, Silyn-Roberts H, Cassey P, Grim T, Rutila J, Moskat C, Hauber ME (2011) Journal of the Royal Society Interface (First Cite online). doi: 10.1098/rsif.2011.0207.