Host genotype and age have no effect on rejection of parasitic eggs.

Egg rejection belongs to a widely used host tactic to prevent the costs incurred by avian brood parasitism. However, the genetic basis of this behaviour and the effect of host age on the probability of rejecting the parasitic egg remain largely unknown. Here, we used a set of 15 polymorphic microsatellite loci, including a previously detected candidate locus (Ase64), to link genotypes of female great reed warblers (Acrocephalus arundinaceus), a known rejecter, with their egg rejection responses in two host populations. We also tested whether host female age, as a measure of the experience with own eggs, plays a role in rejection of common cuckoo (Cuculus canorus) eggs. We failed to find any consistent association of egg rejection responses with host female genotypes or age. It seems that host decisions on egg rejection show high levels of phenotypic plasticity and are likely to depend on the spatiotemporal variation in the parasitism pressure. Future studies exploring the repeatability of host responses towards parasitic eggs and the role of host individual experience with parasitic eggs would greatly improve our understanding of the variations in host behaviours considering the persistence of brood parasitism in host populations with rejecter phenotypes.