As there is geographic and temporal variation in the rate at which reed warblers Acrocephalus scirpaceus are parasitised by the cuckoo Cuculus canorus , phenotypic plasticity of defences against parasitism could be advantageous. Three experiments were conducted using three populations of reed warblers, parasitised by cuckoos to varying degrees, to test if reed warbler defences against parasitism are plastic. In an unparasitised and a rarely parasitised population, attempts to simulate the presence of cuckoos at the nest or in the habitat failed to stimulate an increase in rates of egg rejection. However, three lines of evidence supported the view that both unparasitised and parasitised populations were similarly able to discriminate odd eggs but that there is phenotypic plasticity in the decision to reject those eggs. First, reed warblers at all populations pecked model eggs, thereby indicating recognition of the model egg as a foreign egg, but varied in their tendency to reject them. Second, reed warblers at two populations, one unparasitised and the other frequently parasitised, rejected brown painted reed warbler eggs at the same rate, suggesting that there are no differences between populations in the ability to reject some types of eggs. Finally, rates of rejection decreased seasonally only at the frequently parasitised population. These results suggest that phenotypic plasticity can explain population differences in rates of egg rejection, but do not rule out the possibility that genetic differences also contribute to differences between populations.