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(1) There was no difference in the distinctiveness of egg markings between species that have interacted strongly with cuckoos and species that have not, nor in intra-clutch variation, nor in inter-clutch variation within a species. In Iceland, where they are isolated from cuckoos, the eggs of meadow pipits and pied/white wagtails showed no differences in intra-clutch variation, nor inter-clutch variation, from those in parasitized populations in Britain. Thus there was no evidence that host egg patterns evolve in response to cuckoos. (2) None of the four species tested discriminated against an odd chick (another species) in their nest (chaffinch, reed warbler, reed bunting, dunnock). Hosts therefore evolve discrimination against odd eggs but not against odd chicks. (3) The variation in rejection of unlike eggs among different species of suitable cuckoo hosts is not related to the current costs or benefits of rejecting cuckoo eggs. We suggest that the variation represents snap shots in evolutionary time of different stages of a continuing arms race between the cuckoo and its hosts.

The common cuckoo Cuculus canorus is divided into host-specific races (gentes). Females of each race lay a distinctive egg type that tends to match the host's eggs, for instance, brown and spotted for meadow pipit hosts or plain blue for redstart hosts. The puzzle is how these gentes remain distinct. Here, we provide genetic evidence that gentes are restricted to female lineages, with cross mating by males maintaining the common cuckoo genetically as one species. We show that there is differentiation between gentes in maternally inherited mitochondrial DNA, but not in microsatellite loci of nuclear DNA. This supports recent behavioural evidence that female, but not male, common cuckoos specialize on a particular host, and is consistent with the possibility that genes affecting cuckoo egg type are located on the female-specific W sex chromosome. Our results also support the ideas that common cuckoos often switched hosts during evolution, and that some gentes may have multiple, independent origins, due to colonization by separate ancestral lineages.

On Wicken Fen and nearby watercourses in eastern England, parasitism by cuckoos, Cuculus canorus, declined from 26% and 16% of reed warbler (Acrocephalus scirpaceus) nests in 1985 and 1986, respectively, to 2^6% of nests in 1995^97, owing to a decline in cuckoos. Experiments with model eggs showed that over this 12-year period there was a marked decline in host rejection of non-mimetic eggs, from rejection at 75% of reed warbler nests in 1985^86 to 25% of nests in 1997. Calculations suggest that this decline in host defences is too rapid to re£ect only genetic change, and is more likely to be the outcome of adaptive phenotypic £exibility. Two other results show £exibility in host responses. First, there was a seasonal decline in rejection, which accompanied the seasonal decline in parasitism. Second, although rejection did not vary with proximity to a naturally parasitized nest within the 3.4 km 2 of Wicken Fen and its surrounds, there was no rejection at a small unparasitized population 11km away. Flexible host defences will be advantageous when there are costs of rejection as well as short-term temporal changes and smallscale geographical variation in parasitism rate. Other recent studies reporting changes in host defences may also re£ect phenotypic £exibility rather than evolutionary change.