Cuckoos and cowbirds versus hosts: Co-evolutionary lag and equilibrium

Experiments show co-evolution between the Common Cuckoo Cuculus canorus and its hosts. Both egg mimicry and laying behaviour of cuckoos have evolved i n relation to host defences. In turn, host egg rejection and aggression to cuckoos have evolved in response to parasitism. Selective egg replacement b y cuckoos may also lead to egg mimicry but current evidence for this is wea k. Hosts incur costs of rejection, so below a critical parasitism frequency they do better to accept. This is reflected in phenotypic flexibility in h ost defences in relation to small-scale geographical variation and temporal changes in parasitism rate. The puzzle is why so many hosts accept non-mim etic eggs. There are more acceptors among cowbird hosts in North and South America than among cuckoo hosts in Europe or southern Africa, and cowbird h osts show less intermediate rejection frequencies. One hypothesis is that a cceptor hosts would do better to reject and accept because they are at the start of the co-evolutionary cycle. In support of this: host-parasite syste ms show dynamic changes, calculations suggest that many hosts would indeed do better to reject, and old cowbird hosts are stronger rejectors than are new hosts. An alternative hypothesis is that host acceptance can be an equi librium. In support of this: rejection costs for some hosts are sufficientl y high for acceptance to be best, in Australia a long breeding season reduc es parasitism costs and may explain acceptance there, and Mafia cuckoos may enforce acceptance. Variation in host acceptance is likely to reflect a mi xture of systems at equilibrium and those showing evolutionary lag. Host re sponses to parasite chicks are discussed, particularly how the parasite chi cks manipulate hosts through begging signals.