WHY ARE CUCKOOS HOST SPECIFIC

Teleological evidence of host specificity on cuckoos is their diverse egg morphology, which could have arisen either in response to varying degrees of tolerance shown by different host species or as the result of interspecific and intraspecific egg replacement by female cuckoos. The 2nd alternative provides the more convincing explanation with regard to host specialization in cuckoos. Host defences in these essentially one-on-one host/parasite systems may operate as a density-dependent control on cuckoo numbers rather than as a continuing influence on the morphology of the cuckoo egg. -from Authors