Coevolution is defined as a specialized relationship between species that leads to a reciprocal evolutionary change. Studies of such interactions have led to a better understanding of evolution in general. Brood parasites and their hosts represent particularly suitable systems for studying coevolution. A number of adaptations and counter-adaptations have previously been found on both sides, often leading to a so-called coevolutionary arms race between parasites and their hosts. Many of the previous studies on coevolution have been focusing on within-population dynamics and one-time visit "snap-shot" investigations in these particular populations. However, to acquire a deeper understanding of coevolutionary interactions one needs to investigate both spatial and temporal variation in such relationships. Furthermore, in order to obtain knowledge about the evolutionary potential in a species it is important to collect data on genetic and phenotypic heterogeneity among populations in a metapopulation system. Such an approach will lead to a better knowledge of how to optimally preserve local populations as well as how to conserve the diversity among populations. The proposed project aims to provide fundamentally novel knowledge regarding interactions between brood parasites and their hosts. One important aim is to take into account the influence of human activities on the coevolutionary dynamics through consideration of human disturbance like habitat fragmentation. Effects of climate will also be investigated. More specifically, our goals are to: 1) investigate the evolution and maintenance of sympatric and allopatric cuckoo gentes by making use of molecular analyses and variation in egg characteristics; 2) investigate selection of host pairs by cuckoos (compare egg phenotype and egg size differences between parasitized and non-parasitized hosts); 3) model effects of habitat fragmentation on coevolved adaptations like evolution of egg mimicry in cuckoos, 4) study spatial variation in genetic constitution and local anti-parasite adaptations in two selected cuckoo host species with highly different population structure, and 5) study possible effects of temporal variation in climate parameters on egg characteristics in cuckoos and their hosts. The results from this project will be of relevance for decision makers in their management to preserve genetic diversity on the intraspecific level, and will be of significance for researchers in the fields of evolutionary biology and conservation biology.