Coevolution is defined as specialized, and often intimate, relationships between species that leads to a reciprocal evolutionary change. Studies of coevolutionary interactions have led to a better understanding of evolution in general. Avian brood parasites and their hosts represent particularly suitable systems for studying coevolution. The main aim of the project is to investigate a model parasite-host system over a wide spatiotemporal scale to understand how host-parasite associations arise and evolve. Numerous previous studies have described a suite of adaptations and counter-adaptations in both hosts and parasites, often leading to a so-called coevolutionary arms race between the combatants. Many of the previous studies on coevolution, however, have focused mostly 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. Parasites and hosts drive each other’s evolution, and new host-parasite associations are continuously being formed. Parasites can expand to new host species or change the geographic range over which they exploit their hosts. 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 factors that are important in spatiotemporal variation in host use and expression of coevolved traits, but also on how to optimally preserve local populations as well as how to conserve the diversity among populations.

We have during the stay at CAS constructed a unique dataset consisting of approximately 40,000 clutches containing host and parasite eggs from the whole of Europe sampled during 1819 - 2009. By using this dataset we should be able 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; 5) study possible effects of temporal variation in climate parameters on egg characteristics in cuckoos and their hosts; and 6) disclose factors that may explain spatiotemporal variation in host use by cuckoos in Europe.

The stay at CAS for one full academic year was of invaluable importance to the Brood Parasitism group at NTNU. The stay made it possible for the members, due to the excellent efforts of the excellent staff at CAS, to devote full time focus on developing ideas, discussing projects and publish scientific papers in a very inspiring environment. The stay resulted in numerous publications, submission of a project proposal, discussions concerning future projects and collaboration between group members. In addition, there was substantial media coverage of the project.