Nest site selection in a population of great reed warblers Acrocephalus arundinaceus parasitized by the common cuckoo Cuculus canorus
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- Institutt for biologi 
In avian parasite-host systems, the risk of parasitism is a potential selective agent for nest site selection. According to the spatial habitat structure hypothesis, host individuals that occupy nest site territories close to trees or elevated structures will suffer from higher risk of parasitism. I predict that in a great reed warbler Acrocephalus arundinaceus population frequently parasitized by the common cuckoo Cuculus canorus, natural selection should favour individuals occupying nest site territories further away from these vantage points, which would lead to a preference for such territories. The favoured trait should be the ability to recognize suitable nest site territories and the sufficient determination of territory quality through cues in the habitat structure. Here, I investigate habitat preferences for nest sites in a great reed warbler population parasitized by the common cuckoo breeding in a patch of marshland surrounded by agricultural areas. The main aim was to test predictions derived from the spatial habitat structure hypothesis and determine if the hosts are avoiding nest site characteristics that increase the risk of parasitism. Trees and other elevated structures, which are used as observation posts by the cuckoo and avian predators, were predicted to be the main cues avoided by the host. However, my results show that the most important factor for nest site selection by great reed warbler seems to be the thickness of the reeds, without a relationship to the probability of brood parasitism or predation. Despite the significantly increased risk of parasitism and predation near trees, territories close to trees were not avoided. In my study area, the early arriving great reed warblers might be ecologically trapped, because of the utilization of general cues in the artificial habitat structure, which ultimately could lead the early occupiers into selecting nest sites of low ultimate quality as a result of severe reproductive losses due to parasitism and predation.