Factors influencing the risk of common cuckoo Cuculus canorus parasitism on marsh warblers Acrocephalus palustris

Using multiple logistic regression analysis, we investigated the influence of nest site characteristics, laying date and nest size in marsh warblers Acrocepbalus palustris on the risk of parasitism by common cuckoos cuculus canorus. Marsh warblers breed in more diverse and dense herbaceous vegetation than other cuckoo hosts investigated in comparable studies. The "perch proximity" hypothesis was supported as parasitized nests were situated closer to trees than non-parasitized ones. Furthermore, demonstrated for the first time in a cuckoo host, tree height was an important predictor of parasitism, with higher trees increasing the parasitism odds ratio. The "nest exposure" hypothesis was also supported since parasitized nests had a shorter stand of vegetation in the close vicinity than non-parasitized nests. However, visibility of the nest from the nearest potential cuckoo perch (cuckoo view) was not selected by the model, probably because most nests were well concealed. Laying date, height of nest above ground and the distance from the nest to the nearest edge of the vegetation were not important predictors of parasitism. Though smaller nests tended to be parasitized more frequently than larger ones, nest size only approached significance, making its importance unclear.