The risk of parasitism in relation to the distance from reed warbler nests to cuckoo perches

1. The frequency of cuckoo Cuculus canorus L. parasitism of reed warblers Acrocephalus scirpaceus Hermann, was investigated in the south-eastern part of the Czech Republic in relation to the distance of the nest from the nearest tree, and to the distance of the nest from that of the nearest active conspecific neighbour. 2. We also investigated whether any differences in egg size, clutch size and laying dates existed between the reed warblers breeding at different distances from trees, and if breeding reed warblers avoided choosing nesting sites near trees. Differences in clutch size and laying date between parasitized and unparasitized birds were also investigated. 3. The frequency of parasitism was higher close to trees and far away from the nearest active neighbour. 4. Our data support the prediction that reed warblers should avoid nesting near trees. The density of breeding birds were higher further away from trees. The egg-laying dates of reed warblers breeding further away from trees were earlier. Finally, we found that reed warblers breeding further away from trees laid significantly larger eggs than birds breeding closer to trees.