Great Reed Warbler singing behavior and conspicuous song structures are not nest-location cues for the Common Cuckoo

Abstract

In some systems, brood parasites may be attracted by vocal or visual signals connected with host breeding. We studied a Great Reed Warbler (Acrocephalus arundinaceus) population where annually 30–50% of nests are parasitized by the Common Cuckoo (Cuculus canorus). We observed host males and quantified their song-related behaviors, including time spent singing, distance of movements and time spent singing in particular positions on a reed stem. We predicted that nests of more exposed males (i.e., those spending more time singing, moving a larger total distance, and spending more time on the top of stems) would be more likely to be parasitized than the nests of less exposed males. Additionally, we measured male song characteristics that we assumed to be most audible, and thus potentially the most conspicuous to the Common Cuckoo. We counted the number of “kara” syllables per song and measured their peak frequencies. Since these song structures are of low frequency and thus might be audible at longer distances, we predicted that males producing more kara syllables or uttering kara syllables of lower peak frequencies would also be more parasitized. However, we found that neither male singing behavior nor conspicuous song characteristics were significant predictors of parasitism. Only the visibility of host nests to the parasite, which we treated as a covariate, proved to be significant. Visible nests were more often parasitized than hidden nests. Our findings indicate that the Cuckoo females use nest visibility, or host behavior other than male singing, as the cue to locate host nests.