The number of syllables in Chernobyl cuckoo calls reliably indicate habitat, soil and radiation levels

Cuckoos Cuculus canorus produce calls that consist of a repeated but variable number of syllables that has given name to the species and the family. Here we tested the hypothesis that cuckoo calls are reliable indicators of environmental and individual quality by determining the number of repeated 'cuckoo' syllables in calls in relation to habitat and soil, ionizing radiation, presence of a female cuckoo and local density of male cuckoos at Chernobyl, Ukraine. Males were consistent in their production of syllables, producing more syllables per call when a female or another male arrived. This increase in the number of syllables was larger in males that already produced many syllables in the absence of conspecifics, implying that males of superior quality were still able to increase the number of syllables. Males produced more syllables per call in habitats with black soil and in forests. Furthermore, they produced fewer and more aberrant syllables in radioactively contaminated areas of Chernobyl providing evidence of an effect of environmental perturbation on the number of syllables. These findings are consistent with the hypothesis that the number of syllables is a condition-dependent signal used for attracting mates, repelling competitors, providing information about local environmental conditions for other cuckoos, but also for humans and thus can be used by humans to infer habitat quality.