Can common cuckoos discriminate between neighbours and strangers by their calls?

Common cuckoos, Cuculus canorus, are brood parasites: they lay their eggs in the nests of other bird species, and let these hosts incubate their eggs and feed and rear the nestlings. Although cuckoos do not show parental care, they demonstrate complex social interactions, including territorial behaviours and male-male aggression. Cuckoos have a well-known and simple two-phrase call ('cu' and 'coo'), uttered by males during their breeding season. Previous studies suggested that the 'cu-coo' call of males is individually unique, potentially allowing discrimination between different classes of males. Using playback experiments in a dense population of radiotagged cuckoos, we tested whether neighbouring males are tolerated more than unfamiliar intruders: the classic 'dear enemy' phenomenon. Focal birds responded more aggressively to the calls of unfamiliar simulated intruders (strangers) than to the calls of conspecifics with whom they shared territorial boundaries (familiar neighbours). Cuckoos responded quickly, within, on average, less than half a minute; they often approached the loudspeaker to within 5-10 m, even from up to 80 m away, and used their 'cu-coo' calls in response. Our results showed that cuckoos used their simple call for the discrimination of familiar versus unfamiliar individuals, and did so specifically to defend their own territories. In turn, cuckoos showed tolerance to nearby conspecifics, for example neighbours with overlapping territories, and did not respond to control playbacks. Finally, as typically more than one cuckoo was interested in the playbacks, this study confirmed the opportunity for brood-parasitic birds to socialize during the breeding season.