During the nestling period, brood-parasitic birds stimulate host parents to provide food through complex visual and auditory signals, including emitting supernormal stimuli in the form of begging calls to increase the feeding frequency. However, whether the begging calls of brood-parasitic nestlings act as a universal type of supernormal stimulus signal and their effects on less common host species still require further research. In this study, we used playback recordings to verify the impact of the begging calls of Common Cuckoo nestlings on the parental care behaviour of host Barn Swallow parents. The results showed that male Barn Swallow parents, but not female parents, reduced their feeding frequency in response to both type of cuckoo nestling calls (the begging calls of cuckoo nestlings reared by common/uncommon hosts). Thus, the total feeding frequency in the two experimental groups was significantly lower than the control group. Additionally, brood size, temperature, and weather all affected the feeding frequency in the Barn Swallow. This study supports the idea that the supernormal stimulus of brood-parasitic nestling begging calls does not always work as a universal signal; the behavioural adaptations formed by parasitic birds in response to common hosts may lead to reduced fitness when utilising uncommon hosts, e.g., the Barn Swallow.

During the nestling period, brood-parasitic birds stimulate host parents to provide food through complex visual and auditory signals, including emitting supernormal stimuli in the form of begging calls to increase the feeding frequency. However, whether the begging calls of brood-parasitic nestlings act as a universal type of supernormal stimulus signal and their effects on less common host species still require further research. In this study, we used playback recordings to verify the impact of the begging calls of Common Cuckoo nestlings on the parental care behaviour of Barn Swallow parents. The results showed that the feeding frequency in the two experimental groups (the begging calls of cuckoo nestlings reared by common/uncommon hosts) was significantly lower than the control group. Male Barn Swallow parents, but not female parents, reduced their feeding frequency in response to cuckoo nestling calls. Additionally, the number of nestlings, time of day, and weather were all significantly correlated with the feeding frequency. This study supports the idea that the supernormal stimulus of brood-parasitic nestling begging calls is not a universal signal; the behavioural adaptations formed by parasitic birds in response to common hosts may lead to reduced fitness when utilising uncommon hosts.