Intensity of host nest defense against parasites does not change with different breeding stages

Brood parasitism can force hosts to evolve adaptive defense behaviors. To effectively prevent or mitigate reproductive losses caused by parasites, host birds can adjust their nest defense intensity at different breeding stages. In our study area, the oriental reed warbler (Acrocephalus orientalis) is a common host of the common cuckoo (Cuculus canorus). Previous studies have shown that this host exhibits strong aggressive behaviors toward common cuckoos. To test whether the intensity of nest defense by oriental reed warblers varies between different breeding stages, we compared their behavioral responses toward 3D dummies of the grey common cuckoo and oriental turtle dove (Streptopelia orientalis) during the early incubation and nestling stages. Results showed that there was no significant difference in the defensive behavior of oriental reed warblers against common cuckoos and oriental turtle dove dummies between early incubation and nestling feeding stages. Oriental reed warblers exhibited three different types of response behaviors during both early incubation and nestling feeding stages, which included attack, mobbing calls, and no response. We conclude that the nest defense intensity of oriental reed warblers against common cuckoos did not significantly change with different breeding stages between early incubation and nestling feeding stages. This may provide a new experimental case to fill a gap in understanding the coevolution between the oriental reed warbler (host) and the common cuckoo (parasite).