Defense behavior of two closely related but geographically distant host species against cuckoo parasitism: A next test for the parallel coevolution.

Interactions between avian brood parasites, such as common cuckoos ( Cuculus canorus ), and their hosts are one of the best-studied examples of the coevolutionary arms race. Different stages of this arms race can be seen in different races of common cuckoos and their hosts across their range. However, little is known whether selected populations of two closely related but geographically distant species with probably different coevolutionary histories with the common cuckoo are also at different stages of the arms race. In this study, we tested this prediction experimentally using the same non-mimetic model eggs and three-dimensional (3D) printed models of the gray adult common cuckoo ( Cuculus canorus ). We examined egg recognition and egg rejection and aggression against the common cuckoo in the great reed warbler ( Acrocephalus arundinaceus ) and Oriental reed warbler ( Acrocephalus orientalis ), in Slovakia of Europe and northeast China of Asia. The results showed that the great reed warbler exhibited stronger responses to experimental model eggs and 3D models of the common cuckoo than the Oriental reed warbler. We conclude that both the great reed warbler and Oriental reed warbler have well-developed antiparasitic behaviors against common cuckoos in the studied populations, but with different levels of defense intensity, which may be due to local differences in parasitic pressure and the risk of parasitism. This provides an opportunity to study coevolutionary processes between the brood parasite and its hosts together in both species at large geographical scales.