Costs of breeding far away from neighbors: Isolated host nests are more vulnerable to cuckoo parasitism.

A series of parasitic and anti-parasitic strategies has evolved during the long-term coevolution between cuckoos and their hosts. The first stage of the arms race is host nest choice by cuckoos, followed by nest defense by hosts. This study examined nest defense strategies of the Oriental reed warbler (Acrocephalus orientalis) in relation to parasitism by the common cuckoo (Cuculus canorus). Attack rate of Oriental reed warblers against common cuckoo dummies was 100% and neighboring individuals participated in 87.1% of such attacks. Furthermore, the number of hosts attacking cuckoo dummies was significantly positively correlated with the number of neighbors at a distance from 40 to 70 m, indicating social anti-parasitic behavior. Analysis of nest-site parameters indicated that the distance to the nearest neighboring unparasitized nest was significantly shorter than that of parasitized nests. Our study demonstrated that isolated nests of Oriental reed warblers with distant neighbors were more vulnerable to common cuckoo parasitism suggesting a cost of breeding far away from neighbors. We hypothesize that cooperative behavior within local populations of Oriental reed warblers can probably be considered as an anti-parasitic strategy developed through long-term coevolution. Choice and parasitism of isolated host nests far away from neighbors may be an adaptive parasitic strategy by common cuckoos to increase the probability of successful parasitism.