Nest defence in a cuckoo host: great reed warblers risk themselves equally for their own and parasitic chicks

Hosts of the common cuckoo (Cuculus canorus) may foster a foreign chick instead of raising their own progeny, which incurs a significant cost to their fitness. Chick recognition or discrimination is, however, rare in cuckoo hosts and has been investigated exclusively in relation to nestling provisioning. Here we test for the first time whether hosts differ in the willingness of risk-taking when they care for own or parasitic offspring. We investigated nest defence in great reed warblers (Acrocephalus arundinaceus) against three types of nest intruders while controlling for the number of chicks and the length of the nestling period. The most parsimonious linear mixed-effects model showed that the type of intruder and nest identity significantly explained variation in host aggression. Our results demonstrated that the hosts discriminated two predators from an innocuous species and that some nest owners consistently defended their nests more intensely than others. However, the birds did not differ in their responses in relation to the nest contents, indicating that neither the nestling species, nor the length of previous parental investment influenced the intensity of nest defence. Our findings are, therefore, in accordance with the general scarcity of chick discrimination by cuckoo hosts.