Cuckoo-host co-evolution : portfolios of offence and defence

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In Nature, organisms evolve in response to changes not only in the physical environment but also in their competitors, predators and parasites. The evolutionary battle between the common cuckoo, Cuculus canorus, and its hosts provide a good model system for studying these evolutionary processes in the wild. The cuckoo is a famous cheat: it tricks other species of birds (the hosts) into incubating its eggs and raising its young. The cuckoo lays just one egg per host nest. Soon after hatching, the cuckoo chick ejects the host's eggs and young out of the nest. It then manipulates the hosts to feed it by a remarkable begging call, which sounds like many hungry host young. Given these costs to the host's own reproduction, it is not surprising that hosts have evolved defences, such as rejecting eggs that differ from their own. In response, the cuckoo has evolved into several distinct genetic races, each specialising on one host species and laying an egg type that matches the egg of its particular host species. Cuckoo-host interactions are unlikely to involve just this one line of defence and offence. There is the potential for interactions at several stages, leading to 'portfolios' of adaptations and counteradaptations. For example, mobbing of adult cuckoos can be an effective front-line of host defence, which reduces the chance that the nest is parasitized in the first place. The hawk-like appearance of cuckoos is likely to be a counter-response, which makes close approach potentially risky for the hosts. The outcome of cuckoo counter-responses is that host defences are costly. Egg rejection may redeem the host's reproductive investment but entails the risk that the host rejects its own eggs rather than the parasite egg, while mobbing an adult cuckoo entails potentially dangerous enemy inspection. We have previously shown that hosts vary their defences in relation to local parasitism risk. This proposal addresses two new questions: First, are there interactions between successive lines of host defence? Second, how do hosts assess local parasitism risk? We will study reed warblers, Acrocephalus scirpaceus, the main cuckoo host in marshland. Field experiments, using mounts of adult cuckoos and model cuckoo eggs, will test whether pairs with better surveillance or stronger mobbing responses are more or less likely to reject cuckoo eggs. Recent theory suggests that a stronger front-line of defence reduces the need for subsequent defences, just as a strong lock on the front door precludes the need for a lock on the bedroom door too. We will also investigate nest architecture as a potential defence. In theory, cuckoos should select stronger host nests because the cuckoo chick grows to be more massive than a host brood and also has a more prolonged nestling period. In response, it could pay reed warblers to build more flimsy nests, particularly in sites at high risk of parasitism. We will test how nest design influences the success of parasitized and unparasitised nests, and model the costs and benefits of different nest design at sites of varying parasitism risk. To test whether reed warblers vary nest defences in response to direct cues from adult cuckoos, or indirect cues (eg. distance from potential cuckoo surveillance perches), we will compare defences at sites with and without cuckoos. Experiments will also test whether naïve, young reed warblers can learn to enhance their recognition of the adult cuckoo, as an enemy that can be safely mobbed, by observing the responses of experienced neighbours. This research will improve our understanding of co-evolutionary processes, revealing how adaptations at one stage of an interaction influence those at other stages. It will also enhance our understanding of how individuals use local information to adapt rapidly to environmental change. And it will add to our knowledge of the cuckoo, one of the icons of our countryside.