Host defences against cuckoo parasitism in a changing world

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Charles Darwin likened the natural world to an "entangled bank" in which organisms evolve not only in response to changes in their physical environment but also to ever changing interactions with other organisms - competitors, predators and parasites. The recent decline in cuckoos provides an opportunity to test, by field experiments, how their hosts have adapted to the reduced threat of brood parasitism. The cuckoo is one of Nature's most notorious cheats: it lays its eggs in the nests of other species (one per host nest) and when the young cuckoo hatches it evicts the host's eggs and young. The hosts are then doomed to raise a young cuckoo instead of a brood of their own. This interaction has provoked an "evolutionary arms race": hosts have evolved defences against cuckoos (attacking adult cuckoos and ejecting cuckoo eggs), while cuckoos have evolved tricks to beat these defences (rapid laying to avoid host detection, and eggs that mimic host eggs in size and colour). Our recent work has shown that reed warbler hosts vary their defences in response to parasitism risk.Our first question is: How do reed warblers assess parasitism risk? When parasitism rates are high, reed warblers readily approach adult cuckoos and they are more likely to reject an egg if they see a cuckoo at their nest (a real one or a model cuckoo placed by experiment). Therefore defences are influenced by personal information of local cuckoo activity. However, at low rates of parasitism, reed warblers are more reluctant to approach cuckoos and a cuckoo at the nest is less likely to trigger egg rejection. This suggests that defences are also influenced by other cues. Our experiments have revealed that reed warblers widen their source of information about local cuckoo activity by eavesdropping on neighbours (social information). If they have witnessed neighbours mobbing a cuckoo, they are more likely to approach and mob a cuckoo back at their own nest. However, we do not know whether social information of parasitism risk stimulates egg rejection, either on its own or by priming responses to personal information.We will test how egg rejection by reed warblers (responses to model cuckoo eggs) is influenced by their assessment of parasitism risk (varied by presenting models of adult cuckoos). With controlled experiments, we will vary both personal information of local parasitism risk (a cuckoo at the focal pair's own nest) and social information (a cuckoo plus broadcast of reed warbler mobbing calls on a neighbour's territory). Our second question is: How rapidly are defences mobilised in unparasitised host populations? The recent decline in cuckoos (65% decline in the UK since the 1980's) has resulted in many small, isolated, unparasitised reed warbler populations, where defences (mobbing adult cuckoos and egg rejection) are absent. At twelve such populations of colour-ringed birds, we will present models of adult cuckoos or control, harmless intruders, and add model cuckoo eggs to nests, to test how mobbing and egg rejection defences might be reignited by personal and social information of local cuckoo activity. We will then monitor these populations to test how defences spread to newly-settling pairs.Our third question is: How might a naïve host species first acquire defences? Using the dunnock, Prunella modularis, a host that has no defences against cuckoos, we will test by experiment whether personal and social information of the adult cuckoo as a novel enemy can induce defences against adult cuckoos and stimulate responses to cuckoo eggs.We hope that this study will provide novel insights into how organisms acquire information about threats from enemies in a changing world, how rapidly defences are acquired, and how behavioural change (in this case recognition of a new enemy) might set the stage for evolutionary change by exposing organisms to new selection pressures.