Abstract
Interactions between avian hosts and brood parasites can provide a model for how animals adapt to a changing world. Reed warbler (Acrocephalus scirpaceus) hosts employ costly defenses to combat parasitism by common cuckoos (Cuculus canorus). During the past three decades cuckoos have declined markedly across England, reducing parasitism at our study site (Wicken Fen) from 24% of reed warbler nests in 1985 to 1% in 2012. Here we show with experiments that host mobbing and egg rejection defenses have tracked this decline in local parasitism risk: the proportion of reed warbler pairs mobbing adult cuckoos (assessed by responses to cuckoo mounts and models) has declined from 90% to 38%, and the proportion rejecting nonmimetic cuckoo eggs (assessed by responses to model eggs) has declined from 61% to 11%. This is despite no change in response to other nest enemies or mimetic model eggs. Individual variation in both defenses is predicted by parasitism risk during the host’s egg-laying period. Furthermore, the response of our study population to temporal variation in parasitism risk can also explain spatial variation in egg rejection behavior in other populations across Europe. We suggest that spatial and temporal variation in parasitism risk has led to the evolution of plasticity in reed warbler defenses.
Highlights
Alternative genotypes can change in frequency over time as selective pressures vary (Grant and Grant 2002; Pulido and Berthold 2010), behavioral modification in response to environmental cues is often likely to be a species’ most effective adaptation in a rapidly changing world (Nussey et al 2007)
We suggest that spatial and temporal variation in parasitism risk has led to the evolution of plasticity in reed warbler defenses
DECLINING CUCKOOS AND HOST DEFENSES The causes of the marked decline in cuckoos in Britain are not known
Summary
We reported a decline in cuckoo parasitism and cuckoo egg rejection by reed warblers during the first 12 years of our study (1985–1997; Brooke et al 1998). Reed warblers are more likely to accept “mimetic” model eggs (Davies and Brooke 1988) and host rejection of “nonmimetic” model eggs predicts the degree of cuckoo egg mimicry across different cuckoo host-races as modeled by bird vision (Stoddard and Stevens 2011). Response to these model eggs provides a valid, standard measure of host rejection across the years. A sample of pairs was presented with a model or mount of an adult cuckoo (5 min at the nest after first approach to 1 m) before a mimetic model egg was added to the clutch, to test whether the sight of a cuckoo at the nest stimulated egg rejection
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