Recognition errors and probability of parasitism determine whether reed warblers should accept or reject mimetic cuckoo eggs

Experimental parasitism was combined with a signal-detection model of the host’s decision. The model treats rejection as a choice under uncertainty in which the host risks discarding its own egg, and predicts the threshold that maximises host success given local parasitism rates and the degree of mimicry.

Studied in

Acrocephalus scirpaceus, Cuculus canorus

Sample size

experimental parasitism trials combined with a decision model

Host acceptance of well-matched eggs was consistent with a rational threshold rather than with failure to perceive a difference. Hosts became more likely to reject when local parasitism risk was higher, and cues indicating a cuckoo nearby shifted the threshold in the predicted direction.

• A model fitted to one well-studied host–parasite system; the parameter values are not general.
• Host perceptual limits and decision thresholds are hard to separate from behaviour alone.

• Brood parasitism

  A host accepting a well-matched egg is not necessarily fooled. Rejecting risks throwing out its own egg, so where parasitism is rare, accepting pays — and the same host rejects more readily where cuckoos are common.