Cuckoos versus reed warblers: adaptations and counteradaptations

Model eggs of varying appearance were placed in wild reed warbler nests and the host response — acceptance, ejection or nest desertion — recorded. Egg appearance, timing of the placement, and whether an adult cuckoo had been seen near the nest were varied across trials.

Studied in

Cuculus canorus, Acrocephalus scirpaceus

Sample size

model eggs placed in wild reed warbler nests across seasons

Hosts rejected eggs that differed from their own and accepted closely matching ones, with rejection rates rising as the mismatch increased. Rejection was also more likely when a cuckoo had been seen near the nest, indicating that the decision uses more than the egg itself.

• One host species in one region; hosts elsewhere differ substantially in how readily they reject.
• Model eggs are not cuckoo eggs, and match human-assessed appearance rather than avian visual perception.

• Putting a forged egg in a real nest

  Cuckoo eggs resemble the eggs of their hosts. Is that resemblance doing anything, or does it just look striking to us?

• Brood parasitism

  Hosts reject eggs that do not match theirs, and accept ones that do. That pressure is what produces cuckoo eggs resembling a particular host’s — and it can be tested by putting model eggs in real nests.