The call of a common cuckoo is always a welcome, sound but our best-known summer visitor is far less popular with some of our resident birds, thanks to its devious breeding strategy.
The cuckoo is a brood parasite and lays its eggs in the nests of other birds, cleverly avoiding the time-consuming duties of nest-building, incubation and chick-rearing. In the UK, the most common hosts are dunnocks, meadow pipits, pied wagtails and reed warblers. A female cuckoo will stake out a prospective nest from a nearby vantage point, waiting for it to be left unattended before swooping in to deposit her own egg, which is patterned to mimic the eggs of her chosen host. The mimicry isn’t perfect but is usually good enough to fool the unsuspecting parents-to-be, and when the cuckoo chick hatches it will push all the other eggs and chicks out of the nest so that it has the undivided attention of its new foster parents. Meanwhile, the cuckoo’s real mother is busily depositing more of her eggs elsewhere.
Each cuckoo female specialises in one particular host species; she will only ever lay one type of egg, and she will always target her egg-laying appropriately – after all, an egg laid in the wrong host’s nest would not blend in, and would quickly be rejected. There are actually several different types of female cuckoo, and each type is a distinct evolutionary lineage; they all belong to the same species but are genetically coded to produce eggs that match a specific target host. The genes for egg pattern and colour are thought to be carried on the female chromosome, so they are passed down from mother to daughter regardless of who the female mates with.
This means that a ‘reed warbler’ cuckoo always produces eggs with a reed warbler pattern, and she knows which nests to target thanks to the process of imprinting. As a young chick, she will have learned to recognise the song and appearance of her foster mother; and as a returning adult she will seek out the nests of females that match this mental image, and lay her eggs accordingly.