Question

The colour and markings of the eggs of the common cuckoo (Cuculus canorus) are similar to those of the eggs of the great reed warbler (Acrocephalus arundinaceus). Which one or more of the following options explain(s) this similarity?

• A. The two species are closely related and belong to the same genus
• B. The two species compete for the same insect larvae to feed their chicks
• C. The common cuckoo is a brood parasite of the great reed warbler
• D. The great reed warbler is a predator of the common cuckoo

Hint:

Ask which relationship would give a bird an evolutionary reason to copy another species' egg pattern; only a parasite laying its egg in the host's nest needs that egg to pass as the host's own.

Answer 1

The Correct Option is C

Cuckoo eggs and great reed warbler eggs look alike, and the question asks which relationship between the two birds would actually cause that kind of egg mimicry to evolve.

1. Closely related, same genus: the common cuckoo sits in the cuckoo family, while the great reed warbler sits in a completely different songbird family. They are not close relatives, so shared genes from a recent common ancestor cannot be the reason their eggs match.
2. Compete for the same insect larvae: sharing a food source might affect population numbers or foraging behaviour, but it gives no evolutionary reason for one species' egg pattern to start resembling the other's egg pattern.
3. Common cuckoo is a brood parasite of the great reed warbler: this fits perfectly. The cuckoo sneaks its egg into the warbler's nest and needs the warbler parents to accept and raise it. Any cuckoo egg that looks too different gets spotted and thrown out, so over many generations only cuckoo eggs that closely mimic the warbler's own egg colour and pattern survive to be raised. That selection pressure is exactly why the two eggs end up looking alike.
4. Great reed warbler preys on the common cuckoo: warblers eat insects, not other birds, so this predator relationship does not exist, and even a real predator-prey link would not explain why one species' eggs mimic the other's.

Only the brood parasite relationship creates a real selective pressure for egg mimicry: the cuckoo's reproductive success depends on the host failing to notice the foreign egg, so cuckoo eggs are pushed toward looking like the host's own eggs generation after generation.

Let's summarize:

• Egg mimicry needs a reason for one species to benefit from copying another's egg pattern.
• Only brood parasitism, where the cuckoo's egg must fool the host into raising it, supplies that reason.

So the similarity in egg appearance is explained by the common cuckoo being a brood parasite of the great reed warbler.