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

Step 1: Understanding the Question.
We are told that cuckoo eggs and great reed warbler eggs look alike in colour and markings, and asked to explain why this resemblance exists.

Step 2: Key Concept or Approach.
The common cuckoo is an obligate brood parasite. Instead of building its own nest and raising its own chicks, a female cuckoo lays her eggs directly into the nests of host species, such as the great reed warbler, and lets the host parents raise the cuckoo chick as if it were their own. Because the host can reject an egg that looks obviously foreign, natural selection has driven a coevolutionary arms race: hosts get better at spotting odd eggs, and cuckoos get better at mimicking the exact colour and markings of the host's own eggs so their egg escapes detection.

Step 3: Detailed Explanation.
Option (A) claims the two species are closely related and belong to the same genus. This is factually wrong: the common cuckoo belongs to the cuckoo family Cuculidae, while the great reed warbler belongs to the warbler family Acrocephalidae, so they are not close relatives and do not share a genus. Shared ancestry is not why their eggs look alike.
Option (B) claims the two species compete for the same insect larvae to feed their chicks. Even if some dietary overlap exists, competition for food has nothing to do with why an egg's colour and markings would evolve to match another species' egg pattern, so this does not explain the similarity.
Option (C) says the common cuckoo is a brood parasite of the great reed warbler. This is exactly the mechanism that explains the resemblance: selection pressure from the host's egg-rejection behaviour has pushed cuckoo eggs (in this particular cuckoo lineage, or gens, that specialises on great reed warblers) to mimic the host's egg appearance closely, so the warbler parents fail to notice and reject the foreign egg.
Option (D) claims the great reed warbler is a predator of the common cuckoo. Warblers are small insectivorous songbirds, they do not prey on cuckoos, and even if they did, being a predator of a species has no logical link to why that species' eggs would resemble the predator's own eggs.

Step 4: Final Answer.
The egg mimicry exists because the common cuckoo is a brood parasite of the great reed warbler, and matching the host's egg pattern helps the parasite's egg avoid detection and rejection.
\[ \boxed{\text{The common cuckoo is a brood parasite of the great reed warbler}} \]