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Common Cuckoos Removed Red Model Eggs in 92% of Recorded Choices

Common cuckoo perched on a cable
Research · Behaviour

Female common cuckoos (Cuculus canorus) strongly preferred red over green model eggs when choosing an egg to remove from experimental nests, according to a study published in March 2026. The work tested whether colour hue, rather than only brightness, influences the egg-removal behaviour that often accompanies brood parasitism.

The field experiments were carried out in 2022 in reed habitat at Sifangtuozi Farm in western Jilin, northeast China, where oriental reed warblers (Acrocephalus orientalis) are a main host of the cuckoo. Researchers ran 41 trials, placing red and green model eggs in experimental nests near active warbler nests and using video recordings to document the cuckoos’ choices.

Red eggs were removed far more often

The cuckoos preferred the red model egg in 92% of recorded choices. The researchers therefore conclude that females can use chromatic information — differences in hue — when deciding which egg to remove from a host nest.

Previous experiments had shown that common cuckoos can respond to differences in egg brightness, but the role of hue had not been directly tested. Red and green were chosen because natural eggshell colour is largely produced by two pigments, biliverdin and protoporphyrin IX, which contribute to variation along blue-green to brown-red colour ranges.

Visibility alone did not explain the preference

The team also used models of avian vision to estimate how conspicuous the two egg colours would appear against the nest lining. Those analyses made it unlikely that the red eggs were preferred simply because they were more visually conspicuous in the nests.

The authors therefore consider another possibility: the attraction to red may resemble the well-known tendency of some birds to respond strongly to red-coloured food. They present this as a hypothesis rather than a demonstrated explanation for the behaviour.

Sources and image credit

  1. Wang, L., Luo, H., Liang, W., Mari, L., & Šulc, M. (2026). Egg colour influences egg removal by common cuckoos (Cuculus canorus). Behavioural Processes, 238, 105367. https://doi.org/10.1016/j.beproc.2026.105367Primary research article. Red and green model eggs were presented in experimental nests near Oriental Reed Warbler nests in western Jilin; common cuckoos removed red model eggs in 92% of recorded choices. Avian visual modelling made higher conspicuousness an unlikely explanation, while attraction to red similar to responses to red food was proposed as a hypothesis.

Image: Ivan Vezenković — common cuckoo (Cuculus canorus), photographed 24 April 2024; Wikimedia Commons; representative species image, not a cuckoo from the Jilin experiment; cropped to 16:9 · CC BY-SA 4.0 · Source

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