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Red-vented Bulbuls Rejected Large Foreign Eggs but Accepted Most Host-sized Eggs

Red-vented bulbul perched in natural vegetation
Research · Behaviour

Red-vented bulbuls (Pycnonotus cafer) in Pakistan use several cues when responding to brood parasitism, according to an experimental study published on 2 February 2026. The species is a common host of the Jacobin cuckoo (Clamator jacobinus), which lays its eggs in the nests of other birds.

Researchers experimentally introduced foreign eggs and nestlings of different sizes into bulbul nests to test how the hosts distinguished them from their own offspring.

Large foreign eggs were consistently rejected

The bulbuls accepted most foreign eggs that were similar in size to their own, but consistently rejected large eggs. The authors conclude that egg size acts as an important recognition cue for this host species.

Nestling responses were linked more to skin colour than size

Responses after hatching were different. Nestling discrimination appeared to depend more on skin colour than on size: nestlings whose skin colour did not match the host young were left unfed. The authors explicitly caution, however, that this part of the experiment was based on a small sample and should therefore be interpreted carefully.

Hatching order also affected the response

In a few experiments, bulbuls rejected their own clutch when a foreign egg hatched first. The researchers therefore identify hatching order as another factor that can affect host responses during brood parasitism.

Whether parents were present in the nest also affected rejection behaviour. The birds did not rely on a single recognition rule throughout the breeding cycle; visual, temporal and behavioural cues all influenced their response.

Sources and image credit

  1. Sadam, A., Hu, H., & Liang, W. (2026). Egg recognition and nestling discrimination in a cuckoo host. Biological Journal of the Linnean Society, 147(2), blaf146. https://doi.org/10.1093/biolinnean/blaf146Primary peer-reviewed research article, published 2 February 2026. Red-vented bulbuls accepted most host-sized foreign eggs but consistently rejected large eggs; nestling discrimination appeared linked to skin colour but was based on a small sample, and hatching order also affected host response.

Image: Mildeep / Wikimedia Commons — red-vented bulbul (Pycnonotus cafer) photographed in Nepal on 19 October 2020. Illustrative photograph, not a bird from the experiment. · CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ · Source

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