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Poorer Condition Delayed Moult in Common Bulbuls, While Diet Shaped Its Speed

Common Bulbul perched in Souss-Massa National Park, Morocco
Research · Physiology

A diet experiment in Jos, Nigeria, showed that different phases of moult in common bulbuls (Pycnonotus barbatus) depended on resources available at different times. Body condition shortly before moult influenced when feather replacement began, whereas diet during moult strongly affected how quickly the process was completed.

The researchers followed 40 adult wild-caught bulbuls housed in outdoor aviaries and compared them with free-living birds from the same area. Before the expected moult season, captive birds received either fruit or an invertebrate diet. The study used body mass as a practical proxy for body condition in this species. During the final three months before moult, the invertebrate-fed birds became 5.8% lighter on average than the fruit-fed group. They did not begin moulting on schedule; once their diet was changed to a mixture of fruit and invertebrates and their condition improved, moult began.

Condition just before moult influenced when it started

Condition during the final three months before moult mattered most for when feather replacement began. Birds left in poorer condition by the invertebrate diet started about 33 days later than fruit-fed birds. Condition 6–3 months before the expected moult, by contrast, did not clearly affect either the start date or the duration.

This gives the first part of the biological interpretation. Moult showed a capital-dependent component: reserves built up before feather replacement helped determine whether a bird was ready to start. The term does not mean that the entire moult was fuelled from stored reserves, only that condition accumulated before moult mattered for its initiation.

Nutrients acquired during moult influenced speed

Once moult had begun, the important factor changed. Fruit-fed birds started earlier but took about 185 days to complete moult. Birds switched from the invertebrate diet to a mixed fruit-and-invertebrate diet completed it in about 134 days — roughly 52 days faster — while free-living bulbuls finished in about 98 days on average.

The authors describe this as an income-dependent component: resources acquired while moult was actually under way helped determine the rate of feather replacement. Fruit alone provided enough energy and nutrients for feather growth, but the mixed diet, including animal protein, was associated with substantially faster moult. Free-living birds were faster still, showing that the experimental mixed diet did not reproduce all of the nutritional or environmental conditions available in the wild.

Together, the results argue against treating moult as purely a capital or purely an income process. In common bulbuls, stored condition immediately before moult helped set the start, while ongoing food quality helped set the speed. The authors also note that moult retained a strong seasonal schedule in captive birds even without breeding, suggesting that seasonal environmental cues and the availability of particular nutrients help organise this stage of the annual cycle in a tropical species that can breed year-round.

Sources and image credit

  1. Barshep, Y., Bitrus, K. Z., Tieleman, B. I., & Nwaogu, C. J. (2026). Diet-manipulated body condition affects onset and speed of moult in common bulbuls in a tropical environment. Journal of Avian Biology, 2026(1), e03522. https://doi.org/10.1002/jav.03522Primary open-access research article first published 23 January 2026. Forty adult wild-caught Common Bulbuls were housed in outdoor aviaries in Jos, Nigeria, under experimental diets and compared with free-living birds from the same area. Moult onset was best predicted by diet-manipulated condition shortly before moult, while moult duration was best predicted by diet during moult.

Image: Charles J. Sharp / Wikimedia Commons — Common Bulbul in Souss-Massa National Park, Morocco; representative species image, not a study bird · CC BY-SA 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

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