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Eurasian Hoopoe Preen Gland Showed the Hallmarks of a Specialized Symbiotic Organ

Two photographs of a juvenile Eurasian hoopoe at a nest-box entrance
Research: Disease, parasites & microbiology, Reproduction & development

A 2026 study of the Eurasian hoopoe (Upupa epops) concludes that its uropygial, or preen, gland has the hallmarks of a specialized symbiotic organ: a structure that provides favourable niches for beneficial bacteria while keeping them compartmentalized away from living host tissues. The researchers compared wild breeding populations in southern Spain and southwestern Switzerland, combining microbiome analyses with detailed studies of the gland’s internal structure.

They analysed 342 samples of uropygial secretions and eggshells from breeding females and nestlings, collected in Spain in 2010 and Switzerland in 2014. Bacterial communities in the gland secretions were more similar between the Spanish and Swiss populations than the bacterial communities on eggshells, which are more exposed to the surrounding environment. The authors interpret this as evidence that the gland helps select and maintain a particular community of symbiotic bacteria.

The gland’s anatomy isolated bacteria from living tissue

The anatomical study used glands from nine birds found killed by predators in the wild and eight that died accidentally in a captive breeding population. Histology and fluorescence microscopy showed that in breeding females and nestlings, the gland was strongly compartmentalized by specialized tissue, with bacteria concentrated in particular internal niches rather than entering the surrounding tissues. These structural features were absent in males and non-breeding females that lacked the seasonal symbiosis.

Diagram of the hoopoe uropygial gland, symbiotic bacteria and secretion on eggshells
Structure and proposed function of the hoopoe uropygial gland. Nesting females apply the dark secretion to the eggshell, while bacteria are compartmentalized within the gland. Figure 1, unchanged: Martín-Vivaldi et al. (2026), CC BY 4.0.

The secretion carried antimicrobial symbionts

The dark secretion produced by breeding females and nestlings contains a complex bacterial community that includes antimicrobial-producing symbionts. These bacteria produce compounds that inhibit other microbes, helping explain why cultivating them can be useful to the host. Females spread the secretion over their eggs.

Sources and image credit

  1. Martín-Vivaldi, M., Martínez-García, Á., Peralta-Sánchez, J. M., Schaub, M., Arlettaz, R., Martín-Platero, A. M., Martínez-Renau, E., Barón, M. D., Ruiz-Rodríguez, M., López-Hernández, E., Martínez-Bueno, M., Valdivia, E., & Soler, J. J. (2026). The uropygial gland of the European hoopoe as a symbiotic organ. Animal Microbiome, 8, 55.

Image: Charles J. Sharp / Sharp Photography / Wikimedia Commons. Two photographs of a juvenile Eurasian hoopoe in a nest box at Kondor Tanya near Kecskemét, Hungary, 2 September 2019. Representative species image; not from the study. Cropped. CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/). · CC BY-SA 4.0 · Source

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