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Common Kestrels Had About Twice the Tracheal Mucous-Gland Density of White Storks

Male common kestrel in flight in Türkiye
Research · Anatomy & morphology

Common kestrels (Falco tinnunculus) had about twice the density of intraepithelial mucous glands in the trachea as white storks (Ciconia ciconia) in a comparative study of respiratory histology — the microscopic anatomy of tissues. Long-legged buzzards (Buteo rufinus) were intermediate. The study compared the trachea and lungs of three adult males of each species in Türkiye.

Mucous-gland density differed strongly among the three samples

The clearest quantitative difference was in clusters of mucus-producing gland cells embedded within the tracheal epithelium. Mean density was 5.70 glands per 0.1 mm² in white storks, 7.67 in long-legged buzzards and 11.32 in common kestrels. These multicellular glands were distinguished from individual goblet cells, which also produce mucus.

Histochemical staining — chemical stains used to reveal particular substances in tissues — showed that the tracheal glands contained both acidic and neutral mucins. These mucus components contribute to the moist protective lining of the respiratory tract.

The basic tracheal and lung architecture was shared

All three species had a ciliated, pseudostratified columnar tracheal epithelium with goblet cells and C-shaped rings of hyaline cartilage supporting the airway. The lungs likewise followed the characteristic avian plan: primary and secondary bronchi led into parabronchi, atria and air capillaries, where gas exchange occurs.

Cells morphologically compatible with type I pneumocytes formed part of the blood–air barrier, while type II pneumocytes associated with surfactant production were identified around the atria. The overall lung organisation was therefore broadly similar despite the difference in tracheal gland density.

The functional explanation remains hypothetical

The authors suggest that the larger number of tracheal mucous glands in the two predatory birds, especially the kestrel, could relate to respiratory rate, environmental exposure or the need for mucosal protection. They also discuss whether the lower gland density in white storks could relate to gliding migration and the species’ reliance on bill-clattering rather than vocal communication.

These are hypotheses rather than demonstrated adaptations. The study measured anatomy, not respiratory performance, and included only three adult males of each species. Functional experiments and broader sampling would therefore be needed before the gland differences can be assigned a specific ecological cause.

Sources and image credit

  1. Ermutlu, D. G., & İlhan Aksu, S. (2026). Comparative histology of the trachea and lung in migratory and predatory birds: the white stork (Ciconia ciconia), long-legged buzzard (Buteo rufinus), and common kestrel (Falco tinnunculus). BMC Veterinary Research.Primary open-access Article in Press published 19 September 2026. Full ten-page article checked. Three adult males of each species were examined. Mean intraepithelial tracheal mucous-gland density was 5.70 per 0.1 mm² in white storks, 7.67 in long-legged buzzards and 11.32 in common kestrels; functional interpretations were explicitly presented as hypotheses.

Image: Adult male common kestrel (Falco tinnunculus) in flight in Seyhan, Adana, Türkiye, photographed 6 April 2020. Photo: Zeynel Cebeci. Source: Wikimedia Commons. CC BY-SA 4.0. Representative male; not one of the study birds. · CC BY-SA 4.0 · Source

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