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Kestrels Had More Densely Packed Mucous Glands Than Storks

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

Mucus keeps the lining of the airways moist and protected. But birds do not necessarily have the same density of glands producing it. Researchers in Türkiye found roughly twice as many mucous glands in an equal area of windpipe lining in common kestrels (Falco tinnunculus) as in white storks (Ciconia ciconia). Long-legged buzzards (Buteo rufinus) fell between the two.

The researchers examined windpipe and lung tissue under a microscope from three adult males of each species. The birds had arrived at a rehabilitation centre with severe wing injuries and had been euthanised because they could not regain their ability to fly.

More glands in the birds of prey

The difference concerned small clusters of mucus-producing cells within the epithelium, the cell layer lining the windpipe. In an area of 0.1 square millimetres, researchers counted an average of 11.3 glands in common kestrels, 7.7 in long-legged buzzards and 5.7 in white storks.

All three species also had individual mucus-producing cells, known as goblet cells, which were excluded from the gland counts. These individual cells were more prominent in storks. The difference therefore does not mean that stork airways lack protective mucus.

The glands in all three species contained both acidic and neutral mucins, substances that form part of mucus. The windpipe itself had a lining bearing tiny cilia and was supported by C-shaped cartilage rings.

Similar lungs, different ways of life

The lungs also shared the same basic structure. Air travels through bronchi into smaller tubes called parabronchi. Small chambers called atria lead from these towards the extremely fine air capillaries. Here, air and blood come close enough for oxygen and carbon dioxide to be exchanged.

The researchers found cells resembling type I pneumocytes, which form the thin barrier between air and blood. Around the atria, they also found cells resembling type II pneumocytes. These produce surfactant, a substance that lowers surface tension on the lung’s inner surface. The birds therefore shared both the basic lung structure and cell types despite differing in windpipe gland density.

The researchers suggest that more glands in the birds of prey may be linked to breathing rate or exposure to substances that irritate the airways. They also point to the stork’s energy-saving gliding flight during migration and its use of bill-clattering for communication as possible parts of the explanation.

These links remain hypotheses. The study reveals tissue differences in the nine birds but did not measure how those differences affect breathing or airway protection.

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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