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Six European Woodpeckers Shared a Common Eyelid Plan but Showed Microscopic Variation

Great spotted woodpecker foraging under a branch in Wrocław, Poland
Research · Anatomy & morphology

A Polish comparative anatomical study of six European woodpeckers found the same basic microscopic organization in the upper and lower eyelids, alongside descriptive variation in epithelial thickness, skin folds, melanin distribution and parts of the nictitating membrane. The study included gray-headed woodpecker (Picus canus), European green woodpecker (Picus viridis), black woodpecker (Dryocopus martius), great spotted woodpecker (Dendrocopos major), middle spotted woodpecker (Dendrocoptes medius) and lesser spotted woodpecker (Dryobates minor).

The researchers examined the upper eyelid, lower eyelid and nictitating membrane — the mobile third eyelid that can sweep across the eye — in 40 wild woodpeckers found dead in forest and parkland habitats between 2020 and 2023. Great spotted woodpecker accounted for 18 birds and European green woodpecker for 17; lesser spotted woodpecker was represented by two, and gray-headed, black and middle spotted woodpeckers by one each. The tissue was studied with routine histology—microscopic examination of thin tissue sections—and histochemical stains that make particular tissue components easier to distinguish.

The lower eyelid combined mechanical support with local immune tissue

Across all six species, the outer eyelid surface was keratinized and the conjunctival surface contained mucus-producing goblet cells. The lower eyelid contained a compact fibrous tarsal plate that provides structural support. It also contained conjunctiva-associated lymphoid tissue (CALT), made up of lymphoid nodules or aggregates and diffuse lymphocytes. The authors interpret this tissue as a possible site of local immune surveillance on the ocular surface.

The nictitating membrane had a folded organization, with regional variation in epithelial structure, pigmentation and the shape of its leading edge and folds. Together with the eyelids, these structures could contribute to tear-film distribution, removal of small particles and mechanical protection when woodpeckers climb, drill and forage on tree trunks.

Small samples make the species differences tentative

The study provides the first comparative microscopic baseline for these ocular structures in European woodpeckers, but the authors are careful not to turn the observed differences into fixed species traits. The strongly unequal sample sizes — especially the species represented by only one or two birds — mean the interspecific differences should be treated as descriptive observations from the examined material.

Sources and image credit

  1. Klećkowska-Nawrot, J., Kroczak-Zdańkowska, A., Urantówka, A., Zaniewicz, G., Chrószcz, A., & Poradowski, D. (2026). Histological Diversity of Eyelids and the Nictitating Membrane in Six Woodpecker Species (Picidae). Veterinary Sciences, 13(7), 702. https://doi.org/10.3390/vetsci13070702Primary open-access article published 17 July 2026 under CC BY 4.0. Forty post-mortem wild woodpeckers from six European species were examined histologically. Great spotted woodpecker contributed 18 birds and European green woodpecker 17, while three species were represented by one bird each. The study documented a common eyelid plan, lower-eyelid fibrous tarsal support and conjunctiva-associated lymphoid tissue, plus descriptive variation in the nictitating membrane and other microscopic features.

Image: hedera.baltica / Flickr / Wikimedia Commons — great spotted woodpecker (Dendrocopos major) foraging under a branch in Wrocław, Poland, on 12 February 2021. Representative image of one of the six focal species; not a bird examined in the 2026 histology study. · CC BY-SA 2.0 — https://creativecommons.org/licenses/by-sa/2.0/ · Source

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Anatomy & morphology

SPECIES IN THIS STORY

Species in this story

Black Woodpecker Dryocopus martius Explore species Lesser Spotted Woodpecker Dryobates minor Explore species Great Spotted Woodpecker Dendrocopos major Explore species

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