Histological Diversity of Eyelids and the Nictitating Membrane in Six Woodpecker Species (Picidae).

Woodpeckers (Picidae) are exposed to repeated mechanical loading and particulate material during climbing, drilling, pecking, and foraging on tree trunks, but the microscopic organization of their eyelids and nictitating membrane remains poorly known. This study asked whether European woodpeckers share a common histological pattern of ocular adnexa and whether selected structural features may be interpreted in the context of ocular surface protection. The upper eyelid, lower eyelid, and nictitating membrane were examined in 40 birds representing six European woodpecker species using routine histological and histochemical methods. In all of these species, the eyelids showed a layered organization with a keratinized anterior palpebral surface, non-keratinized conjunctival surface with goblet cells, marginal zone, musculofibrous stroma, and no tarsal glands. Species-related differences were observed in the examined material in epithelial thickness, skin fold development, melanin pigment distribution, and marginal zone organization. The lower eyelid contained a compact fibrous tarsal plate and CALT represented by lymphoid nodules and diffuse lymphocytes, with morphologically identified HEV-like venules located within or close to lymphoid tissue. The nictitating membrane showed regional variation in the marginal plait, leading edge, epithelial surfaces, stromal pigmentation, and fold organization. These findings provide the first comparative histological baseline for the eyelids and the nictitating membrane of European Picidae and support the cautious interpretation that these structures may contribute to mechanical, mucosal, and immune protection of the ocular surface in woodpeckers. Because sample sizes were unequal, these interspecific observations should be treated as descriptive, sample-level findings rather than definitive species-level traits.