Anatomical and histomorphological characterization of the gastrointestinal tract (esophagus, stomach, and ileum) of the shining sunbird (Cinnyris habessinicus): A paradigm for the nectarivorous niche in sunbirds.

The shining sunbird (Cinnyris habessinicus) is a specialized nectarivore with unique gastrointestinal adaptations to its dietary habit. This study aimed to characterize the morpho-functional features of the gastrointestinal tract of C. habessinicus using gross anatomical, histological, histochemical, morphometric, and scanning electron microscopy (SEM) techniques. Morphometric analysis revealed regional variations in tunics thickness: the esophagus had the thickest tunica serosa (55.67 ± 2.4 µm), the proventriculus had the thickest tunics mucosa (884.55 ± 13.05 µm), and the gizzard had the thickest tunics submucosa and tunica muscularis (47.48 ± 3.5 µm and 526.33 ± 20.55 µm, respectively). Anatomically, the esophagus was situated on the right side of the neck and showed prominent mucosal infoldings. Numerous oval-shaped mucous glands were identified within the esophagus intraepithelial layer, at the mucosa-submucosa interface, and in the submucosa, exhibiting strong reactivity for acid mucins. The proventricular mucosa contained superficial and deep polymorphic glands. The ventriculus was thin-walled and was internally lined by a keratinized koilin membrane. The ileum featured numerous tongue-shaped villi composed of absorptive enterocytes and goblet cells surrounding central lacteals. SEM analysis further revealed the surface ultrastructure of the alimentary canal: the esophagus had leaf-like mucosal folds with glandular openings; the proventriculus contained dome-shaped glandular lobules and scattered gastric orifices; the ventriculus showed undulating mucosal ridges and numerous glandular pores; and the ileum presented radially arranged pyramidal villi and mucous orifices. Histochemical staining demonstrated regional differences in acid mucin distribution and collagen localization. In conclusion, the structural and histochemical results provide new information about the nectarivorous feeding strategy of C. habessinicus by revealing its intricate gastrointestinal adaptations.