<p>The current study aimed to identify and compare the histological and histochemical features of the duodenum in two bird species with different feeding habits: the carnivorous slender-billed gull and the grainivorous cockatiel. We utilized general histological techniques, including Hematoxylin and Eosin (H&amp;E), Mallory's triple stain (MTS), and Azan (AZ) stain, to examine the overall histological characteristics. Additionally, this study represents the first application of several histochemical techniques—bromophenol blue (BP), Sudan black B (SB), Alcian blue (AB), and periodic acid-Schiff (PAS)—to highlight the histochemical features of the duodenal cells and glands. The villi displayed varying numbers, lengths, and shapes between the two birds. In the cockatiel, the villi were covered with numerous simple columnar epithelial goblet cells. The components of the lamina propria were more diverse in the slender-billed gull compared to the cockatiel. The cockatiel also exhibited a lymphoid nodule due to the changing number of villi, while such nodules were absent in the slender-billed gull. Additionally, secretory glandular units were present beneath the villi, containing enteroendocrine cells. The cockatiel also had a thin, muscular layer. The histochemical analysis revealed positive AB and PAS responses in the intestinal epithelial cells and glands of both birds, indicating mucous production. Additionally, there was a weakly positive response for PB and a negative for SB in both birds, suggesting a lack of protein and fat secretion. Histochemical techniques reveal the duodenum's adaptation to two distinct bird feeding habits, indicating its crucial role in the digestion process of birds with varying dietary preferences.</p>