Morphological description of the alimentary canal and adnexal glands in Amazilia tzacatl, Amazilia saucerottei, Amazilia amabilis and Anthrachotorax nigricollis species.

The hummingbird family (Trochilidae) includes the smallest and most metabolically active vertebrates. They have a high energy demand because of their extraordinarily high metabolic rates during hovering while looking for food. The morphology of the digestive apparatus is related to the feeding habits of the species. The anatomy and histology of the digestive apparatus in these birds have not been thoroughly described except for their tongue. Therefore, this study aimed to describe the gross anatomy and histology of the alimentary canal and adnexal glands in four species from the hummingbird family: Amazilia tzacatl (n = 2), Amazilia saucerottei (n = 1), Amazilia amabilis (n = 1) and Anthracothorax nigricollis (n = 1). The alimentary canal was found to be very short. The epithelium of the oesophagus and crop showed variable degrees of keratinization and parakeratotic areas as normal conditions. A dorsal crop was observed as a differential characteristic of these birds. Like other birds, the ventricular mucosa in hummingbirds was covered and protected by the cuticle and showed a tunica muscularis constituted by three muscle layers. There was no isthmus between the proventriculus and ventriculus. The intestine presents a well-differentiated duodenum and jejunum. However, no ileum nor caeca were identified. The intestinal villi length, base width, crypt depth and area showed differences among the specimens studied among the small and large intestines. In addition, variations in thickness were observed in the smooth muscle tunica along the intestine. In all the studied species, the liver was composed of two lobes (right and left), and no gall bladder was observed during gross inspection or in histological sections. Finally, the pancreas was observed as a diffused organ forming islets related to all the small intestines. Some anatomical differences were observed among the studied species, mainly concerning Anthracothorax nigricollis. Hummingbirds showed very interesting and distinctive morphological characteristics. Hummingbirds possess unique and intriguing morphological characteristics. Future comparative studies related to the anatomy, histology and function of the digestive apparatus of hummingbirds are required. Expanding our understanding of the digestive morphophysiology in these bird species is crucial. However, it is necessary to conduct more comprehensive studies encompassing a wider range of hummingbird species and including a larger number of individuals to obtain more conclusive findings.