**Abstract**

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in both the central nervous system and peripheral nervous system and play a crucial role in modulating cellular activity and function. While these receptors have been extensively studied in mammals, their presence and role in avian species remain a relatively unexplored area of research. Nonetheless, several studies have suggested the existence of multiple functional muscarinic receptors in various avian species, including the vestibular periphery of pigeons, retinal cells, intestinal smooth muscles, dorsal root ganglia, developing hearts in chickens, and avian salt glands. Despite this, only the M2-M5 subtypes have been characterized, except for some studies that suggest the existence of functional M1 receptors in avian species, such as in the dorsal root ganglia, retina, heart, and vestibular periphery. In this paper, we review the distribution of avian muscarinic receptor subtypes, the characterization of muscarinic acetylcholine receptors in various organs and organ systems, and the sequence similarity of mAChR 2 and mAChR 3 between various birds and animals. Given the current gaps in our understanding, more research is needed to investigate further the function and expression of mAChRs in avian species.

**Keywords:** avian, muscarinic receptors, acetyl choline, birds, update, homology modeling, phylogeny tree, organs
