Final answer:
In birds, the end of the small intestine and the beginning of the large intestine is marked by the ileocecal valve, connecting the ileum to the cecum of the large intestine.
Step-by-step explanation:
The anatomic landmark that demarcates the end of the small intestine and the beginning of the large intestine in birds is the ileocecal valve (or sphincter). In avian anatomy, as in mammals, the small intestine consists of three parts: the duodenum, jejunum, and ileum. The ileum is the final segment of the small intestine and is thicker and more vascular than the jejunum. It connects to the large intestine at the ileocecal valve, which is at the junction with the cecum, marking the transition from the small to the large intestine, or colon.
This sphincter muscle allows the passage of digested material from the ileum into the cecum, while preventing backflow into the small intestine.