The Comparative Anatomy of the Upper Digestive System in 26 Species of Zoophagous-Polyphagous Palearctic Birds

Birds are the most diversified organisms on Earth, with species covering various niches in each major biome, being essential to understand the modern ecosystem. This study concentrates on the diversification of the anatomical structure of the upper digestive tract for 26 species of zoophage-polyphagous birds and the anatomical differences in the digestive system to reveal aspects related to their evolution and diversification. The trophic spectrum of the selected birds includes several categories of food, or, as in the case of strictly carnivorous birds, to a single food category. After performing the dissections, the digestive tract was separated from the carcass and each digestive segment was measured and analysed. In this study, it was demonstrated that the birds' feeding behaviour influence the macroscopic particularities of the digestive system, more visible in the cranial portion (oropharyngeal cavity, esophagus, proventriculus and gizzard), with little descriptive information in the literature. The tongue is poorly developed and immobile in piscivorous birds, while the tongue of insectivorous birds is long and moves considerably away from the tip of the bill. The esophagus was stretchable and presents longitudinal folds on its entire surface in piscivorous species and not extensible in insectivorous birds.