The plasticity of the digestive system (DS) of birds allows them to use a wide range of feeds, which is especially important for migratory birds. Some fatty acids (FAs) included in the spectrum of polyunsaturated FAs (PUFAs) are not synthesized in the bird organism, and are supplied only with food. They determine the level of unsaturation of lipids, and are essential for the organism. Among other important functions of these FAs are energy: they affect the energy metabolism of muscle cells. This is what determines the demands of migratory birds to the fodder base of migration stops. The largest number of general lipids among investigated species of invertebrates living in the Azov-Black Sea region ponds is found in the organisms N. zonata, A. salina, H. diversicolor, I. baltica (4.6–4.0%). The highest content of PUFAs was revealed to be present in the lipids of mollusks H. acuta and Th. astrachanicus: 32.87–35.73 % of the total amount of FAs. The content of PUFAs in the organism of Chironomis depends on the degree of water salinity. The unsaturation coefficient of FAs (K1) is revealed to be the highest in Mollusk lipids (H. acuta — 1.361; Th. astrachanicus — 1.610) and some Polychaeta. These types of invertebrates are the main source of intake of essential PUFAs by the organism of the waders at migratory stops in the Azov-Black Sea region.
The results of the macro-micromorphological structure of the digestive system of the waders and comparative analysis with the migrants of the Aves are presented. It was revealed that the digestive system of waders at the anatomical level has a universal structure typical for representatives of class Aves. As a result of histological studies of the structure of the digestive tract, it was found that the feature of the wall of the waders small intestine is the dense location of the crypt in its own plate of the mucous membrane throughout its length. High proliferative capacity of cambial crypt cells and their multilayered location provide high secretory and regenerative activity of enterocytes, which helps to restore the mucous membrane and intensify the digestive processes, especially during the active feeding of the waders at the migration stopover points. At this time, the length and mass of the intestine, the mass of the stomach and the liver increase, what is considered as a reaction to a large number of feeds in the intensive feeding of birds and indicates the plasticity of their digestive system. It is shown that the change in the morphometric parameters of the waders digestive system organs depending on the migration situation is an integral part of the adaptive mechanism of the migratory birds, which provides the basic need of the organism - fat accumulation. Th e content of general liver lipids, abdominal fat and thoracic muscles in 6 species of tundra warblers with varying degrees of fat accumulation at the migration stopover points in the Azov-Black Sea region was studied.