A total of 87 Mute Swan (Cygnus olor) faecal samples, 77 Bewick's Swan (Cygnus bewickii)
samples and 72 Whooper Swan (Cygnus cygnus) samples were analyzed. The materials were
collected in 2014-2019, during observations of spring migration sites on the southern shore of
the eastern part of the Gulf of Finland (from the Neva Bay to the Kurgalsky Peninsula). The Mute
Swan diet consisted mostly of soft plant food, particularly green macroalgae, and there was also
a significant proportion of diatoms found in the droppings. Other groups of macroalgae, stems
of pondweeds and thin root papillae of aquatic vascular plants were present in small quantities.
In contrast, diatoms were found in an extremely small proportion of the faecal samples
collected for the other swan species. The diet of these species consisted mainly of coarse forage,
such as rhizomes, and seedlings of Phragmites australis and Bolboschoenus maritimus, as well as
sedges and Juncus sp. The remains of thick reed rhizomes were found only in the Whooper Swan
samples, whereas the remains of reed seedlings were more common in the Bewick's Swans’
faeces. Seasonal variation in the diet was described. Following arrival at the end of February to
the first half of March, all species feed on the remains of Сladophora sp. from the previous
summer, which overwintered on the bottom in the form of algae mats. From late March to the
first half of April, reed root papillae and diatoms appear in the Mute Swan diet, and the
Whooper and Bewick's Swans feed on rhizomes and seedlings of reeds and other semisubmerged plants. Young growth of Stuckenia pectinata, Zannichellia palustris and other
submerged vegetation are consumed by all species, mainly in the second half of April — early
May.