Limiting factors and breeding success of semiaquatic and aquatic birds in the conditions of mountain-floodplain water regime in the Selenga river delta (Eastern Siberia)

The Selenga river delta is characterized by the specific living conditions of semiaquatic and aquatic birds. Based on many years of work (1972-1985), we have revealed the limiting factors determining the breeding success of the birds in the Selenga river delta (Lake Baikal). Valley of the river has a well-defined mountain-floodplain water regime. Very strong changes in water levels during one nesting season between the periods of laying and hatching lead to significant loss of clutches, which decrease the breeding success of birds. Only the use of special adaptations (sideways building, repeat breeding and high dynamic spatial structure) allows birds to maintain a sufficient level of breeding. We revealed the peculiarities of bird breeding during the small (11-year) intra-century climate cycle in 1973-1982 on a key site in the central part of the Selenga river delta with an area of about 100 km2. To determine all the parameters of the nesting cycle of birds, we used nest control at discount areas of different sizes and configurations, but not less than 1.0 ha, which allowed recording the death of eggs in clutches and the emergence of new nests. The article identified the main limiting factors that determine the success of nesting season of different bird species in the conditions of mountain-floodplain water regime. In general, the reproductive success of colonial species is higher than that of non-colonial birds, an average of 9.0%. The types of adaptation to nesting used by these groups under unstable environmental conditions, despite significant differences in ecology, are the same. However, in colonial species they work more efficiently. The total death of eggs in different species by season was 29.8-75.8% and was associated with the size of a particular species and the characteristics of its ecological niche. The main adaptations of colonial bird species during nesting under extremely dynamic environmental conditions are nesting in small colonies, characterized by high synchronization of reproduction and high dynamic spatial structure. Large colonies can be quickly formed in areas optimal for nesting, since the groups of nesting birds that survived during the flooding of the territory are at the same time a kind of “information centers”. Such surviving colonies advertise the most productive sites, which are the centers of “feed activity” and at the same time facilitate the search for places that ensure successful nesting. Coloniality of birds in freshwater bodies of water is an effective reproductive strategy, ensuring the use of plentiful, but unevenly distributed and constantly moving feed objects in space.