FACTORS INFLUENCING THE NUMBER OF BREEDING WATER BIRDS IN LAKE ENGURE, LATVIA

Lake Engure has attracted great interest since the middle of the 19th century when the water level of the lake was lowered by construction of a canal. The lake area decreased from 90 km2 to about 45 km2, and favourable conditions were created for development of emergent vegetation (presently covers 58.6% of area). A nature protection regime was established in 1957. The lake was included in the Important Bird Area list in 1994, in the Ramsar lists in 1995, and as part of the Natura 2000 network in 2004. The present paper deals mostly with data collected by the Laboratory of Ornithology, Institute of Biology, in the period 1948–2011. Direct and indirect impacts of climate, human activities, water level, vegetation succession, anthropogenic and natural food and predation on water birds (coot, waders, ducks, larids and grebes), as well as relationships between these water bird groups, were demonstrated. The impact of vegetation succession via overgrowing of meadows with reed and bushes, merging of the mosaic of small stands of emergents into huge reed dominated massives (thus reducing areas of habitats suitable for water birds) is described. The relation of Black-headed Gull population size with availability/unavailability of anthropogenic food and the associated changes in breeding duck numbers, as well as predation structure are discussed.