No abstract

The Black-headed Gull population in Latvia increased from 10,000 pairs in the early 1940s to 110,000 pairs in 1986, then decreased to about 40,000 pairs in 1994 and continues declining. The population changes have been more pronounced at coastal sites than at inland ones. Simultaneous changes in breeding success (a significant decline of average number of fledglings/pair since 1974), diet and feeding flights have taken place. Changes in the availability of anthropogenic food, appearance of new predators (mainly the American mink Mustela vison) and decrease of suitable nesting habitat are considered local reasons for the recent population decline.

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 km2to 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.

ABS1'RACTIn summer 1997 a project of duck research using radio-telemetry was carried out on the Lake Engure, Latvia. 10 females of Mallards, 10 females of Shovelers, 9 females of Pochards and 6 females of Tufted Ducks were radio -tracked during the period from the hatching of their ducklings till fledging. Information about effect of the harness-type radio-transmitters on different species of ducks has been collected. For 15 ducks home range sizes and distances between nests and range locations were calculated. Mallards used to move further from their nest sites; one Mallard brood after one week spent near the nest site dislocated 8 km to the Southern part of the lake during one day. Problems of estimating habitat use and habitat selection of duck broods have also been discussed.