Skrattmåsens Larus ridibundus populationsutveckling i Sverige under de senaste 25 åren

Abstract

Although starting its northward spread in Sweden earlier, the Black-headed Gull Larus ridibundus showed an almost explosive increase after the Second World War. However, during the last two decades the population has decreased strongly, especially in the agricultural districts of South and South Central Sweden, i.e. those areas which held the majority of the Swedish population. In this paper, data have been compiled by province from various sources (mainly papers and bird reports in local ornithological journals) in an attempt at estimating the size of this decrease and at finding any geographical pattern that might shed light on its cause, or causes. In the main agricultural areas, the decrease of the Black-headed Gull population has been in the order of 70% to 80%, on the island of Öland in the Baltic even more than 95%. Several reports indicate that the decrease started in the species' core areas in the mid-1970s when most of the large colonies diminished or disappeared, but somewhat later in less extreme agricultural areas. The reason for this recent decline is not known. Both deterioration of breeding habitat and predation by mink Mustela vison may explain a certain re-distribution of colonies from eutrophic lakes to offshore islands (and cities and industrial waste land); heavy predation by mink on gull chicks and subsequent abandonment of colonies have been reported. However, parents have abandoned chicks at colonies without documented mink predation, and chick starvation and poor fledging success have also been recorded, however most often in dry years.

Population trends for Swedish breeding birds

“…However, this shift in habitat has not compensated for the large-scale fall in its numbers, the reasons for which have not been fully established. Suggested factors are changes in agriculture, increased predation, diseases and deterioration of winter habitats (Källander 1996). The sharp decline of the Sedge Warbler over the 30year period is generally thought to be due to more adverse conditions during migration and in its winter quarters in Africa (Foppen et al 1999).…”

Häckfagelfaunan i östra Smålands ytterskärgård 1990—2000

“…Tidigare har det ansetts att det skett en förskjutning av skrattmåsens populationer från inlandet till kusten så att antalet kolonier har blivit flera längs kusten. Detta gäller såväl i östra Småland som en stor del av Sverige (Sieurin 1992, Källander 1996. Orsakerna kan bl.a.…”

“…Orsakerna kan bl.a. vara försämrade häckningslokaler (igenväxning och mänsklig påverkan), ökad predation samt födobrist under häckningstid med försämrad ungproduktion som följd (Källander 1996). 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Skrattmås.…”

Överlevnad och produktion hos skrattmås Larus ridibundus

“…I denna notis skall vi komplettera informationen med en analys av hur de tva nyckelfaktorerna, ungproduktion och overlevnad, paverkar bestandsutvecklingen . Det kunde konstateras att skrattmasbestandet kring 1995 pa manga hall i landet endast utgjorde 20% av vad det varit 20 ar tidigare (Kallander 1996a). Glutz & Bauer (1982) redovisar varden for overlevnaden under det fOrsta levnadsaret, d. v.s.…”