TEXEIRA R (2014) Rise and fall of a breeding population of Black Swan Cygnus atratus in the Netherlands; an invasive alien species?. LIMOSA 87 (4): 185-192.

This paper describes the rise and fall of a regional population of non-native Black Swans near Bergen op Zoom, NoordBrabant, The Netherlands. The main breeding areas are shallow freshwater lakes in reclaimed former estuarine areas and marshy fens on the higher, sandy plateau of the Brabantse Wal. In summer the swans feed on submerged vegetation and in winter on low lying grasslands in polders. Most nests were found in spring, but sometimes young were seen far into autumn; such late breeding attempts are easily missed in regular breeding surveys.
The first Black Swan was seen near Bergen op Zoom in 1978, but it took 10 years before two were seen together and until 1991 before breeding was established. The breeding population then grew rapidly to some 20 pairs in 1995, whereafter it more or less stabilised until 2005-2007, forming 25-30% of the national population. At that time a rapid decline set in and in 2013 no breeding pairs were found anymore. Of the 274 breeding pairs found in total, 175 occurred in the Markiezaat, a protected reclaimed area.
The majority of the confirmed breeding attempts was found here as well, the last in 2009. The number of wintering birds around Bergen op Zoom was found to peak between 2005 and 2009, when the breeding population was already declining, although the figures are incomplete. Numbers of wintering Black Swans in The Netherlands as a whole also peaked from 2005 to 2009.
The most likely cause of the decline of the Black Swan around Bergen op Zoom and in The Netherlands as a whole is lack of adaptation to winter weather. Other possible factors are the nitrification of the lakes in the reclaimed areas, which reduces water clarity and the growth of submerged vegetation, low reproductive success, disturbance by farmers on grassland in winter, and probably to a lesser extent illegal hunting and lack of adequate anti-predator strategies. Based on this study it is probably more appropriate to speak of a formerly invasive introduced species, than of an invasive alien species as is done in a recent review of exotic bird species occurring in the Netherlands.

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