VAN DIJK AJ (2023) Breeding Whooper Swans Cygnus cygnus in the province of Drenthe closely followed: observations on distribution, breeding and moulting in 2005-2021. LIMOSA 96 (1): 17-19.

In 2005 the first Whooper Swan Cygnus cygnus pair bred in the Netherlands, in a small marshy lowland peat area near Wapserveen, in the province of Drenthe (52o50’N, 6o12E’). Since then, 1-2 pairs bred annually. In 2006, 2007, 2011 and 2017 breeding was successful and fledged cygnets were ringed with colour rings (except in 2006 when young were not ringed). In 2010 the breeding pair was replaced by a pair consisting of a three year old female, hatched in Wapserveen in 2007, and an unknown (unringed) male. In 2009, the composition of the couple changed again. The male stayed but paired with his own daughter born in Wapserveen in 2017. In years the pair bred successfully, they stayed year-round at the breeding site (Wapserveen), an area of 40 km2 in the brook valley of the Vledder, Wapserveense and Steenwijker Aa. After unsuccessful breeding, summer displacements were observed 22 km to the northeast to the wetland Diependal near Smilde, but the birds returned to Wapserveen to spend the winter. In 2012 a second pair settled in the nature reserve Drents-Friese Wold. The male of this pair had hatched in Wapserveen in 2007, the female was of unknown origin (unringed). In 2012-18 this pair built nests and in two years 1-2 eggs were laid. Only in 2018 the pair produced one young at an unknown nesting site. In 2020-21 a new pair settled in the peat moor area of Fochteloërveen and in 2021 it successfully raised four chicks.
Breeding was mainly been studied in Wapserveen. The territory was occupied in March and between 17 March and 12 April yearlings left the breeding pair. Mute Swans Cygnus olor were successfully chased away from breeding area up to at least 900 m from the nest and in 2005-18 seven Mute Swan nests were abandoned. On the basis of this behaviour the territory was determined at c. 400 ha. Building or rebuilding of nests took on average 12 days (Tab. 1). Incubation started between 25 March and 9 May (median date 16 April) and the average incubation period was 35.0 days (SD=3.0, N=12). Only the female incubated. Average clutch size was 5.4 (SD=1.6, N=13). Egg dimensions were 112.4 x 70.6 mm (range 105.3-118.2 x 68.2-72.1, N=30; excluding two dwarf eggs 89.0 x 60.3 and 68.4 x 52.1 mm). Cygnets were cared for by both parents and reared in swampy grasslands near the nest site. During breeding and rearing the male almost daily made feeding trips during 10-150 (max. 180) minutes to nearby agricultural grassland or marshland or to bathe in a pool. In 2005- 20 an average of 2.3 (SD=2.0, N=34) chicks hatched in 15 clutches. Eleven clutches (73%) produced chicks. Of the 34 hatched chicks, only nine fledged (26.5%), a fledging success of 0.6 (SD 1.1). Causes for the low chick survival and long rearing period are not clear. Food quality could play a role, although the preferred food plants by Whooper Swans and their offspring, such as Water Horsetail, were widely distributed. Breeding success in 2005-09 was higher than in 2010-18, perhaps as the males in 2005-09 was more aggressive compared to the male in 2010-18. In addition, inbreeding could play a role in the low fledging success. Observations that support the idea of inbreeding include a cygnet with stunted wings in 2011, six unfertilized eggs in 2015-16 and two dwarf eggs in 2017. Inbreeding might occur as exemplified by the Wapserveen breeding pair consisting of a father and a daughter. Observations of ringed swans in Wapserveen indicate that eight of nine fledged young survived their first winter (89%), seven their second (78%) and six their third winter (67%).
In four successful breeding years the Wapserveen pair moulted primaries at or near the breeding site. Birds started their moult at the time the young had fledged on average on 30 August (SD=6.6, N=4, Tab. 2). Onset of moulting in females was 2-13 days ahead of males, but in 2011 moult periods overlapped completely. In unsuccessful breeding years moult started on average on 18 July (SD=18.7, N=11). The moult period of both partners on average was 37 days (SD=4.4, N=15). After moult the pair returned to the breeding site between 17 September and 2 December. The moulting site of the pair of Drents-Friese Wold is unknown.

[dutch summary]