KLEYHEEG E, DIRKEN S †, VAN BEUSEKOM R, EGGENHUIZEN T, JONKERS D, KOFFIJBERG K, MAJOOR F & NAGTEGAAL J (2020) Moulting Ruddy Shelducks Tadorna ferruginea in the Netherlands: numbers, origin and ecology. LIMOSA 93 (1): 1-14.

The Ruddy Shelduck has experienced rapid increases in the Netherlands during summer, when birds congregate for wing moult. In 2018 the four key moulting sites combined hosted 1935 individuals (Fig. 1), which is in sharp contrast to the rather small feral population of 10-30 breeding pairs in the Netherlands (2013-2015). Speculations that such large moulting numbers could originate from the native breeding range in Southeast Europe or Central Asia initiated a large-scale colour-marking program, including the use of GPS-GSM transmitters (Tab. 1). Sightings and tracks of marked birds revealed that the majority of moulting birds originated from two distinct breeding populations, one in Northrhine-Westphalia in Germany and one in Bavaria, Baden-Württemberg (Germany) and Switzerland (Fig. 2). Both populations established from escaped birds and according to our data the size of the breeding population must be much larger than previously thought. According to our conservative estimates, the Dutch-German breeding population of Ruddy Shelduck consisted of at least 3600 individuals in 2018. Ruddy Shelducks arrive at Dutch moulting sites shortly before the onset of primary and secondary moult and depart quickly after completion of wing moult (Figs. 4 and 5), indicating that they undertake migrations to specific moulting sites. Some individuals returned to the moulting sites annually, while others exchanged with a moulting site at Lake Constance in southern Germany and other sites in the Netherlands (Fig. 7). The four key moulting sites in the Netherlands have in common that they are situated in large shallow freshwater bodies, all characterized by little human disturbance, large congregations of other waterbirds, small islands or dams to rest on, and usually also with extensive stands of submerged macrophytes. The establishment of Dutch moulting sites and the distances birds travel to reach them suggest that the availability of suitable sites is limited in Northwest and Central Europe. So far, we have been unable to find a connection between Dutch moulting sites and wild breeding populations farther east. Our study does, however, show that this feral population has developed a fascinating natural behaviour of collective movements in an important part of their annual cycle.

[pdf only for members] [dutch summary]