Taken by surprise: arctic conditions in the Wadden Sea increase mortality of wintering waterbirds and force most birds to escape further south

Arctic weather conditions during the last two weeks led to an almost completely frozen Wadden Sea, which may have severe effects on wintering waterbirds staying during this cold spell, as experts of the joint Dutch-German-Danish monitoring group of migratory birds (JMMB) expect.

Waterbirds suffer from bad food availability in combination with the very cold temperatures, which some will not survive. However, most birds managed to escape quickly after the cold spell started further southwest to west-European coasts to reach milder weather conditions.

In contrast to the previous years this winter started very mildly and without frost. Due to that fact large numbers of waterbirds like Barnacle Geese, Shelducks, Wigeons, Oystercatchers, Grey Plovers, Red Knots, Dunlins, Curlews and even Golden Plovers and Lapwings remained in the Wadden Sea and tried to overwinter. In addition, the recent storms pushed an unusually large number of Kittiwakes and Little Gulls to the Wadden Sea coast and some individuals of the very rare gulls like Glaucous and Iceland Gulls were observed.

This winter’s late sudden cold caught the birds by surprise. The cold spell started on 26th of January and within a few days the entire Wadden Sea from Denmark to the Netherlands was frozen. All higher tidal flats are covered now by a thick layer of ice and many birds concentrate on temporarily ice-free lower tidal mudflats, where they can feed. This caused the escape of the most birds from the Wadden Sea. Especially Geese and Wigeons, the vegetarians, flew to milder regions as well as shorebirds, which numbers dropped rapidly. Thousands of Dark-bellied Brent Geese arrived in the northern Wadden Sea in late January, but on the arriving date the low temperatures, ice and snow cover forced them to return back within a few days.

These extreme temperatures pose a danger for the birds. After the coldest night from 6th to 7th of February with temperatures below minus 15°C more than 90 dead shorebirds were found by volunteers of “Schutzstation Wattenmeer” at the coast of Schleswig-Holstein, among them 70 Oystercatchers. This shows that quite a number of birds did not find enough food and did not have any fat reserves left and, therefore, froze to death. Normally, birds with good body conditions and fat reserves should survive minus 15°C easily as well as arctic periods of several weeks.

If the cold spell continues, it is expected that more winter mortality among birds may occur. Especially for Oystercatcher this would be another negative impact on its population level, which dropped by almost 50% within the last 20 years. This species has suffered limited food stocks in large parts of the Wadden Sea as a result of shellfish fisheries in the Dutch Wadden Sea. Moreover, breeding success has been very poor in the past years, as revealed by the new Trilateral Monitoring and Assessment Program on breeding success.

About the Wadden Sea:
The International Wadden Sea area, with its 14.7 thousand km ², is the most significant resting, moulting and wintering ground for waterbirds and waders along East-Atlantic flyway between the Arctic and South Africa. In 2009 the high national and international protection status of the territory was recognized by inscription of the Wadden Sea into the UNESCO World Heritage list.

About JMMB:
The Joint Monitoring of Migratory Birds (JMMB) program is carried out in the framework of the Trilateral Monitoring and Assessment Program (TMAP), and constitutes a long-term monitoring program, coordinated by the Common Wadden Sea Secretariat. It covers a large connected eco-region stretching from Den Helder in The Netherlands to Esbjerg in Denmark; regular ground counts for most species and areas plus aerial counts for seaducks involves hundreds of observers and several institutes and agencies.

Source: CWSS
Photo: Tidal flats covered with ice © G. Reichert/ NPV Niedersächsisches Wattenmeer