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Following Sandwich Terns over sea

Wageningen Marine Research

31-May-2020 –

How do Sandwich Terns fly over the North Sea and the Wadden Sea? And where exactly do they catch their fish? Scientists from Wageningen University & Research (WUR) and the University of Amsterdam are trying to map the flight and foraging behaviour of large Terns by equipping fifteen Texel birds with transmitters.

The Dutch Ministry of Agriculture, Nature and Food Quality finances the station survey. The underlying question is: what effects can sand replenishment for coastal reinforcement in the foreshores of the North Sea and outer deltas of the Wadden Sea have on the foraging locations of large Terns? The researchers want to generate insights into which water depths, at which sandbanks and in which channels the Terns catch their fish. Because sand replenishment changes the depths and shapes of banks and channels, the consequences for large Terns can then be determined.

Reading out and mapping data

On 19 and 20 May, researchers from WUR, Bureau Waardenburg and Sovon gave the fifteen Texel birds a GPS tracker from the University of Amsterdam (uva-bits.nl). In the breeding colony Wagejot antennas have been set up that read the data from the trackers and send them to the researchers. A second set of antennas has been set up in De Slufter on Texel. When the colony leaves the breeding area Wagejot, many Terns use De Slufter as a resting place. Here the data of the birds are read when they no longer enter the colony. According to researcher Martin Baptist of Wageningen Marine Research, the transmitters display the flight behaviour of the birds in great detail. “We get their GPS position, flight altitude, speed and even the temperature of the tracker every 13 seconds. Because the trackers also have an accelerometer on board, we can determine when they make a dive down. If the tracker also cools down suddenly, we know exactly where they are plunging into the sea to catch fish,” says Baptist.


Flying Sandwich Tern with transmitter (Source: Hugh Jansman)

Area managers Natuurmonumenten (nature reserve Wagejot) and Staatsbosbeheer (Staatsbosbeheer) (De Slufter) are happy to participate in the research into the Sandwich Terns. After all, about half of the Dutch population breeds on Texel, and a better understanding of flight and foraging behaviour contributes positively to management advice and nature policy goals.

Colours

The fifteen emitted birds have also been given a blue colour ring. This has a three-letter code on it, starting with TX and then another letter. Throughout Europe and even in Africa color-rings are read remotely by researchers and fanatical birdwatchers with a good telescope. The researchers learn from the rings how the birds migrate and they hope that their Texel Terns (TX) will be seen a lot.


Sandwich Terns in the sky (Source: Hugh Jansman)

Iconic birds of the Wadden Sea

Sandwich Terns are iconic birds of the Wadden Sea. They fly above clear seawater and hunt fish by diving. There are only a few breeding colonies in the Netherlands, of which the colony Wagejot on Texel is the most important; it is one of the largest in Europe. The birds return from their wintering grounds in Africa from the end of March back to the Netherlands to breed. From the end of April, they form densely packed breeding colonies on sparsely vegetated sandbanks or shell banks. They lay one or two eggs in a dimple. After 22-26 days the eggs are hatched. The chicks cannot fly for the first 25-35 days. During that period, the parents fly back and forth to the sea to always bring one fish to a chick. The bigger the chicks become, the bigger the fish the parent birds offer. If the chicks are quick to fly, they go with the parents to learn to fish at sea themselves. On Texel, the Terns often gather in De Slufter. In August and September, the parents migrate with young to southern destinations, some all the way to Cape of Good Hope in South Africa.

large Terns in the Wagejot nature reserve near Oosterend (Source: Hugh Jansman).

More information  (interesting to look at! (Roelof Heringa, MBP)

Text: Martin Baptist, Cecile Leuverink, Wageningen Marine Research

Pictures: Martin Baptist (lead photo: Big star with attached transmitter); Hugh Jansman

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