New food source in the Wadden Sea for bar-tailed godwits

As the bar-tailed godwits prepare for their 4,000-kilometre journey to their breeding grounds in Siberia, researchers have observed a remarkable change. Field observations and droppings analyses show that many bar-tailed godwits at Balgzand are consuming large quantities of mud shrimps (Corophium sp.). Roeland Bom is one of the researchers involved. “Normally, bar-tailed godwits probe deeply into the mudflats with their long bills to catch worms. Now we are seeing them making shallower feeding movements and swallowing far more frequently.”

In a hurry

There is not yet a full explanation, but climate change may be affecting the seasonal availability of prey. Climate change is already creating challenges for bar-tailed godwits. Earlier snowmelt in Siberia causes insects to emerge in large numbers sooner, even though these insects are a vital food source for bar-tailed godwit chicks. To keep pace with this earlier spring, the birds must migrate more quickly while also shortening their stopover in the Wadden Sea. “They arrive directly from their wintering grounds in Africa and have only a few weeks to build up sufficient reserves for the long flight to Siberia”, explains Bom. “During this period, rest and undisturbed foraging conditions are just as important as the food itself.”

Wadden Sea crucial for refuelling

Previous research by Eldar Rakhimberdiev and other NIOZ colleagues has already shown that the success of bar-tailed godwits is often determined before they even arrive at their breeding grounds: here in the Wadden Sea, where they refuel for the next stage of their migration. Bom says: “The behaviour of the bar-tailed godwits at Balgzand raises new questions about how migratory birds respond to changes in the Wadden Sea and to climate change in the Arctic. Were mud shrimps simply not available during this period in previous years? Or have other food sources become less nutritious?”

10 per cent of the world population

Balgzand is a highly suitable location for bar-tailed godwits because the dyke area experiences very little human disturbance at either high or low tide. Thanks to the WATLAS tracking system – an advanced network of transmitters that reveals how birds move through the Wadden Sea – it has also become clear that the number of bar-tailed godwits at this site has increased dramatically. In May 1995, around 8,000 bar-tailed godwits gathered at Balgzand and this year there are more than 40,000. That represents over 10 per cent of the global population. Bom: “Our research also provides valuable insights for conservation and management. How can we work together to safeguard the peace and quiet at Balgzand during this crucial period?”

Come and see for yourself

The researchers invite people to come and witness the swirling flocks of bar-tailed godwits for themselves. Bom: “It’s a magnificent sight. From the dyke, the bar-tailed godwits can be observed very well without disturbing them. It is also where you sense the urgency. These birds are working incredibly hard for 12 hours a day to build up enough fat reserves within just a few weeks. Around 20 May, they will depart for their breeding grounds in Siberia.”

Publication

The study ‘Time-stressed Bar-tailed Godwits exploit novel prey to fuel’ by NIOZ and BirdEyes will be published in IBIS and is part of the Waakvogels project, in which six migratory bird species reveal the state of the Wadden Sea.

Text: Royal Netherlands Institute for Sea Research (NIOZ)
Images: Jan van de Kam (lead photo: bar-tailed godwits)