Stronger winds, less food for shorebirds

For their analysis, the researchers looked at the rise and fall of the average sea level under different wind directions and wind forces. While winds from the east somewhat ‘drain’ the Wadden Sea, thus ensuring that more mudflats are available for birds such as the red knot, westerly storms cause the average water level to rise. “During the strongest westerly winds, which we analyzed between September and November, we saw that at low tide there was over 50 percent less dry foraging area available for wading birds”, says Keuning. “When we looked specifically at the areas where cockles, Baltic clams and mudsnails live, we found that during a strong westerly storm, red knots had up to 44 percent less of this favorite food available.”

Headwinds

Using tiny radio transmitters that could pinpoint the locations of birds to within a meter – the so-called WATLAS system – the researchers also gained an initial impression of how the birds reacted to different wind directions. This showed that, in calm conditions, the red knots seem to prefer to shelter from high tide around Griend, close to rich feeding grounds. Only when water levels were at their most extreme, did red knots fly to the more distant islet Richel, despite strong headwinds. There they are safer from predators such as peregrine falcons. In strong winds, the birds at Griend are pushed strongly towards the dunes, where danger always lurks.

Fewer red knots

These initial analyses do not yet reveal a clear effect of storms on the food intake or survival of red knots. Further research is needed to provide clarity. Nevertheless, ecologist Allert Bijleveld, the research coordinator, already sees a clear warning in this work. “In the long term, we see a clear link between food availability in the Wadden Sea and the numbers of red knots. So if storms from the west occur more frequently, as climate models predict, this will in all likelihood also have an effect on the presence or survival of red knots in the Wadden Sea”, says Bijleveld.

Unexpected effect

“The potential effect of increasing westerly storms on red knots and possibly other wading birds as well, is an underexposed effect of climate change”, says PhD candidate Evy Gobbens, who co-conducted the research. “Research into climate change often focuses on the average sea level rise. For the time being, this rise is very modest in the Wadden Sea. But the increasing storms do show what this sea level rise could potentially cause. Wading birds will have to search for food in a smaller area for a shorter period of time and will therefore face more competition. In that sense, this research gives a preview of what wading birds can expect if climate change continues at this rate.”

Text and images: Royal Netherlands Institute for Sea Research (NIOZ)