During the last decade the composition within the Wadden Sea’s waterbirds has drastically changed. Bivalve predators as Red Knot, Calidris canutus, Oystercatcher, Haematopus ostralegus and Common Eider, Somateria mollissima have severely declined in numbers, whereas polychaete predators have increased (e.g. Bar-tailed Godwit Limosa lapponica, Dunlin Calidris alpina and Grey Plover Pluvialis squatarola. Clearly, this suggests that the observed changes in number of birds present are linked to changes in the benthic fauna. Fortunately due to long term monitoring of the numbers of shorebirds and their benthic prey more understanding is gained how these numbers are influenced. Indeed, very recently, the dramatic decline of Red Knots from the Wadden Sea could be attributed to the decline in food abundance, which had been derived from the NIOZ 15-yr-long benthic survey throughout the western Dutch Wadden Sea. With this long term survey, it became apparent that shellfish stocks largely collapsed due to dredging, while polychaete stocks remained at a constant level or even increased. The changes in food stocks are likely to be responsible for the changes in species abundance that rely on the intertidal Wadden Sea area. The question remains, whether the increase of worm-eating birds are in fact driven by a possible increase of food abundance.
Shorebirds forage on benthic prey during low tide on intertidal mudflats which differ in the degree of food availability, food quality and predation risk. To decide where to forage, migratory shorebirds trade off the costs and benefits of different habitats to maximize their survival. To measure the available densities of the benthic prey the NIOZ started a long-term benthic research effort that began in 1988. In 2008 this effort has expanded and the entire Dutch Wadden Sea area is being sampled in a fixed 500m grid and all polychaete species are included (previously the larger polychaetes were only counted) and analyzed in the lab. This enables us to create a food landscape of the entire Wadden Sea per benthic species. Historically our research group has gained much understanding of the foraging ecology of the Red Knot (Calidris canutus) and in 2001 a new study species has been included in our work, the Bar-tailed Godwit (Limosa lapponica). This species is predominantly a polychaete feeder and thus provides us with a mirror species to the already well known Red Knot.
Bar-tailed godwit experiment
This movie shows an example of a foraging experiment of Bar-tailed Godwits (Limosa lapponica). The experiment was designed to study the functional response of female Bar-tailed Godwits foraging on their favourite prey, the Lugworm (Arenicola marina). The functional response describes the relation between the predator and their predator. During observations in the field we discovered a large effect of burying depth of the prey and the intake rate. In this experiment we manipulated the density of prey and the depth of these prey, to study these effects in detail. In this particular trial, the bird is foraging on a patch which contains only three Lugworms, at a depth of 20 cm. As you can see, the bird puts a lot of effort into finding and extracting the prey.