De Levende Natuur nummer 6 van 2024 (English summary)

Abundant food for Pied Avocets in the shallows of Marker Wadden

Pied Avocets (Recurvirostra avosetta) search for food in the muddy shallows of Marker Wadden, a large human-made archipelago in lake Markermeer, comprised of basins with mudflats and shallow waters. Basins are either kept closed or are in open contact with the surrounding bays and gullies. What food might avocets and other waders encounter in these basins? Do potential prey types differ between basins that are designed differently?
Over half of the present biomass (>0.5 g/m2) consisted of lake flies/non-biting midges (Chironomidae, mostly larvae) and small Oligochaeta worms. Larvae and worms were especially abundant in benthic samples. The aquatic fauna consisted predominantly of chironomids, scuds/side swimmers (Gammaridae), water boatmen (Corixidae) and opossum shrimps (Mysida). Basin design had no apparent effect on the abundance of chironomids, but water boatmen were more abundant in closed basins and scuds in open basins.
At locations with foraging avocets and other waders, chironomids were most numerous, with many water boatmen and scuds as well. In April to July the fauna at foraging sites of waders closely resembled the benthic fauna at the sampling transects. In late summer they especially used locations where aquatic fauna was abundant. Avocets used mostly closed basin, but in 2021 when scuds were abundant, they used open basins as well. Chironomid larvae seem to be an important pillar in the attractiveness of Marker Wadden as a habitat for waders. 

Marsh islands paradise for growing fish

Marker Wadden was constructed as wetland islands with gradual land-water transitions. This means a substantial increase in shallow, nutrient-rich water. Marker Wadden is already an important nursery for fish, and thus an important food area for fish-eating birds like spoonbills, herons, grebes, and common terns.
The shallow waters of Marker Wadden support favorable habitats for many species of fish. Especially for young fish, the shelter within and around the islands and the nutrient-rich water, which warms quickly in the spring, provide an ideal nursery area. Juvenile fish alternately utilize the diversity of habitats in the gradual land-water transitions with riparian and aquatic plants between the islands and the deeper water around the islands. Fish are mobile and change habitats quickly, each species in its own way. By sampling fish every 2 to 3 weeks during the growing season in a standardized manner, we gained good insight into the use of the islands by both large and small fish throughout the year. This variation also largely determines where and when small fish are available as food for fish-eating birds The large numbers of three-spined sticklebacks in early spring and juvenile perch and pikeperch during the summer seem to play a key role in this.

Nieuw Land offers terns new breeding and foraging areas

Due to construction of islands, the past 25 years the lake IJsselmeer area has offered ample breeding habitat for common terns. However, food supply was not guaranteed in each of those years. When between 2016 and 2022, the artificial islands of Marker Wadden and Trintelzand were constructed in National Park Nieuw Land, it again was rapidly colonised by a substantial breeding population of common terns. By 2023 the now abundantly available sandy islands in the archipelago attracted up to 2,300 common tern pairs. We studied their breeding success, feeding range (with transmitters) and food choice (with wildlife cameras at their nests). Encouragingly since 2017 the breeding success was good with on average over one fledgling per pair. Only in 2023 the breeding success was low due to avian influenza and a summer storm that flooded most of the breeding sites. All bio-logged terns foraged on the large lakes of IJsselmeer and Markermeer, mostly within 30 kilometres from the colonies. Occasionally they went further than 40 km. The tern chicks benefited from a diet of smelt, young perch and young zander. We discovered that the newly created sheltered and shallow zones within and around Marker Wadden added foraging opportunities to the abundant deeper waters of the surrounding lakes. Although absolute numbers were relative low, however the densities in the sheltered lagoons and gullies were higher than on open waters of the lakes.

Spoonbills benefit from new shallows

The Eurasian spoonbill Platalea leucorodia is an icon of National Park New Land located in the centre of The Netherlands in the IJsselmeer area. In the national park there are a number of large-scale wetland development projects providing feeding and resting habitat for wading birds such as spoonbills. In 2016/2017 two additional shallow habitats with islands (Marker Wadden and Trintelzand) were created in lake Markermeer, a rather deep lake south of lake IJsselmeer. In 2021 the nearby wetland Oostvaardersplassen was partly drained (reset) to improve marsh vegetation succession. Due to these events a large area of shallow habitat became available. By tagging four spoonbills with transmitters and additional bird counts, we studied habitat use and the connectivity between wetlands in National Park New Land. Habitat use of spoonbills varied between years and within season. For example, in the first year of the reset, Oostvaardersplassen offered perfect foraging conditions for spoonbills, while in subsequent years the human-made archipelago Marker Wadden was favoured for feeding. The single spoonbill colony in the park is located on a dam in the harbour of Lelystad in striking distance from the archipelago. In this archipelago, the spoonbills used all locations with shallow waters and did not show a clear preference for sandy or clay bottoms. Peak numbers of spoonbills coincided with fish spawning periods, however large numbers of spoonbills were also present when there was less fish in the lagoons. This might be explained by the fact that later in the season fishes were less common but larger. Most important potential prey in April-May were 5-6 cm long three-spined sticklebacks, and fry and fingerlings such as 4 to 5 cm long perches in June. Eurasian spoonbills show the importance of the availability of multiple foraging areas and options in the National Park New Land.

Water table and availability of seeds of pioneer vegetation determine possibilities for foraging common teal in winter

In winter ‘22/’23 and ‘23/’24 the authors have studied common teal in National Park Nieuw Land (NPNL), with focus on Marker Wadden and Oostvaardersplassen. Extensive areas of extreme shallow water (0-10cm), freshwater mud and the productivity of seeds of pioneer vegetation proved key to the presence of teal. Vegetation was mapped in both areas and both years and showed marked differences, related to plant succession and the water table during summer. The food of teal was studied by microscopic analysis of droppings.
Preliminary results show the importance of pioneer species as Chenopodium rubrum, Persicaria lapathifolia, Rorippa palustris, Ranunculus sceleratus, Atriplex prostrata and Rumex maritimus. Part of the seeds prove to pass the digestive tract undigested which contributes to plant dispersal. Again, water table determined the accessibility of the seeds and thereby the attractiveness of the wetlands for this species. In the years of study maximum numbers of 75,000 teal were present in NPNL. Telemetry of 17 teal showed that foraging occurred at night, thereby avoiding the avian predators like white-tailed eagle, peregrine falcon, and harriers. Marker Wadden had no foxes and fewer avian predators than Oostvaardersplassen which was reason for part of the teal to spend the daylight hours there. Teal were faithful to their foraging grounds and returned after one season, frequently exchanging between the wetlands within NPNL.
The authors make a plea for a more natural cause in water level, especially lower in summer than at present, to allow more room for bare soil then and vegetation development. Lake Peipsi at the border of Estonia and the Russian Federation is a challenging example for effects of a more natural water table. Besides for pioneers, also marsh plants as reed, cattail, sedges and rush take profit of such a regime of fluctuating water tables, both within as between years. Teal is considered a flagship species to indicate natural dynamics of freshwater wetlands.

Semi-open swamp vegetation ideal for avocet chicks

Pied avocets Recurvirostra avosetta were one of the first breeding birds that colonized the human-made archipelago Marker Wadden in a freshwater lake in the Netherlands. Initial studies indicated avocets can raise multiple chicks so the fresh water mudflats are likely productive. To inform management decisions we studied the home range and habitat use of avocet families. We followed eight adult avocets equipped with a GPS-tracker between 2021 and 2023 to study the movements of their families. All families moved small distances, on average 200 meters, between their nests on sandy levees to the rearing areas in adjacent sparsely vegetated silt basins. The rearing areas were characterized by shallow water at the boundary between sparse marsh vegetation and water. This habitat can also be found along the sandy levees, but avocets preferred shallows over silt rather than over sand. During the study period there were few or no exposed mudflats. The rearing area of the families averaged 1.3 hectares. This is relatively small and might suggest that habitat quality is good for rearing chicks. The size did increase as the chicks grew older. This is a combined effect of chicks venturing off further and parents feeding at larger distances from their young. Interestingly, the home ranges of avocet families hardly overlapped. Avocet chicks use a combination of suitable foraging habitat in shallows over silt and shelter for potential predators in open marsh vegetation.