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More Than 500 Pied Avocets Used New Marker Wadden Mudflats, but the Annual Maximum Fell Below 100

Pied avocet swimming with its upcurved bill clearly visible
Conservation · Restoration

Newly created freshwater mudflats at Marker Wadden, Netherlands in Lake Markermeer initially provided feeding habitat for large numbers of pied avocets (Recurvirostra avosetta). More than 500 birds were recorded early in the archipelago’s development, but the annual maximum had fallen below 100 by 2023.

Researchers followed staging avocets from 2017 to 2023 and monitored habitat development in 12 constructed basins. The study asked how basin age, size, design and changing vegetation and water conditions affected the birds’ use of this human-made wetland.

Open mudflats disappeared as the basins aged

The estimated area of unvegetated mudflat fell from 318 hectares in 2018 to 22 hectares in 2022. As the basins aged, vegetation spread and the mosaic of shallow water and mud became smaller. Basin age had the strongest negative relationship with avocet numbers, while larger basins supported more birds.

The authors conclude that natural succession was one major reason the site became less attractive. Avocets were most numerous where extensive shallow mud remained bare or only sparsely vegetated.

Higher water levels reinforced the loss of feeding habitat

Several basin levees were breached during development, connecting them permanently to the surrounding lake. Water levels then rose in most of these basins, reducing the amount of shallow water and exposed mud. Avocets were generally more numerous in closed basins, but the difference became much smaller once the actual habitat conditions were taken into account.

The biological point was therefore not simply whether a basin was labelled open or closed. What mattered most was whether its design and water regime maintained a broad mosaic of shallow water, mud and little vegetation.

Young artificial wetlands can provide valuable but temporary habitat

Marker Wadden was built in phases, so new early-successional mudflat appeared while older basins were already becoming vegetated. The authors argue that this prolonged the period in which the archipelago could support large numbers of avocets.

For freshwater restoration aimed at avocets, the study supports creating and actively maintaining large areas of shallow, muddy habitat with little vegetation. The seven-year record also shows why newly created wetlands cannot be assumed to remain equally suitable without continued water and vegetation management.

Sources and image credit

  1. Artificial freshwater mudflats temporarily provide substantial feeding habitat for waders such as Pied Avocets Recurvirostra avosettaPrimary peer-reviewed open-access research article by Verkuil, Dreef, van Horssen and van der Winden in Bird Conservation International, published online 12 January 2026. Used for the 2017–2023 counts, habitat measurements, models, observed decline and management recommendations.

Image: David Merrett / Wikimedia Commons — pied avocet swimming at Titchwell Marsh, Norfolk, England, 21 April 2009. Representative photograph; not taken at Marker Wadden. · CC BY 2.0 — https://creativecommons.org/licenses/by/2.0/ · Source

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