Drought reduces the value of both artificial and natural wetlands for gulls breeding in the Mediterranean region

Increasing water extraction and climate change combine to increase the impact of droughts in Mediterranean wetlands. Natural wetlands are being increasingly transformed into artificial wetlands, notably ricefields and aquaculture ponds. Since waterbirds are highly dependent on aquatic resources throughout their life cycles, changes in wetland habitats can significantly affect their use and movements. Studies relying on count data can highlight the importance of artificial wetlands for waterbirds, but analysis of movements using GPS tracking data facilitates a full assessment of their value compared to natural wetlands. We used GPS-tracking to study the habitat use of nesting black-headed gulls, Chroicocephalus ridibundus, in the iconic Doñana wetlands in Spain. In this area, the flooding of remaining natural wetlands has been reduced by overextraction of groundwater and by prolonged drought, whereas adjacent fish ponds have maintained stable water levels. We tagged 65 adults, and studied habitat selection of nesting birds (n = 30 bird-years), combining flooding information from Landsat images with land-use maps. Size of colonies, fledging success and proportion of tagged birds that nested all increased in years of higher rainfall. In the wetter 2024, when flooded natural marshes were available, gull home ranges shifted to include those areas. When away from the nest, fish ponds were the preferred habitat in 2022 and 2023, and natural marshes in 2024. Although ricefields are used by gulls in winter they are almost unused during the nesting period. In the driest year 2022, home range sizes were smallest. But due to successive drought years, availability of freshwater habitats was even lower in 2023, and gulls responded to rainfall events by flying up to 54 km to exploit other, distant, temporarily flooded, agricultural habitats. The importance of artificial wetlands for waterbird conservation can be easily overestimated from count data alone. The value of extensive fish ponds as a breeding habitat depends on maintaining the availability of natural marshes as a foraging habitat. This requires reducing impacts from groundwater extraction. Given recent abandonment of aquaculture, some fish ponds should be restored into natural marshes.