When and Where Do Waterbirds Need Water? A General Approach for Inferring Candidate Restoration Areas From Spatio-Temporal Variation in Surface Water Availability

Abstract

Aim: Globally, about 850 bird species depend on surface water habitats either year-round or during migration. However, large-scale analyses examining how different groups of waterbirds are associated with surface water across various regions and during different seasons are lacking. This study analyzes this relationship using high-resolution satellite imagery to determine when and where surface water and associated resources might be improved. Location: Western Palearctic (Europe, Middle East, Northern Africa). Methods: We correlated monthly counts of 40 waterbird species with surface water availability in the Western Palearctic at a 100 x 100 km grid cell resolution. We then identified where and when surface water or associated resources might be limiting for waterbird numbers, using waterbird count data and the most recent release of high-resolution satellite imagery from the Copernicus project. Results: Surface water availability was higher in the East-Atlantic flyway than in the Black Sea-Mediterranean flyway, but differences were small, while bird abundances (after correction for observation effort) were comparable. The relationship between surface water and waterbird abundances was typically positive and slightly stronger in the East-Atlantic flyway. However, it was negative during summer in the northern regions, where many waterbirds breed. Correlations were stronger for ducks and other strictly wetland-dependent birds, reflecting their more exclusively aquatic feeding behaviour. Crowdedness, calculated as the number of birds per surface water area, was higher in autumn compared to spring migration. Main Conclusions: Our results confirm that the importance of surface water availability for waterbirds varies seasonally and geographically. In addition, our maps integrating bird counts and surface water help to strategically prioritise regions where water availability could be limiting and high crowdedness implies a vulnerability for waterbirds, for example, in terms of resource competition or disease transmission. These include parts of the Iberian Peninsula, the UK, northern-central Europe, the Levant and Northern Africa.