Large‐scale depth‐related seasonal distribution patterns of a benthic‐feeding sea duck in two contrasting marine systems

Moulting and overwintering Common Scoter Melanitta nigra aggregate in largely undisturbed, shallow‐water marine areas, preying upon sessile benthic organisms (mainly bivalves), which do not reproduce during this period of exploitation. Assuming even prey distribution, we predict that Common Scoters would aggregate to moult in shallowest waters with most accessible prey in July, where diving costs were minimal, but would disperse to deeper (i.e. comparatively less profitable) waters through the season as Common Scoter numbers increase and (potentially) as their prey are depleted in winter. To test these hypotheses, we used multiple aerial survey count data to study Common Scoter distribution patterns in Aalborg Bugt, Denmark (in relatively sheltered areas subject to restricted tidal influence), and along the more exposed, highly tidal Schleswig‐Holstein North Sea coast in the German Bight. Despite these physical differences, Common Scoters displayed similar distribution patterns in both areas, showing significant increases in mean water depth (from 6.3 m in July to 9.8 m in March in Denmark, 6.5–10.3 m in Germany), number of flock units (432 to 1614; 48 to 581) and the percentage of 3 × 3 km grid squares occupied by birds as the season progressed (15% to 44% of 628 grid cells; 1% to 39% of 408 grid cells). The results support our hypotheses that these consumers distribute themselves to maximize their nutritional and energetic intake, while minimizing costs of gaining food in two contrasting marine environments, but we require sequential sampling of their food supply at differing water depths to confirm the causes of these observed patterns.