Pochard Aythya ferina use of chironomid-rich feeding habitat in winter.

Abstract

The feeding distribution of Pochard on two flooded gravel pits was studied at the ARC Wildfowl Centre in January/February 1989, when peak wintering numbers were present. On the deeper lake (mean depth 2.3 m) 99% of feeding occurred in the shallowest 10% of the lake, where depth averaged 1.6 m. In the other shallower lake (mean depth 1.5 m) favoured feeding areas averaged 0.9 m deep. At this lake only 15% of the available habitat at this depth was used by feeding Pochard, much shallow water being ignored. Benthic sampling produced no submerged macrophytes and revealed that only oligochaete worms and chironomid larvae were numerous in the sediments. Oligochaetes were similarly distributed in both feeding and non-feeding areas. Chironomid larvae were present in significantly higher numbers and biomass/m² in feeding areas. Hence it would appear that Pochard prefer to feed in shallow water and can select prey-rich areas, thus maximizing their food intake whilst minimizing their energy expenditure. Two out of 3 Pochard shot at the site in December 1989 which had food present in their upper digestive tracts contained only (large) chironomid larvae. The Pochard's ability to exploit patches rich in chironomid larvae is discussed in relation to the relatively low nutrient status of flooded gravel pits.