Effect of depth and open waters on site selection by wintering waterfowl in freshwater wetlands

Abstract

Most attempts to understand site selection by wintering waterfowl in the tropics have centered on food abundance. However, when wintering waterfowl are confronted with a choice of shifting mosaics along the Central Asian Indian migratory flyway, the wetland characteristics favoring selection of some wetlands over the others have been less understood. To obtain an insight into the influence of wetland characteristics like depth, water-spread, open-waters and macrophyte cover on waterbird site selection, a year long study was conducted on 2 lakes situated 3.5 km apart but harboring a differential waterbird population. Curtorim Lake (CuL) (10 ha) and Maina Lake (ML) (7 ha) had a maximum depth of 5.5 m. Their dominant vegetation included Utricularia aurea, Nymphoides indicum and Nymphaea nouchali. The weed Salvinia molesta, present at ML consistently reduced the open-waters to below 45 percent. At CuL, open waters ranged from 42-100 percent. CuL and ML housed 26 and 22 species of waterbirds respectively of which 18 were common to both lakes. These included pintail ducks, cotton teals, lesser whistling teals, garganeys, pheasant-tailed jacanas and bronze-winged jacanas. The innate wetland attributes showed pronounced variations between the lakes. This played an important role in structuring the waterbird community namely ducks and waterfowl. At both lakes, density of ducks and waterfowl increased significantly with depth and water-spread. At Curtorim Lake, waterbird density decreased significantly when open-waters exceeded 65 percent but increased as openwaters decreased to 40 percent. Both parameters remained stable thereafter. At ML, although a negative relation was observed between the waterbirds and open waters it was not statistically significant owing to the consistently high vegetation/weed cover throughout the year. When open-waters fell below 30 percent, anatids deserted the lake. This indicates that a) 40-60 percent open-water is essential for making lakes waterbird friendly b) innate wetland characters like depth and vegetation cover greatly influence site selection by waterbirds.