Distribution of the White-headed Duck Oxyura leucocephala is Affected by Environmental Factors in a Mediterranean Wetland

The White-headed Duck Oxyura leucocephala is a globally endangered diving duck that breeds in a few wetlands in Spain, Asia, and northern Africa. Little is known about how environmental variables affect the duck's distribution, so information is required to protect wintering and breeding areas. We attempted to assess which morphometric (open water surface and shoreline development index; [SDI]) and limnological variables (chironomid larvae biomass, macrophyte seed density, macrophyte cover, Secchi disk depth, conductivity, water depth, and chlorophyll a) were important in predicting suitable habitat conditions for this duck in an important wetland for the species in southeastern Spain. Our study included 2 periods with contrasting hydrological conditions: ”wet” (winter 2003 to summer 2004) and ”dry” (spring to summer 2005). Limnological variables were measured, and bird censuses were performed once a month in each water body, while morphometric variables were estimated from aerial photographs. In the wet year, the probability of the presence of wintering ducks increased with a greater open water surface area. In spring, the likelihood of the species occurring increased with the SDI, biomass of chironomid larvae, and level of eutrophication, and decreased with an increasing open water surface area. In summer, a high density of macrophyte seeds, high water transparency, and greater macrophyte cover were positively correlated with the presence of ducks. In the dry year, water depth and chironomid biomass were positively related to the bird's presence in spring and summer. White-headed Ducks were selective regarding the conditions of the water bodies, having different habitat requirements in different stages of their annual cycle. In stressful hydrological conditions (severe drought), ducks sought relatively deeper water bodies independent of their morphometry. We propose suggestions for the species' conservation in terms of local habitat management strategies.