Nest-site measurements
The design employed in this study was a stratified case-control with actual nest-sites as the cases, and random nest-sites as the controls, and strata defined by the sampled wetlands within and outside of the lake basin (Maxson, Fieberg & Riggs,2008). Data at each nest-site followed descriptions of various authors such as Dwyer & Tanner (1992) and Wu, Zha, Zhang & Yang (2009).
The following four variables were measured (or assessed)at each active nest-site(one having an egg and/or chick),as well as at the random nest-sites: mean water depth and mean vegetation height- over four samples (at each cardinal direction) at 1.5 m from the centre of the nest, distance from the water edge, and grazing intensity (see criteria below). A random nest-site for each active nest was selected to compare characteristics of active nests (presence) with those of randomly-selected sites, also known as unused areas or absent nests. The random nest-sites were placed arbitrarily at 15-25m in any direction of the actual nest.
Grazing intensity score was assessed around the nest as either i)none (no faecal remains, vegetation intact, no spoors), ii)low (scattered faecal remains, vegetation slightly grazed, scattered spoors),iii) medium (faecal remains evident, vegetation grazing more evident) or iv) high (faecal remains around the nest, dense spoors and intensive grazing evidence).