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).