Numerical Distribution of Siberian Cranes and Their Relation to Food and Water Depth in Sha Hu at Poyang Lake, China

Poyang Lake is the largest freshwater lake in China and globally famous for its birdlife.The rich food resources provided by this wetland's emergent and submerged aquatic plant diversity is a major reason that hundreds of thousands of migratory birds travel to Poyang Lake every winter;on average more than 400 000 waterbirds make Poyang Lake their winter home.Poyang Lake is extremely important to Critically Endangered Siberian Cranes Grus leucogeranus,as 3 000-4 000 Siberian Cranes,or over 98% of the world's population of this species spend winter here each year.The International Crane Foundation and the Poyang Lake National Nature Reserve collaborated to carry out a monitoring program from 1999 to 2010 in Sha Hu,one of nine sub-lakes of the Poyang Lake National Nature Reserve in Jiangxi Province of China.The objectives of the monitoring were to: ① record the number and location of the Siberian Cranes;② determine the biomass of their aquatic food plants;and ③ better understand relationship between foraging Siberian Cranes and their aquatic food plants as well water depth.Tubers of Vallisneria spp.were the main food of the Siberian Cranes during winter.Vegetation transects were set up to record numbers and biomass of Vallisneria spp.and their tubers.High vantage points that covered the whole Sha Hu were used to record number of Siberian Cranes and the water depth where the birds were foraging throughout the winter.The location of the birds was also mapped.During the 12 years,overall average number of Siberian Cranes was 46 birds,with winters in 2002,2006 and 2009 over 90 birds.Within a winter,numbers of Siberian Cranes from November to January were relatively stable and the peak number of cranes occurred in December(120 birds).Siberian Cranes were seen mostly in the middle part by south of the lake,while very few recorded in northern and southern parts.The cranes were almost exclusively seen in the submerged vegetation communities dominated by Vallisneria spp.and Potamogeton spp.From 1999 to 2010,the average dry weight of Vallisneria tubers at Sha Hu was 5.92 g/m2,in 2005,2006 and 2008 about or over 12.00 g/m2,while was 0.10 g/m2in 2010.For the 12 years of data,the correlation between the number of Siberian Cranes during December and January and tuber dry weight was not significant,with r=0.231,indicating Vallisneria tubers,at least alone,were likely not a limiting factor to the wintering Siberian Cranes.Areas with water depth of 5-27 cm had the highest number of the cranes(58%),followed by areas with water depth of 40-45 cm(31%).In total,91% of the Siberian Cranes were found in areas with water depth of 5-45 cm.The big flood at Poyang Lake in summer of 2010 had a significant impact on aquatic plant food,forcing Siberian Cranes seeking alternative food sources in sedge/grass zones.