An inference of the wading depths of the cranes in wintering wetlands through photographic sampling: a case study of black-necked cranes (Grus nigricollis) in Caohai Wetland, China

Abstract

AbstractSeveral overwintering crane species like Black-necked cranes (Grus nigricollis) mainly depends on wetlands for foraging or roosting, preferring stable water depths. Understating the wading depths of cranes is a pressing concern for the better management of wetlands and robust conservation of the overwintering cranes populations. Yet, practically it is very difficult to precisely measure the wading depths due to several limitations like, inaccessibility and sampling difficulties. In the current study a sample of 1288 wading photographs of cranes, based on the constancy of proportion of tibiotarsus to tarsometatarsus was taken in Caohai wetland situated in the Northwest Guizhou Province of southwest China. Caohai Wetland is the biggest overwintering wetland around the world, and a very promising area to infer the exact ranges of wading depths of these cranes. The ratio of tibiotarsus to tarsometatarsus of the cranes standing on the lands and the ratio of tibiotarsus to the rest of tarsometatarsus of the cranes standing in water were measured; to infer the projection depths of the part of tarsometatarsus submerged in water, which was used to calculate the accurate depth range of the cranes by comparing with the referenced values. According to the results, 89.92% of individuals preferred the water depths from (0–20] cm, 76.11% preferred (0–10] cm% while water depths for 49.04% individuals was (0–5] cm. Thus we conclude that the Black-necked Cranes prefer a depth range of (0–10] cm. This water depth may be relative to the swamps around the lake, which are very important foraging and roosting sites for cranes, For better habitat management, we strongly recommend paying special attention to swamps with specific water depth ranges, particularly when the water level changes.