Nondestructive or non-disturbance sampling is always recommended for investigating animal ecology (Richards et al., 2014). In our study, the ratio of tibiotarsus and tarsometatarsus when the cranes were standing on land and the ratio of tibiotarsus and tarsometatarsus submerged in water were verified using the photographic sampling, and both ratios obeyed the normal distributions and had the similar density functions, which represented that the overall inference hypothesis in our study was reasonable and feasible. When the land ratio has been calculated, it was very critical for sequent calculations about how to get the wading depth projection value r.
Our results showed that the Black-necked Cranes preferred to choose the shallow water areas with the water depth less than 20 cm, especially less than 10 cm and they nearly do not prefer to the water depths higher than 30 cm. In field investigation in Caohai, the Black-necked Cranes almost never drowned their whole tarsometatarsus when choosing wading habitats. This is similar to the field observation results from Dashanbao and Napahai in Yunnan (Li and Li, 2005), as well as similar to the overwintering Black-necked Cranes found on the sides of lakes, reservoirs or river shoals in Tibet (Lei et al., 2012). Similarly with the other crane species, for example, the Siberian Cranes (Grus leucogeranus), a closely related species to the Black-necked Cranes, overwinter in Shahu and Poyang Lake. Wu et al. (2013) estimated that 58% of the white cranes moved in the water depths of 5–27 cm and the Siberian Cranes were also mainly in shallow water with depths below the tarsometatarsus (Wu et al., 2013). He et al. (2002) observed that small groups or family of Siberian Cranes were mainly scattered in some scattered shallow water pools with the depth of the water mostly 2 ~ 10 cm in Momoge Nature Reserve, Jilin Province, China, but for large flocks of cranes, the water depth was relatively deeper, ranging from 10–30 cm (He et al., 2002). Lv et al. (2007) reported that the wading water depth of the Red-crowned Crane (Grus japonensis) was generally below 30 cm and few cranes were wading at the higher depth of 30 cm (Lv et al., 2007), and the research results of the common cranes (Grus grus) and the Sandhill Cranes (Grus canadensis) (JM et al., 2004; Folk et al., 1990)were in agreement with our results. All the aforementioned results were derived from subjective estimation; yet, out results are based on relatively big sample size and fine derivations.
Field observations in our study showed that, the Black-necked Cranes overwintering in Caohai Lake mainly preferred to stay in the lacustrine regions, sedge meadows, shallow meadows, barren grasslands, corn fields, and vegetable fields. Shallow swamps and sedge meadows, provides a vast range of plant communities that cranes can consume, In addition with abundant supply of snail and small fishes that can serve as rich protein sources for these birds (Wu and Li, 1985; Li, 1999). Therefore, the Black-necked Cranes were observed to frequently preferred wading in these shallow areas with the water depth of (0–10)cm.
Currently the authorities are planning to raise the water level of Caohai from 2171.7m-2173m, to expand the water surface area upto 33 km2 (Ran et al., 2017). The overall water level will be raised by more than 1m, which will drastically affect the current distribution of foraging and roosting sites of the Black-necked Cranes. During the increased and high water levels, it becomes difficult for cranes to locate the food. Our study in this regards provide reliable guidance to better maintain the water levels and habitat management for the Black-necked cranes at Caohai and the other overwintering sites. Based on the results of the current study we strongly recommend to maintain the stable water levels, and to predict the formation of new suitable overwintering habitats. Furthermore, we believe slow adjustments of water level can gradually enhance the cranes adaptability; and ultimately the Blacked-necked cranes habitats can be better protected and managed, both quantitatively and qualitatively.