Satellite tracking reveals a new migration route of black-necked cranes (Grus nigricollis) in Qinghai-Tibet Plateau

Study area

Nyingchi Prefecture (26°52′–30°40″N, 92°09″–98°47″E) is located in southeast Tibet, within the middle and lower reaches of the Yarlung Zangbo River. The average altitude of this area is 3,100 m. Nyingchi Prefecture has a tropical humid and sub-humid climate, high vegetation coverage, rich water resources, and annual precipitation of 650–2,000 mm (Zhang, 2006). A variety of rare wildlife is distributed across this area (e.g., the snow leopard, Panthera uncia; black stork, Ciconia nigra; and black-necked crane) (Sun, Yang & Zuo, 2003). Previous studies have indicated that Nyingchi is an important wintering area for black-necked cranes (Tsamchu, Gu & Bishop, 1994; Han & Guo, 2018).

Satellite tracking

In March 2016, seven black-necked cranes were safely captured in Nyingchi Prefecture using a pole trap or a mist net and fitted with solar-powered GPS-GSM satellite transmitters (models HQBP3622 and HQLN0421, Hunan Global Messenger Technology Company) and engraved color rings (Mi, Møller & Guo, 2018). The entire process, from capture to release, was less than 10 min. The HQBP3622 transmitter weighed 22 g and was attached to the back of the birds using a Teflon strip. The HQLN0421 transmitter weighed 40 g and was attached to the left leg, and the inner diameter of the transmitter was 20 mm (Mi, Møller & Guo, 2018; Wang et al., 2020). The data were transmitted through GSM (Global System for Mobile Communication) hourly/every three hours and consisted of longitude, latitude, instantaneous speed, course, altitude, temperature, voltage, and precision. Two individuals failed to send signals due to transmitter failure, but the tracking data of five individuals was successfully received. The weight of the two transmitters was less than 3% of the cranes body weight, and thus is believed to have minimal impact on behavior (Barron, Brawn & Weatherhead, 2010; Bodey et al., 2018).

Data analysis

The accuracy of the tracking data was divided into five grades: A (within 5 m), B (within 5–10 m), C (within 10–20 m), D (within 20–100 m), and invalid (Wen, Ren & Xing, 2017). In this study, we only used locations categorized as A, B, and C. The migration routes and heat maps were created, and the cumulative migration distance was calculated via the geometry function in ArcGIS 10.2. The start and end of the migration time were defined as the date from which the individuals departed from and arrived at the breeding/summering or wintering sites (Wang et al., 2018). Migration duration was calculated as the time between the individual’s start and end of the migration. Stopover duration was defined the days individual spent at the most important stopover sites (spring: Basom Lake; autumn: Shazhuyu River) for each migration season. Two migration parameters were previously defined by Abrahms et al. (2017) and Buechley et al. (2018): migration straightness (direct distance/cumulative distance) and migration speed (cumulative distance/migration duration). We calculated corresponding migration parameters based on the above definition. Boxplots were used to show the migration patterns (Deng et al., 2019). P value was calculated to compare migration parameters between seasons by T test. The sites passed during the migration process involved two time zones, UTC+6 and UTC+7. The local sunrise and sunset times were sourced from an online database (https://richurimo.51240.com/) to determine the daily migration periods. Sunrise and sunset were approximately 07:00 and 20:00 during the spring migration and 08:00 and 19:00 during the autumn migration. Heat maps were used to show the density of points by a density function (Zhao et al., 2014; Perrot et al., 2015). Using the data during migration to draw the heat maps, it can be found that the areas with highly dense of points on the migration route are important stopover sites. Stopover sites were defined as places where a bird stopped for more than two days, while roosting sites were defined as a place where a bird stopped for less than two days (Kölzsch et al., 2015). These points were identified from speeds equal to zero, and fly points were identified by speeds greater than 10 km/h (Mi, Møller & Guo, 2018). The conservation efficiency was determined by the proportion of stopover and roosting site points in protected areas (World Database on Protected Areas: https://www.iucn.org/theme/protected-areas/our-work/world-database-protected-areas). All data are presented as the mean ± SD.

Ethical note

The study is executed according to Chinese laws on bird capture and handling (Administrative licensing (01401), National Forestry and Grassland Administration, China).

Data collation

From 2016 to 2019, 51,280 positions were obtained, of which 49,324 with accuracies A, B, and C were used in this study (details in Table 1). Among the five successfully tracked cranes, three (No. 1, No. 2, and No. 3) were only tracked for one spring migration in 2016. One bird (No. 4) was tracked for three annual cycles from 2016 to 2018, but we only used data before the 2017 spring migration due to missing data. Bird No. 5 was tracked for four spring migrations and three autumn migrations, between 2016 and 2019. In total, nine spring migrations and four autumn migrations were tracked from five individuals.

Breeding/summering areas and migration patterns

Among the five black-necked cranes wintering in Nyingchi Prefecture, four (No. 1, No. 2, No. 3, and No. 5) spent their breeding/summering season in Qinghai Lake, and the remaining bird (No. 4) spent its breeding/summering season in the Jinzihai Wetland of Dulan County, Qinghai Province (Fig. 1). Their migration routes were highly consistent both intra- and inter-annually in our study (Fig. 1).

From March 12 to April 10, the black-necked cranes underwent spring migration, which lasted 8.7 ± 4.6 days and covered a distance of 1,182.5 ± 90.4 km. From September 26 to October 31, the cranes had their autumn migration, which lasted 30 ± 10.6 days and covered 1,455.7 ± 138 km (Table 2; Fig. 2). The straightness metrics differed significantly between the migration seasons (spring: 0.80 ± 0.07, autumn: 0.66 ± 0.05; p = 0.009). The migration speed also differed significantly (spring: 181.8 ± 93.2 km/day, autumn: 52.9 ± 12.7 km/day; p = 0.027) (Fig. 2).

Approximately 96.6% (172 / 178) of the fly points with a velocity >10 km/h in the spring migration were between 07:00 and 20:00, and 87.4% (118 / 135) of the fly points with a velocity >10 km/h in the autumn migration were between 08:00 and 19:00. These results indicate that the black-necked cranes mainly migrate in the daytime (Fig. 3).

Stopover sites

Twenty-one stopover and roosting sites were identified for the spring and autumn migrations (Table S1), with three spring stopover sites (Basom, Yalong, and Shazhuyu) and three autumn stopover sites (Shazhuyu, Tsaidam, and Xiongqu Rivers; Fig. 1). The heat maps (Fig. 1) identified Basom Lake as the most important stopover site during the spring migration—there were 686 (38.5%) points located in the lake, and the cranes used it as a stopover site in seven of nine spring migrations (stopover duration = 4.4 ± 3.7 days in March and April; Fig. 2). Basom Lake (29°30′N, 93°36′E) is located in southeast Tibet, within the upper valley of the Ba River. This lake has abundant biological and water resources, with an area of approximately 26.5 km² and an altitude of 3,464 m (Yang, 2019). It is suitable for the subsistence of the black-necked crane. For the autumn migration, the Shazhuyu River was the most important stopover site—there were 1,891 (76.3%) points located in the river, and it was used as stopover site in three of four autumn migrations (stopover duration = 26.3 ± 10.7 days in September and October; Fig. 2). Shazhuyu River (36°33′N, 99°22′E) is located within the Shazhuyu Basin of Gonghe County, Qinghai Province. This river is close to Qinghai Lake and can be used as a stopover site for the black-necked crane during early autumn migration. Seventeen stopover and roosting sites (81%) were located in or near the valley and lakeside swamps, and four were located in meadow swamps (19%; details in Table S1).

Conservation gap

We found that only 17.7% (516/2,914) of the black-necked cranes’ stopover and roosting site points were in protected areas (Fig. 4; Table 3). During migration, the cranes stopped at three nature reserves: the Yarlung Zangbo Grand Canyon Nature Reserve in Tibet (66 points, 12.8%), with a cumulative stopover duration of 9 days; the Sanjiangyuan National Park in Qinghai (432 points, 83.7%), with a cumulative stopover duration of 43 days; and the Changshagongma Nature Reserve in Sichuan (18 points, 3.5%), with cumulative stopover duration of 4 days. The most important stopover sites, Basom Lake and the Shazhuyu River, were not located within nature reserves.