Mangrove Habitats in Zhanjiang of South China: A Potentially High- Risk Environment for Breeding Birds

This study was done in Zhanjiang Mangrove National Nature Reserve, located at the southernmost tip of mainland China (20°15′–21°55′ N, 109°40′–110°55′ E). Patches of mangroves are scattered along the coastal mudflats of Leizhou Peninsula in southwest Guangdong Province that faces the South China Sea in the east, with the Beibu Gulf in the west connected to Guangxi and with the Qiongzhou Strait in the south facing Hainan Island across the sea, covering an area of approximately 200 km2. The reserve is located in the transition zone from the northern tropics to the southern subtropics and is influenced by the monsoon and marine climates. The average annual temperature is 23 ℃, with a maximum temperature of 38.8 ℃ and a minimum temperature of −1.4 ℃. The average annual precipitation is 1534.6 mm, with rainfall concentrated between April and September, which is also the typhoon season (Zou et al. 2008). The coastal tidal patterns are varied, with an average tidal height of approximately 2 m. From August to December 2019, artificial nest experiments were set up in the Jiulongshan and the Caoyang mangrove areas in the Zhanjiang Mangrove National Nature Reserve to study the nest predation rates and predator types in the mangrove forests of the two sites. Artificial nests were woven from rattan (Figure 1A). The artificial nests were placed both in trees and on the ground, with the height of the nests on the trees being approximately 2 m from the ground. The locations of the artificial nests were randomly chosen, with a distance of approximately 30 m or more between any two nests. There was 0% coverage for all nests that were placed 2 m above ground on trees and 0.5 m above the ground. The only difference between the artificial nest experiments at the two sites was that quail eggs were placed in the artificial nests in the Jiulongshan mangrove area, whereas pigeon eggs were placed in the artificial nests in the Caoyang mangrove area. Crab predation on bird eggs or chicks has been reported (Yang et al. 2014; Kwon et al. 2018), and many types of crabs have been identified in the mangrove reserves of the study area. Therefore, the predation of bird nests by crabs in the Caoyang mangrove area were investigated, i.e., two munia (Lonchura spp.) eggs, or a munia egg and a pigeon egg, were placed 10 cm from a crab hole and the predation of bird eggs by crabs was observed and recorded. We observed all experimental nests or experimental eggs continuously for 7 days (the period within which all the observation nests were predated) and recorded the outcome of the experimental eggs. In addition, some nests were videotaped to record the predator type. The results showed that the nest predation rates in the Jiulongshan mangrove area in the reserve were substantially higher than those in the Caoyang mangrove area during the first three days for both the artificial nests on trees and on the ground. However, the nest predation rates were 100% for the experimental nests in both mangroves within seven days. There were diverse predators, including the greater coucal (<i>Centropus sinensis</i>), Oriental magpie-robin (<i>Copsychus saularis</i>), white-breasted waterhen (<i>Amaurornis phoenicurus</i>), mangrove crabs (<i>Chiromantes dehaani</i>), and rats. Our findings suggest that the diverse predator types and high predation risk in mangroves may be an important reason why few birds breed in the mangrove.