DISCUSSION
The impact of invasive plants on native species cannot be generalised. With the increase in invasion time, some native species will gradually adapt to invasive plants, where they activity, forage, or build habitats, more native birds will also find use of invasive plants (Schirmel et al., 2016; Blackburn et al., 2011; Graves, 2019). Different birds have different adaptive abilities, and a few birds with flexible behaviours and a wide range of diets can adapt and take advantage of new invasive habitats faster and better (Dylewski et al., 2019). After Smooth Cordgrass invaded from the east coast to the west coast of the United States, as time went on, birds, such as the Clapper RailRallus crepitans , Merida Wren Cistothrus meridae and Song Sparrow Melospiza melodia , also began entering and reproduction in invasive habitats (Nordby et al., 2009; Delach, 2006; Lampert et al., 2014). In Chongming Dongtan, Shanghai, China, the invasion of Smooth Cordgrass attracted the nonnative species of Marsh GrassbirdHelopsaltes pryeri to enter and nest in it (Ma et al., 2011). It has also been recorded in Yancheng, Jiangsu, that the native bird, Plain Prinia Prinia inornata , adapts to and utilises Smooth Cordgrass and enters it and sings during the breeding season (Chen et al., 2022). However, invasive plants may also be potential ecological traps. Although native birds have entered Smooth Cordgrass to reproduce, their nests are more likely to be destroyed by mammals, and the reproductive success rate is significantly reduced (Nordby et al., 2009; Fisher et al., 2011; Hao et al., 2015). Moreover, the courtship calls of native bird entered Smooth Cordgrass have changed, which greatly reduces the courtship success rate (Chen et al., 2022).
Studies and reports have shown that the Reed Parrotbill is a bird that is highly dependent on reed wetlands for survival, and all stages of its life cycle are completed in reeds (Xiong et al., 2013). It prefers the use of a mixture of old, new, and dense reeds (Boulord et al., 2011). During the whole overwintering period, we found that the Reed Parrotbill entered Smooth Cordgrass flocking and chirping. There used to be a large area of reeds in that area, and the invasive Smooth Cordgrass is located on the other side of the unsurfaced road near the coast. On the other hand, Smooth Cordgrass may have invaded at least 40 years, dead and new Smooth Cordgrass are mixed. Dense vegetation can also support behaviours such as standing and singing. We conjecture that the original reed habitat was destroyed by human interference and the Reed Parrotbill forced to enter the Smooth Cordgrass. The Reed Parrotbill in this area passivily entered Smooth Cordgrass, but initiatively evolution behaviors to adapt the Smooth Cordgrass. It can be determined that Smooth Cordgrass provide an important overwintering place for the Reed Parrotbill. At present, we have not found the nests of the Smooth Cordgrass, but the native species of the Vinous-throated Parrotbill has been nesting in the Smooth Cordgrass (the same family with the Reed Parrotbill, and they both rely on reeds to nest) (Chen, 2020). We speculate that if the Smooth Cordgrass continue to exist and there is no suitable reed habitat, the Reed Parrotbill will also nest in Smooth Cordgrass. However, it still needs further observation and research.
At present, the Chinese government mainly takes physical measures, such as manual uprooting, mowing, shading, flooding, and burning, supplemented by chemical measures to suppress the spread of Smooth Cordgrass to completely remove it (Xie et al., 2019; Tang et al., 2021). According to the experiment in the removal project area, the species and number of birds increased significantly after Smooth Cordgrass was completely removed, but it had a greater negative impact on macrobenthos (Ma et al., 2017). In addition, the use of physical methods has the disadvantage of requiring multiple removals, the cost is relatively high, and it easily causes secondary intrusions (Zhao et al., 2022). However, simple removal of chronic invasive plants may sometimes not be an appropriate management measure. On the east coast of the United States, with an increased history of invasive Smooth Cordgrass and hybridisation with native Spartina, the Clapper Rail has become acclimated to the new Spartina and has become increasingly dependent on this habitat. The removal and management of Smooth Cordgrass has even seriously threatened the normal reproduction of this species (Lampert et al., 2014). The discovery that this specific bird Reed Parrotbill flocking and singing in the invasive Smooth Cordgrass suggests that coastal birds in China may be repeating what happened in North America, and simple removal may not be suitable for the long-term invasion of Smooth Cordgrass. We need more intensive and comprehensive research to find suitable ways to govern Smooth Cordgrass.