Unlocking urban green spaces: Retrofitting potential green roofs to enhance bird connectivity and comprehensive ecological benefits in high-density areas

Despite widespread enthusiasm for green roofs (GRs) as an eco-friendly solution to various urban issues, studies regarding GRs in the context of bird conservation remain limited. In this study, the Nanjing urban area (NJUA), a typical high-density area in China, was evaluated to determine where rooftops can be potentially retrofitted to become GRs. This process of identifying potential green roofs (PGRs) started with site identification and continued with a benefit assessment that was constructed by synthesizing machine learning and multiple models. It was found that: (1) Of 39.80 km² of roof patches identified, approximately 26.38 % of the building space was suitable for GR retrofitting, i.e., PGR. (2) The core bird flight corridor in the NJUA occupied about 5.42 % of the overall area. In another 65.54 % of the urban space, birds could not move freely. In all, 2.01 km² of PGRs would improve bird connectivity. (3) Of this 2.01 km² of PGRs, 0.99 km² (1178 patches) was considered medium to high ecosystem service benefit area, and these patches were considered to be the most highly effective stepping stones that should be prioritized for retrofitting. (4) In simulations, the high-efficiency PGRs in NJUA were shown to be effective in enhancing bird corridor space. If the PGRs were all retrofitted, bird activity space would be expected to grow by 12.42 %, resulting in a 48.69 km² enhancement, though core corridor space would only increase by about 4.09 %. These results provide scientific evidence that the application of GRs would be beneficial in highly urbanized areas as well as a methodological reference for PGR retrofitting efforts in high-density areas globally.