Abstract: Long-term and systematic biodiversity monitoring data is the basis for achieving goals of biodiversity conservation. The development of citizen science has promoted the large-scale data collection on birds with less resource investment. In this study, six years birdwatching data and a one-year formalized bird monitoring data were used to compare the similarities and differences between the traditional line transect method and birdwatching method, and the advantages and disadvantages of the two survey methods were explored for further promoting the application of public science birdwatching in bird diversity research and protection. To do this, birdwatching data were collected from July 2015 to June 2021 from public birdwatching data platforms, and the comprehensive survey of bird species and their distribution by fixed-distance line transects was conducted from July 2020 to June 2021 in Laoshan, Nanjing. List length was used as an indicator of the professionalism of the investigators. Mann-Whitney test was used to assess the differences in the counted number of bird species per survey trip by using the two different methods. By resampling the data within each group of observers, two hundred bootstrap samples were produced for each group and 95% confidence intervals were calculated for the detection rates. The results show that 214 bird species were recorded by the two methods, of which 83 species were only recorded by birdwatching activities. From July 2020 to June 2021, 125 species were recorded by birdwatching and 131 species were recorded by the line transect method. Comparing the detection rates of species between the two methods, the largest differences in detection rates were for breeding birds, uncommon raptors, and rare bird species in the study area. The monitoring intensity of transect method was 11, and the birdwatching was between 2.4 and 8. The peak of birdwatching activity was concentrated in May-June. The average number of bird species surveyed by birdwatchers per trip (23.57±20.04) was greater than that monitored by the line transect method (19.81±6.84). In the short term, neither of the two survey methods can completely record all bird species in the study area. In the long term, birdwatching data can fill the gap of traditional bird monitoring to a certain extent. However, spontaneous birdwatching activities are easily influenced by birdwatchers' preferences, with biases in time, space, and species identification, and by problems such as insufficient monitoring intensity and inaccurate data recording. Therefore, the design and specification need to be strengthened to reduce the biases in citizen science data sets and then provide a proper data base for bird research and conservation in a broader field.

Key words: biodiversity, scientific monitoring, citizen science, avian diversity, birdwatching, fixed-distance line transect, bootstrapping method