[Objectives] The study aims to test whether there is a significant difference in the cognitive level between adult and sub-adult Black-tailed Gull Larus crassirostris. [Methods] From March to May 2023 and March to May 2024, we conducted experimental tests on Black-tailed Gulls (Fig. 2a) in the Heishi Reef sea area (Fig. 1) of Dalian, Liaoning Province, China. The experimental equipment consisted of two transparent acrylic boxes for four experimental approaches (Fig. 3), and the performance of Black-tailed Gulls in these four experiments was captured and recorded by a camera (Fig. 4). A total of 120 tests were performed on sub-adult and adult Black-tailed Gulls (Table 1), and all test options had two different possible outcomes (failure or success). The number of successes for both adult and sub-adult Black-tailed Gulls was counted and analyzed using SPSS 27.0 software, the Mann-Whitney U test was used to compare whether there was a significant difference between the two groups of Black-tailed Gulls in terms of cognitive ability. [Results] The results showed that the success rate of sub-adults was significantly lower than that of adult Black-tailed Gulls in all experiments (P < 0.05) (Table 2), indicating that there were significant differences in cognitive ability between the two groups of individuals. [Conclusion] Our experimental results show that Black-tailed Gulls are able to solve problems by observing and pulling on a string and have cognitive abilities, but there are age differences in cognitive levels among animals at different age stages, which has important reference value for people to understand the degree of intelligence development in animals, especially birds.