Azure-winged magpies solve string-pulling tasks by partial understanding of the physical cognition

String-pulling is one of the most widely used paradigms in animal cognition research. We investigated how azure-winged magpies Cyanopica cyanus solve multiple-string problems that they have never encountered before. In Experiment 1, the strings were arranged in parallel, slanted, or crossed to investigate what rules azure-winged magpies use to solve multiple spatial relations of strings. Experiment 2 assessed whether the subjects understood the connection between the string and the bait while taking advantage of broken strings. In Experiment 3, the subjects were confronted with strings of different lengths attached to rewards in order to explore whether the string length, as a proxy for the pulling efficiency or reward distance, was crucial for the birds' choice of which string to pull. Generally, the birds were successful in tasks where the reward was close to the correct string's end, and they relied on a "proximity rule" in most cases. The results showed that azure-winged magpies had a partial understanding of the physical principles underlying the string-pulling but were stumped by complex spatial relations. They likely relied on simple strategies such as the proximity rule to solve the tasks. The effects of individual difference and experiential learning on string-pulling performance are also discussed.