Tag identification is an important tool in RFID systems with applications for monitoring and tracking. A RFID reader recognizes tags through communication over a shared wireless channel. When multiple tags simultaneously transmit their IDs to a reader, the tag signals collide and this collision disturbs the reader's identification process. Therefore, tag collision arbitration for passive RFID tags is a significant issue for fast identification. This paper presents two adaptive tag anti-collision protocols, an Adaptive Query Splitting protocol (AQS), which is an improvement on the query tree protocol and an Adaptive Binary Splitting protocol (ABS), which is based on the binary tree protocol, which is a de facto standard for RFID anti-collision protocols. To reduce collisions and identify tags efficiently, adaptive splitting protocols use information obtained from the last process of tag identification. Our performance evaluation shows that AQS and ABS outperform other tree based tag anti-collision protocols.