Six birds. According to relevant public information, there are 6 birds left on the tree, 6 of which fly in and 6 fly out. Simply calculate, the flying birds and the remaining birds add up to 12 cicadas, and subtract the 6 birds that fly in to get 6 cicadas. According to the calculation results, we know that there are six birds in the tree. This is an easy-to-make-mistake problem in first-grade mathematics: pushing backward the application problem and training children's ability to push down the calculation.