Terns usually will aggregate in the same reproduction area during breeding season. The color pattern of eggs may match the background, and it is important for breeding adults to distinguish their own eggs, especially in colonies with high density. This research was carried out at three islands, Huolung Shoal, Baisha and Jishan Islets of Penghu from May to August of 2007 and 2008. The experiment was divided into three parts, egg moving (EM) experiment, nest modification (NM) experiment and egg switching (ES) experiment. Terns spent more time for searching their own nests of each of the experiment situation. In the EM experiment 69.7% Black-naped Tern adults settled on original nests without eggs in 2007; 52.0% Roseate Tern in 2007 and 29.6 % in 2008 did the same. In the NM experiment, the behavior patterns of returning adults were significantly different between each operation and the control group. Adults were confused by the interference of the experimental conditions. In some occasions, adults settled on incorrect eggs quietly in ES experiment. In conclusion, terns have low ability of egg recognition but high ability of nest recognition. Terns presumably can recognize nests with the nearby environmental landmarks, but not be able to recognize eggs.