When your bird reaches the maximum level for its current stage of evolution, you can level it up even higher after evolving it.

Birds with star ratings from 1 to 3 can evolve three times, and the highest level they reach is 60, whereas 4-5 star birds can evolve five times up to level 100.

You can level up birds with a 5-star rating even higher by awakening them. Feel free to learn more here.