The meaning of sentence 13 is unclear. What change should Derek make to improve the clarity of this sentence?

(12) Birds are some of the most unusual sleepers in the animal world. (13) Some have to migrate thousands of miles every year, and it leaves very little time for sleep. (14) The thrush is one bird that has found a way to survive with minimal sleep. (15) This bird lands briefly for short “micro-naps,” each lasting mere seconds, which is just long enough for it to re-energize for the next phase of the journey. (16) Some birds can even enter a form of sleep while flying, similar to the way underwater mammals can let half of their brain sleep while the other half is awake.