The "hitchhiker" whose warning cries help a rhino (31-Down) avoid poachers, as featured in The New York Times crossword puzzle on September 5, 2023, is the tickbird. This bird is known for its symbiotic relationship with large mammals like rhinos, where it alerts them to danger such as approaching poachers by making warning cries

. The tickbird, also known as an oxpecker, feeds on ticks and other parasites found on the rhino's skin, effectively "hitchhiking" on the animal while providing a protective service through its alarm calls. This mutualistic behavior helps the rhino avoid threats, including poachers, by responding to the bird's warning signals

. Thus, the answer to the NYT crossword clue "‘Hitchhiker’ whose warning cries help a 31-Down avoid poachers" is TICKBIRD.