One day in 2009, Frank Rheindt was wandering up a forested mountainside on an Indonesian island when the skies opened up. He had spent months planning this trip, days finding a charter boat that would carry him to this remote place, and hours plodding uphill, but the local tour guides insisted that the rain would make the search impossible.

Reluctantly, Dr. Rheindt agreed to head to lower ground.

But on the way down, even with the deluge, he was startled by the sight of a thrush sitting on a log. Dr. Rheindt, an ornithologist, knew that a thrush shouldn’t have been on that island, and that the species normally would seek shelter from the rain.

A little farther along, he heard the distinctive call of a grasshopper warbler, an endangered bird that’s normally hard to spot. “I could tell from the sound that it was a grasshopper warbler, but different than I was used to,” he said. “That’s when I knew I was going to come up again.”