A long-term study of the Great Skua seabird shows how hard it can be to separate the natural and the man-made worlds. 100 years ago, hunting pushed the large brown gull to the brink of extinction. It was eventually saved by fishing fleets that threw mountains of unsalable "bycatch" into the sea. Now that new fishing rules are shrinking that bycatch, the Skua are feeding on smaller birds whose colonies may soon be at risk. NPR's John Nielsen reports.