The wing collection survey offers more clues. This cooperative effort between woodcock hunters, the U.S. Fish and Wildlife Service (USFWS), and state wildlife agencies provides data on where, when, and how many woodcock are harvested each year. Hunters are asked to mail a wing to the USFWS from each bird they kill, indicating the date and location of the harvest. State and federal biologists sex and age each wing, which gives them a peek at annual recruitment (the number of young produced each year) and, consequently, population trends.