Most upland bird hunters are familiar with the ups and downs of ruffed grouse populations. Such cycles of abundance and scarcity are typical of all grouses, ptarmigans, and even prairie chickens. The fluctuations are usually blamed on the weather-even sun spots-or on parallel cycles in the abundance and scarcity of predators.
But Peter J. Hudson of the University of Stirling in Scotland had a hunch that predator cycles were more reactive than causative. He believed parasitic worms are more central to grouse cycles than are foxes or hawks.
In his long-term study, Hudson and a team of graduate students medicated 3,000 birds in three different populations of Scottish red grouse. The treated birds thrived, even when most others died during the "down cycle."
After the fallen populations recovered, Hudson left two of his test groups untreated while continuing to medicate the third. Most of the untreated birds perished when red grouse numbers plummeted four years later, but Hudson's medicated birds continued to flourish.
Grouse numbers may still be affected by weather and predators, but the triggering mechanism for declines seems to be parasitic infestations. As Hudson summed up, "This is the first time we've proven what causes a population cycle."