Tread quietly along a river in the old-growth forests of Primorye, Russia, and you may be lucky enough to spot a Blakiston's fish owl (Bubo blakistoni), one of the world’s largest and rarest owls. Weighing up to 4.5 kg, with a 1.8-meter wingspan, the hulking predator feasts on salmon and other fish that migrate up the rivers from the Sea of Japan. Logging and mining have begun to impinge on the endangered owls’ habitat throughout Eurasia; to learn more about how these activities might affect the birds, researchers set out on foot to hunt for owl nests over in a 20,213 square-kilometer region of the central Sikhote-Alin Mountains. After locating 14 nests by sight, listening for calls or searching for owl tracks and feathers in the snow, they searched for clues about what made the local real estate attractive. Not surprisingly, the birds tend to locate near a river for easy access to fish, and build their nests in trees old enough to contain large decayed cavities. The researchers also discovered something unexpected: The owls tend to nest in areas where old-growth trees have died and fallen into nearby streams, creating the deep pools and complex, shallow channels that provide ideal spawning ground for fish. These logjams also help fish survive in streams through the winter, the researchers report this month in the journal Oryx. In other words: Ancient trees provide owls not only shelter, but year-round nourishment.