TIRED, DIRTY AND HUNGRY after a day of travel and photography, my husband and I arrived at the Yellowstone campground too late in the day to find a spot, even in overflow. A sympathetic park attendant directed us down a rough, rarely used road to a parking lot at the back of beyond. Climbing out of the truck, we decided to check out the surrounding forest before fixing supper. We could hear a bird calling ahead. As we stepped into a clearing, we spied a young great grey owl ensconced at the top of a broken tree trunk. Although nearly completely fledged, it had enough remaining baby fluff on its head to give it a comical look as it sat squawking. As we watched, an adult great grey glided silently out of the dense trees at the forest edge and delivered a nice, plump vole to junior. All thoughts of our own meal forgotten, we hunkered down in the brush. The adult owl returned to a perch at the clearing edge, sat staring at the tall grass below its feet and plunged, talons first. Barely hesitating, it rose back into the air with another furry tidbit for the little feathered pig that was still calling for food. We watched the adult repeat this process several times, until it became too dark for us to find our way back to the truck without sustaining injury. The cuts and bruises were worth it. Great grey owls never fail to enchant me with their silent flight, beautiful markings and deadly hunting accuracy. They are northern birds, ranging from Alaska across to western Quebec, and south into northern Minnesota. They also live in high-elevation mountain forests as far south as central California and northwest Wyoming. Other great greys make their homes across northern Eurasia. The scientific name, Strix nebulosa, refers to the bird's nebulous or "misty" colouration. Those fluffy feathers keep the owl warm and make it look deceptively bulky. An armful to hold, it weighs only 1 to 1.5 kilograms (two to three pounds). By length, it is the largest owl in North America, and it has the biggest wingspan at 140 centimetres (55 inches). However, both the snowy owl and the great horned owl weigh more. Great grey owls in Canada and Alaska dine almost exclusively on mouse-like voles. Hunting night and day, the owl often waits on a perch until spying or hearing its prey. In winter, it relies almost totally on its excellent hearing to detect the squeaks of voles scurrying about in their snow tunnels. The parabolic discs of feathers on the owl's face gather sound, and its asymmetric ear openings let it precisely locate the source of the noise, through up to half a metre (1.5 feet) of snow, and from more than 25 metres (27 yards) away. Having pinpointed its prey, the owl plunges feet first into the snow after it. Scientists report that a great grey can crash through snow crust that will support an adult person. Dependence on one food source has drawbacks. Small rodent populations are cyclic, reaching a peak and then declining sharply. When food is scarce, great greys and other owl species travel out of their normal winter range in migrations known as irruptions. The winter of 2004-2005 saw one of the largest recorded irruptions. People as far south as Minnesota and Wisconsin and as far east as Ontario and Quebec were treated to a rare glimpse of this great grey ghost. Many called it the thrill of a lifetime. If voles, also known as meadow mice, abound where you live, watch for great grey owls, as well. - REBECCA L. GRAMBO