As days get shorter, and nights grow longer, the cast of characters on my bird feeders changes. A mixed flock of pine siskins, purple finches, juncos and a few red-breasted nuthatches has joined the usual crowd of chickadees, titmice, nuthatches, cardinals, song sparrows and goldfinches.But it is a tiny winter bird that captures my attention as fall morphs into winter. Its call comes from high in one of the few conifers in the yard. The high-pitched voice is distinctive. It's the call of a golden-crowned kinglet -- "zee, zee, zee."Like many birds, kinglets are often heard before they're seen. When I looked up, I spotted two tiny greenish birds hovering as they gleaned insects from the tree's evergreen needles. Kinglets weigh about 6 grams -- a bit less than a quarter-ounce and about half the weight of a chickadee. I usually need binoculars to see them well enough to identify them.Golden-crowned and ruby-crowned kinglets are two of the tiniest songbirds in North America. Some pass through as they work their way south; some winter as far north as Maine. Kinglets are small, olive-colored birds. Both species are about 4 inches long, they have wing bars, and their bellies are lighter than their bodies.Look to their heads for differences between these two insectivorous songbirds. Black stripes border the fiery orange top of a male golden-crown's head. The female's crown is yellow. Both sexes also wear a broad white eyebrow stripe, so the head has a distinctly striped appearance.Ruby-crowns are duller. Females lack the red hat that characterizes males, though the male's red feathers are seldom visible. Only when males get excited or agitated, perhaps by the presence of a hawk, do they erect these feathers and display the colorful ruby crown. The keys to recognizing ruby-crowns are the white eye-ring and the absence of stripes on the head.Kinglets nest in the coniferous forests of the Rocky Mountains and across the northern U.S. and Canada, though golden-crowns also nest south along the spine of the Appalachians. Remarkably, these tiny birds lay clutches of seven to nine eggs. They accommodate such large clutches by arranging the eggs in two layers. Furthermore, the nest is usually so deep that the incubating parent is concealed beneath the rim of the nest. Incubation lasts 12 to 15 days, and two broods are typical.Because kinglets are so small and only visit backyards occasionally, they're unfamiliar to many people. But if you know what to look for, they're easy to spot. Be alert for active tiny birds often associating in loose flocks with chickadees, titmice, brown creepers and fall warblers.Kinglets forage at the tips of branches and sometimes hover while gleaning small invertebrates and egg cases from hard-to-reach leaves and twigs. Sometimes they venture to the ground and forage amid the leaf litter. Wherever they are, though, kinglets seem to move constantly and flick their wings as they move from limb to limb. Seldom does a kinglet perch quietly to allow a birder a leisurely look. These very behaviors, coupled with the distinctive head markings, make identifying kinglets manageable.But exactly what are kinglets eating high in the treetops in midwinter? Biologist Bernd Heinrich answered that question in his classic Winter World (2003, HarperCollins). Working in the frigid north woods of Maine, Heinrich was equally befuddled. How could such tiny birds overwinter in places where nighttime temperatures often plunged below zero?So Heinrich collected (killed) some kinglets at dusk when he knew their bellies would be full. His first discovery was that the kinglet's body temperature immediately after death was 111 degrees. And its belly was filled with 39 inchworm caterpillars - in the middle of winter! Until then, no one knew or even suspected that some moths overwinter in the treetops as caterpillars.Because kinglets eat invertebrates almost exclusively, they ignore the seeds we offer in feeders. They may, however, eat suet or peanut butter/suet mixes. Chilly fall mornings and hard frosts send insects to their winter refuges, so watch for kinglets at suet feeders.Send questions and comments to Dr. Scott Shalaway, 2222 Fish Ridge Road, Cameron, WV 26033 or by email at email@example.com.