Just watching the energetic chickadees was making me delightfully tired that late autumn afternoon.

The plump black-and-white birds would fly to the feeder, grab a single sunflower seed, and immediately dash back to the leafless willow to crack open the shell and eat the rich kernel inside. A moment later, the bird repeated the circuit.

A constant interplay of chickadees, titmice, goldfinches and sparrows added to the sense of nonstop action.

I was so captivated by the perpetual motion that I nearly missed the tiny birds flitting among the tawny-colored ferns just a couple of yards beyond the feeders.

Chickadees are small birds, but the olive-green birds nervously working the dormant fern bed were even tinier. I grabbed the binoculars for a closer look. The background birds had white wing bars with matching white eye rings. Their bills were as slight as the birds. I could see a blush of yellow on the edge of the short tail and on the constantly-flicking wing feathers. These diminutive visitors were ruby-crowned kinglets (Regulus calendula).

Kinglets are some of the smallest birds in North America. Only the hummingbirds are smaller. We have two kinglet species in the United States: the ruby-crowned and the golden-crowned. These elfin birds are closely related in size, geographic distribution and behavior.

The colorful crests that give these birds their names are rarely seen. Only males have these bright head feathers, and they are typically hidden except during mating season or when the birds are aggressively defending their territory.

Ruby-crowned kinglets, like the pair I was watching out of our kitchen window, are about 4 inches from the end of their delicate beaks to the tips of their stubby tails. You could put five of these birds in your hand and they would collectively weigh just an ounce. Along with their slightly smaller cousins, the golden-crowned kinglets, they are the miniatures of the songbird family.

The ruby-crowns in the side yard would be nearly invisible given their subtle coloration and the dull browns of the fern bed. But these birds were actively feeding. Their constant movements took them from fern to fern, nabbing bugs too small for me to see. In addition to gleaning insects from the fronds, ruby-crowned kinglets will sometimes hover at the edge of a leaf to snatch the bugs that may be clinging underneath. Occasionally, these birds will also eat sap, seeds or berries as they diversify their diets when cold weather makes finding sufficient insect food impossible.

Every summer, ruby-crowned kinglets breed across Canada and Alaska. They nest in trees, typically conifers, often in semi-open patches in the forest. Their nests are cup-shaped and lined with feathers, spider webs and moss.

The pair engage in predictable and stereotypic parenting rituals. The female is in charge of the home and early care of the babies while the male takes on the role of the provider. The female ruby-crowned kinglet takes care of the nest and lays the eggs, which she incubates for two weeks. The male, meanwhile, will be busy gathering bugs from twigs as well as spider eggs and other protein-rich food for himself and the mother. After the chicks hatch, the job of feeding hungry mouths multiples. The female will stay on the nest for a few days, then join the male in bringing meals to the hungry youngsters.

In less than two weeks, the young birds will fledge. The parents will continue to supplement the diets of the baby birds, but the new kinglets will soon be on their own.

Ruby-crowned kinglets produce a single brood each year. Given the extraordinary size of the clutch, one brood is plenty. Females will lay up to a dozen eggs. The eggs are tiny, but that's still a lot of eggs for such a small bird. A dozen eggs will weigh 80 percent as much as the female's normal body weight. It is the largest ratio of eggs to body weight of any North American bird.

The kinglets in the ferns were flitting about, snatching insects and regaining some of the weight they have lost during their migration from Canadian forests. During the fall migration, kinglets will join mixed flocks of other songbirds for the long journey south.

Perhaps the kinglets I've been watching will spend their winter in the loblolly pine forests of Blackwater National Wildlife Refuge, or maybe they will continue all the way to Honduran rainforests. One thing is certain: This suburban yard is just a way station for these migratory midgets.

The flurry of activity at the feeders continued, but the nearly invisible action in the ferns was the focus of my ongoing attention. Like so many things in life, sometimes it's the little things in the background that are the most interesting.