The early morning air was cool and evening dew hung heavily on the grass. Before us, a lovely stream slipped down a narrow valley. Birdsong filled the still-brightening sky, and all around us were the darting images of birds in flight.

Straight ahead, at the spot where the terrain leveled off, our group leader had spotted a "good bird." A warbler was foraging for breakfast on some branches at eye level about 10 yards ahead.

The bird was almost uniformly gray, on top, from its nape to body, wings and tail. The head was also mostly

gray with a black patch on the forehead, large yellow-white eye rings and short, thin yellow lines that connected those rings to the bill. The bird appeared to be wearing fashionable over-sized glasses.

Underneath, the active warbler was almost uniformly yellow until one got back to the white vent. The bright yellow body had one more distinguishing mark: a series of black streaks across the top of its breast.

It looked like a black necklace against a yellow blouse.

Like most warblers, the Canada warbler (Wilsonia canadensis) is small. The bird is about 5 inches long with an 8-inch wingspan. It weighs less than two-fifths of an ounce.

The bird we were watching was a male. The gray of the females is somewhat muted and the necklace is not nearly as well-defined. This guy was feasting on the myriad insects that were rising on the faint wisps of morning heat.

A small group of us had gotten permission to enter the National Arboretum before normal operating hours. We were there to see the spring migrants that use this 400-plus-acre oasis as a safe stopping point on the flight farther north.

In the case of the Canada warbler, he had started his journey weeks ago in the Andes. His migration route took him up through Central America, west of the Gulf through Mexico and Texas, before he and other members of his species spread out across the Midwest and southern states to continue their flights north.

The bird we were watching was likely far from his destination. As his name suggests, he was most likely heading to Canada to breed.

In the Chesapeake region, breeding pairs of Canada warblers are found where temperatures are cooler. The Appalachian forests from southwest Virginia into Pennsylvania are good spots. Above the Mason-Dixon Line, the temperatures are moderate enough for Canadas to slip down from the mountains and spread across the terrain from New York and New England to the Great Lakes and the eastern half of Canada.

Although they have an extremely long migratory route, Canada warblers get off to a slow start. They are among the last of the spring migrants to arrive; typically in May across the Chesapeake basin. They will also be among the first to return, leaving breeding grounds in late August.

Places like the National Arboretum in Washington, DC, are particularly valuable for migrants like the Canada warbler. Migrating birds are big energy consumers. Frequent stops are needed to refuel and rest.

Here along the Western Hemisphere's most densely developed corridors, migrants couldn't survive without these vital refuges. The cool microclimate of Hickey Run in the Arboretum gives the Canada warbler what it needs: the right temperature, plenty of food and water, and enough space to avoid predators.

The Canada warbler appears to feed exclusively on bugs. Insects are high in protein and fat, which are critical nutritional elements of a long-distance migrant's diet.

The National Arboretum was never envisioned as a migratory bird refuge. It is owned and operated by the U.S. Department of Agriculture, which uses the space for research and to cultivate healthy trees and bushes for the country's farms, orchards and backyards.

The migrants don't care about that, of course. They simply needed a pit stop on the long flight north.

I'm sure that my little group of fellow birders wasn't focused on the arboretum's history, either. We had come to see some beautiful birds.

The Canada warbler certainly fit the bill. I found myself admiring the bird's handsome coloration as well as its skillful gleaning of insects.

All I could hear were bird songs and the faint gurgling of the little stream.

The Arboretum is bordered on the north by U.S. Route 50 and its heavy traffic. At that hour, the road was jammed with bumper-to-bumper commuters.

At the bottom of this little valley, trees and terrain blocked off traffic noise. For a moment, the cars were unseen and unheard.

I refocused my attention on the Canada warbler and considered how wildly improbable it was that my path would cross with this handsome songster. I was lucky enough to be in this accidental refuge at an unorthodox hour to capture this moment in a migration of thousands of miles.

This was no miracle, though. Such wonders abound. I simply had to stop long enough to look carefully and listen mindfully.