We were taking a break from the heat and the sun, sitting under a lovely grape arbor. The day had gotten progressively hotter and the sky milkier. Under the arbor, our eyes adjusted to the less intense light and took in the deeper shades of green that only shadows provide.

As we sat, a tiny flash of blue-gray and white sped away. A moment later, the bird returned, and went back to foraging up and down the outer branches of the vines in search of food.

The bird was white underneath and almost exclusively blue-gray up top. The wings looked a bit darker, as if some black or brown ink had been splashed in. A white eye-ring stood out against a backdrop of gray head and a slender, jet black bill.

The most remarkable thing, though, was the bird's diminutive size. Wings spread wide were just a bit more than 4 inches. My field guide says that I would need to hold five of them in my hands just to get one ounce.

Blue-gray gnatcatchers (Polioptila caerulea) are common in the Chesapeake region during the May–September breeding season. These gnatcatchers spend most of the year on their winter grounds in the U.S. Gulf states, Mexico, Honduras, Guatemala and the Caribbean.

The spring migration sends them broadly throughout the Northern Hemisphere, up to New England and across to the southern Great Lakes and over to the California/Oregon border.

Although there are several gnatcatchers found in North America, the blue-gray is the only migratory species and the only one seen routinely in the Chesapeake states every summer.

As the broad range of habitat suggests, the blue-gray gnatcatcher is flexible about breeding territory. It prefers deciduous forests and streamside thickets where available, but will readily take to live oak, pinyon-juniper and chaparral as its primary habitat in the U.S. Southwest and Mexico.

When the birds arrive, the male and female work together to construct a delicate cup-nest. Made of plant down, spider webs and other gossamer materials, the nests are decorated on the outside with bits of tree lichen. The female will then lay one egg a day for three to six days. These nests can be found on branches ranging from the very low to the high.

Both parents incubate the eggs, although only the female has a true brood patch and she does most of the sitting.

After two weeks, the naked, hungry birds emerge. Both parents feed

the young, but the duty is short-lived. In as little as 11 days, the new generation of gnatcatchers will leave the nest for good.

If the nest is not contaminated with parasites, a most peculiar thing may occur. The male may disassemble the nest, piece by piece, and reconstruct the home a short distance away, complete with lichen. When the building of the recycled nest is complete, the birds will mate again and raise a second brood. Why the birds undertake this odd behavior of recycling their nest is unknown.

The bird we were watching was a female. The only way to tell her sex is her slightly more drab coloration and the lack of the black forehead band that a male displays during breeding season.

Although they are tiny birds, blue-gray gnatcatchers are not especially difficult to spot. When they are foraging on the ends of branches, the birds fan their tails, showing wide white edges bordering black central tail feathers. The tail is rather long, and the bird rapidly swishes it back-and-forth or up-and-down. This combination of contrasting colors and rapid movements will startle the bird's prey. The bird strikes with deadly accuracy when the unfortunate insect makes a move.

Gnatcatchers eat aphids, flies, bees, wasps, butterflies, moths, beetles and ants. They are nearly entirely insectivorous, but will eat sunflower seeds and nut meal at well-placed backyard feeders. Apparently, some free food is just too good to pass up.

As soon as the female gnatcatcher flew off, a male showed up at the grape arbor. We sat and watched a bit longer, enjoying both the cooling shade and the avian show.

These gnatcatchers had traveled as much as 2,000 miles to get here. Years of evolutionary development had proven that the arduous trip was worthwhile. The food here was plentiful and nesting areas abundant. Countless generations had gone into the development of that long, bi-colored tail and its twitching actions. Some equally compelling Darwinian forces were at work on the male's peculiar second-nest construction behavior.

To enjoy these extraordinary forces of nature at work, all we had to do was sit back and watch the show with inquisitive eyes. It is hard to imagine a sweeter lesson in evolutionary biology than watching these gnatcatchers simply go about their lives on a hot summer day.