As we looked skyward, an endless palette of greens blocked our view of the tree canopy, which soared 80 feet or more overhead. Our futile gazes were in search of a hidden bird, drawn by its persistent, penetrating high trill. The song wasn't pretty, but it was unmistakable. The note rose in intensity until it suddenly dropped to nothingness.

We were in the National Arboretum, the 400-plus-acre oasis in Northeast Washington, DC. After recent surgery and a series of medical setbacks, I was taking full advantage of the arboretum's paved paths. I needed the physical therapy involved in the outing and the mental break that I find in birding.

The path led us to a break in the foliage. We craned our necks anew and were rewarded with glimpses of tiny warblers flitting about in nearby treetops.

The voice and habitat alone were enough to convince us that we were looking at northern parulas (Parula americana), but we wanted to take a few minutes to get a better look at these colorful wood warblers.

Northern parulas are tiny birds, weighing about a third of an ounce. They measure just 4.5 inches from their bills to the end of their black tails. When the specimen is that small and 80 feet overheard, you'd better be equipped with good binoculars and cooperative lighting. Fortunately on this early summer's day, we had both.

One of the birds stopped its aerial dance in search of insects and perched obligingly on a terminal twig. He was still far away, but at least he was holding still long enough for closer inspection.

The yellow throat and chest caught my eye first. The head and wings were a cool blue-gray, contrasting with his white belly. The wings sported two white wing bars. Arcs of white also circled his eyes. The yellow of his throat blended seamlessly into the lower bill. The upper bill, in contrast, mimicked the blue-gray of the cap. Across his chest, reddish and black bands interrupted the yellow. His back was olive green.

The females look the same; except they don't have the chest bands and the colors are a bit more subdued.

Like other warblers, parulas are neotropical migrants. These birds head for Central America and the Caribbean each winter. On the return trip north in the spring, northern parulas are looking for a specific habitat.

In the southeastern United States, they will look for tall trees with Spanish moss. Farther north, when the Spanish moss gives out, other epiphytes, particularly old man's beard lichen, serve the same function. The females use the abundant epiphytes as nesting materials. In trees with extensive epiphytes, a nest constructed of such materials doesn't look conspicuous.

In Central America and especially in the Caribbean, loss of habitat is having an impact on population density. In the United States, the loss of epiphytes, primarily to air pollution, strips the forests of an essential evolutionary element. Breeding bird survey data show a distinct band in New York State and lower New England where there are no parulas successfully breeding. Acid rain has killed off the sensitive lichen. And where there is no lichen, there are no parulas.

The area without parulas coincides with the worst cases of acid rain in the nation.

Poorly regulated power plants in the Midwest have spent decades pouring pollution into the upper atmosphere. Prevailing winds carry these plumes of pollution to the Northeast. Sulfur and nitrogen oxides are eventually washed out of the sky, with the damage concentrated in Central New York and across a swath of New England.

The acid rain has a devastating effect on the lakes in the region, robbing them of much of their native aquatic life. The high-pH rainwater has also been lethal to epiphytes. Parulas have lost nesting habitat while humans suffer from smog and soot. Thankfully, regulators have begun to crack down.

The parula in the arboretum didn't face those challenges. He left his perch to resume gleaning insects from the high foliage. Another trill rose to its crescendo before suddenly disappearing.

To some, the abrupt ending of the bird's song suggests this delicate species might suddenly be eliminated by a combination of pollution and habitat loss.

I choose to hear a different meaning. These are tiny but rugged birds built for survival as well as beauty.

They will spend the summer producing another generation. Then they will make the long migration from the Chesapeake to the Caribbean and back. They will have to work around areas that humans have destroyed or despoiled.

But next year they will be back, at the top of the tree, announcing anew their presence. It's the song of a survivor, and I find it inspiring in spite of its noisy nature.