Valley Commentary : A Formal Introduction to a Shy, Gray, Endangered Bird : Seldom seen, the migrating songbird has two strikes against it: It prefers to nest close to rivers and streams and it builds its nest near the ground, close to predators.

When there’s news of endangered species, the chances are that the least Bell’s vireo will pop up in the story.

This is a small gray, unobtrusive bird that flits nervously through the trees as it searches for insects. It has a charming song: a questioning rise, a pause, then an answering fall: Cheedle cheedle chee? Cheedle cheedle chew! Aside from that, there’s little about the bird’s appearance to distinguish it.

What does distinguish it is its precarious status in nature and the fact that it makes its summer home along the Santa Clara River in Ventura County, among other places. As an endangered species, the little bird is the subject of government protection plans in much of Southern California and a restoration program in Ventura County.

The intention here is to introduce people to the bird, which is often written about but, like many once-common migrating songbirds, seldom seen.

The least Bell’s vireo (least because it’s the smallest of several subspecies) may be in worse shape than others. It has two strikes against it. It prefers to nest close to rivers and streams bordered by a thick growth of willow trees with lush vegetation, exactly the kind of riverside habitat in demand for housing and agriculture. As the human population increases, nesting opportunities decline.

The vireo’s other serious problem is predation. It builds its cup-shaped nest close to the ground, so the eggs and the nestlings are vulnerable to cats, dogs and other small animals.

An even greater threat is the brown-headed cowbird, a native bird that lays its eggs in nests of small birds. Cowbirds do not build their own nests but have developed the labor-saving method of conning other birds into raising young cowbirds. The female cowbird spots a bird building its nest, and when a finished nest is unattended, she slips in and lays her own egg.

This brood parasitism, as it’s known, has several interesting features. The cowbird egg usually hatches before the vireo eggs, and the young cowbirds frequently push the vireo chicks out of the nest. Even if they let the “host” chicks stay, baby cowbirds are almost always larger and noisier, so they get fed more frequently. And they grow very fast. It is a strange and pitiful sight to observe a small adult bird feeding a demanding interloper twice its size while its own young are neglected.

Why doesn’t the vireo recognize the strange egg and evict it from the nest or even abandon the nest and build another? On rare occasion it does, building a new floor to the nest on top of the unwanted egg and thus killing it. The normal vireo response, however, is to raise the cowbird as its own.

One scientific explanation is that the least Bell’s vireo did not always have two strikes against it. Its inability to outfox the cowbird is thought to result from historical inexperience with cowbirds. Before the pioneers moved West, the usual range of the cowbird was the Great Plains, where it foraged in the grass for the insects that the great buffalo herds kicked up. (Today, cowbirds follow cows.) The settlers cut down the forests, planted crops and laid out their pastures. Eventually the cowbirds moved in to share territory with the vireo.

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Species of birds that have shared their range with cowbirds for thousands of years have, through evolution, learned to cope. They recognize the parasitic eggs and destroy them. Both parents vigorously attack cowbird females and drive them off.

The least Bell’s vireo was declared an endangered species by the U.S. Fish and Wildlife Service in 1986, and the service’s mandate is to take all possible measures to rescue it from extinction. The vireo is also listed as endangered by the state of California. Once abundant and widespread throughout the Central Valley, the vireo had a range that extended from northern California to northern Baja. Today, the range has shrunk to four counties in southwestern California.

With habitat restoration along rivers and streams, cowbird control and protection of unspoiled riverside areas in places such as the Prado Basin and the Santa Margarita River, perhaps the decline of the vireo has been arrested.

In 1985, there were 300 breeding pairs of least Bell’s vireos in the United States. This year, there are 550. Fish and Wildlife people have done a great job, but despite the increase, it will be many years before the vireo may be safely removed from the list of endangered species.