A male screech owl roosts 6 feet up in a blue spruce, within 5 feet of the sidewalk. People walk by without noticing him, and he serenely ignores the passers-by.

Screech owls are small owls, about 10 inches long with wingspans of 2 feet, and are common in suburban environments.

This year two of my friends report that they have screech owl nests in their yards. In each case, the female tends the chicks in a hole high in a silver maple, and the male roosts in an adjacent blue spruce. The owls are a joy to the families with whom they share a yard.

Screech owls are opportunistic feeders that eat large insects and small mammals, birds, lizards and snakes. They kill their prey immediately and carry it back to the nest. But one species of snake is often brought back to the nest alive.

Texas slender blind snakes, Leptotyphlops dulcis, are rare in Colorado, but are abundant in Kansas, Oklahoma, Texas and Mexico. They are easily mistaken for earthworms, for they are pink to light brown, appear to be segmented and have no eyes. Their maximal length is about 8 inches. They spend most of their time buried in forest litter or soft soil, and they only come to the surface during drenching rains or floods.

Blind snakes have tiny mouths, much too small to bite a human. Their favorite prey is the larvae of ants and termites.

Screech owls kill and eat some blind snakes, but others are brought back to the nest and released. Blind snakes remain in the nest until the chicks fledge and the adults abandon the nest, and then they slither down the tree and return to the soil.

A survey of screech owl nests in Texas revealed that about 20 percent of the nests had one or more blind snakes. Of the nests containing snakes, most had just one, but one nest had five and another nest had 15 snakes.

Fecal matter, regurgitated bones and fur and fragments of animal carcasses attract a large number of insects to owl nests, including houseflies, greenbottle flies and bluebottle flies. These flies eat the material in the nest, but they also lay eggs.

Because fly larvae and blood-sucking insects feed on the chicks, they are a source of nestling mortality. Blind snakes living in owl nests eat the insect larvae that pose a threat to the nestlings.

A comparison of nests with and without snakes revealed that chicks living with snakes grew faster and suffered lower mortality. In nests without snakes, 40 percent of the nestlings died, but in nests with snakes, mortality was reduced to 30 percent.

We know that birds are clever. Birds have learned to remove cardboard lids from milk bottles, to use spider silk as thread, and to use cactus spines and bark flakes as prying tools. But screech owls have gone a step further, establishing a mutualism with a snake.

The blind snake is introduced to a protected environment with abundant food. Owl nestlings gain a resident guardian.

By sparing the life of a normal prey animal and introducing a snake to their nest, these wise little owls have established a mutualism that enhances nest hygiene, nestling growth and survival, and their own reproductive success.

Jeff Mitton (mitton@colorado.edu) is chairman of the Department of Ecology and Evolutionary Biology at the University of Colorado.