Birds have perfect pitch

A great horned owl is calling out behind our house. If it's where it often is, in a neighbor's cottonwood, it's 100 yards away. The deep, slow sound of the owl's courtship comes clearly through the glass of the patio door beside me.

Me, who's always asking for the television remote so I can boost the volume. Me, who can't hear certain warbler species sing from 30 feet away. Why I can and can't hear these birds has something to do with me, but most to do with the birds.

My ears are doing what many male ears do at this time of our lives: I'm losing the ability to hear high-register sounds (and sometimes the mid-range sound of my wife's voice).

The birds are doing what they have evolved to do: singing at a sound frequency that varies with topography and ground cover. You sing at one frequency if you live deep in the woods, and another if you spend your time in tree canopies. Grassland birds such as bobolinks deliver songs in yet other pitches.

In research terms, the wood where the owl lives is a complex vegetative structure. This complexity poses a barrier for efficient passage of higher-frequency sounds. Lower frequencies work in that situation. So, the owl goes low.

The higher frequency songs sung by, for instance, orange-crowned, blackpoll and Cape May warblers, escape my ears. Those thin songs are perfect, however, for passage through the canopy habitat that these birds use.

Various species sharing the same habitat deliver songs at varying frequencies. You, as a bird, want your song to stand out from the crowd. You have your own frequency.

Well, the air is normally still at that time of day, allowing for good sound transmission. Many of the daytime predators are not hunting yet, making singing less hazardous. Eat insects? They're not as available first thing in the morning, so it's a good time for birds to invest in song.

And, many female birds lay their eggs in the morning. The best time for productive copulation is the hour before the egg is laid. If you're going to sing love songs, first thing in the morning is an opportune time.

Bird song frequencies are changing because of us. Songs must be heard above the human din if birds are to successfully share the space we occupy. The ambient sounds with which we fill the day -- planes, trains and automobiles, lawn mowers and leaf blowers -- mask bird songs.

Some birds change the frequency and/or loudness at which they sing to compete with us. They do this at a price: More energy than usual must be invested. That leaves less energy for the other demands of life.

Research has found that fewer bird species and fewer individuals live in noisy areas, such as along a busy commuter route. Why? Because if the noise is overwhelming, the birds can't compete. Studies have shown that these birds give up and leave.