Adjustments to life in a COVID-19 world are not limited to humans. Some birds also have changed behavior.

White-crowned sparrows living in the San Francisco metro area didn't adopt extraordinary behavior. They simply reverted to normal.

It's about traffic noise, masking song notes, forcing the bird to invest more energy into its message.

Research published in the journal Science reported that the observed birds seemed to react to the drop in human noise, vehicular traffic in particular.

After decades of sacrificing song quality for higher volume, these sparrows have switched to songs that closely resemble those of rural cousins. Those are the songs all white-crowns sang in our previous world.

No longer competing with our wheels, the birds' songs once again could be the softer, more complex calls nature gave them. They made the change rapidly, the study showed.

The natural songs also doubled the city birds' communication distance, said Elizabeth Derryberry, a behavioral ecologist at the University of Tennessee, Knoxville, and lead author for the study.

Rural birds had no need to adjust their songs.

Bird song defines territory and courtship appeal, among other things. It has well-defined importance, more than simply pleasing the ear.

Many studies have been made of bird response to the noise we pump into the environment, in particular the sound of road traffic.

Research has suggested that traffic noise is a primary cause of wildlife population declines near roads. (There's a birding clue for you.)

Several years ago researchers from Boise State University created a "phantom road," in southwestern Idaho using an array of speakers to apply traffic noise to a roadless landscape. They directly tested the effect of noise alone on birds using that landscape during autumn migration.

Thirty-one percent of the birds avoided the phantom road area, the researchers reported in an online publication of the National Academy of Sciences.

Birds often stop during migration to rest and refuel. The birds that stayed in the test area showed less ability to improve body condition during the stopover, the report said.

"Our results suggest that noise degrades habitat that is otherwise suitable," the research team wrote. They added that finding birds in a noisy area does not mean lack of impact.

A study in Massachusetts found that a road-effect zone there was approximately 360 yards wide.

Sound is measured in decibels. A whisper is about 15 decibels, normal conversation 60, a lawn mower 90.

Owls hunt with their ears, listening for the rustle of moving prey. A study was done on the influence of background noise on the hunting success of saw-whet owls.

Writing in the Biological Conservation journal in 2016, biologists reported that for every increase of one decibel in background noise the odds of the owl detecting prey fell by 8%. The odds of the owl making a successful kill dropped by 5%.

Sound levels below our normal voices had that impact.

"By the time noise reached 61 decibels — a little louder than normal conversation — the owls completely failed to even notice nearby prey," the research report stated.

This study was inspired by an earlier project with bats, which use echo-location to find prey. The study showed that bats had to increase hunting time when flying near busy highways.

For birds or bats, sound level can significantly reduce the amount of habitat otherwise productive for hunting.

In the continental U.S. more than 80% of our land is less than a mile from a road. In the next 30 years 15 million miles of new road are expected worldwide.

The report came to me via the Birding Community e-Bulletin (tinyurl.com/E-bulletinSIGNUP).

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at startribune.com/wingnut.