“In order to see birds it is necessary to become a part of the silence.”
Robert Lynd

I love birds. I'm fascinated with their ability to fly wherever they want, sometimes for days and days without landing. Bird calls and songs are so beautiful and always get my attention, sometimes pleasantly distracting me away from something mundane or worrisome. Some of my favorite memories are of waking up after a good night's sleep to birds happily singing. And though we may seem as different as night and day, avians and humans may be more connected to each other than you think.

Our eons of co-existence with birds shows in the reverence found for them in art, literature, and music. Our science seeks to mimic their flight. Birds have been our workers in coal mines and our delight in zoos. They've been our lifeline as hunters and as food. Their symbolism leads troops into battle and politicians into elections. They've even led us down the road on the back of a motorcycle jacket.

We 'fly like an eagle,' 'eat like a bird,' 'peck at our food,' and 'take like a duck to water.' We're 'proud as a peacock,' 'dead as a dodo,' 'happy as a lark,' 'crazy as a loon,' 'skinny as a rail,' and a few of us are 'night owls.' Of course sometimes we're 'an ugly duckling' or 'pretty as a peacock,' or maybe an 'eagle eye' or 'graceful as a swan.' I like to 'get my ducks in a row' or I might be a 'sitting duck' before my 'swan song.' But the last thing I'd want to do is take you 'on a wild goose chase' because that would make me a 'bird brain." I'd rather be 'as free as a bird' than 'sing like a bird.'

Which leads me to bird song. It's more than music to our ears – it could mean survival. Acoustic ecologist Gordon Hempton was curious about our sensitivity to audio frequencies in the 2,500-5,000 Hz range. Hertz, often written as Hz, is the measurement of how many cycles per second a sound wave vibrates. The faster a wave vibrates, the higher pitch we hear. For instance, the lowest keys on a piano are very near the lowest limit of sounds we hear at the 20 Hz limit (piano key A0 is 27.5 Hz). Our upper hearing range tops out around 20,000 Hz (or 20 KHz). The highest keys on a piano are smack in the middle of our hearing range, the 2,500-5,000 Hz hyper-sensitive part of our hearing. This is also the frequency range of most bird song and calls.

We humans don't hear all frequencies at the same level. If we graph our hearing sensitivity, the plot looks more like a bell curve. We perceive the lowest and highest pitches less than those in the middle. Our ear canal has a resonant frequency range between 2,500 and 5,000 Hz. A resonant frequency is similar to the natural pitch you get from blowing across the open neck of a bottle or blowing into a horn. Says Hempton of the human range of sound, "it’s a perfect match for bird song. If we hear bird song, then we’re also listening to an area that has food, water, and an extended favorable season long enough to raise the young off the nest . . . Bird song is the number one indicator of habitats prosperous for humans.”

If we respond to birds, do they respond to us? During the early months of the pandemic the State of California was essentially shut down, and the reduction of traffic made San Francisco very quiet. In fact the city hadn't been that quiet in 70 years, according to University of Tennessee behavioral ecologist Elizabeth Derryberry. She led a research team that compared recording of birds in 2020 to earlier ones at the same locations and found that birds' calls in April and May of 2020 were softer and lower in pitch, which helped their range and effectiveness. An article at Science Alert about the study equates the pre-pandemic bird calls with going to a loud party and having to talk loudly over the din just to be heard.

What do birds hear? Most birds are most sensitive in the 1 KHz to 3 KHz range, similar to humans. Because their hearing organs are different and simpler than ours, they do have some limitations. They can't differentiate between two closely pitched sounds as easily as we can. And because their ears are closer together (an inch or less in a lot of cases), they can't sense direction as well as we can (our stereo hearing relies on a sound hitting one ear before other, therefore signaling the brain which direction a sound is coming from). Instead, they fly in circles and use doppler shifts to locate a sound. But, in 2014 researchers found that when a sound hits one ear of a duck, crow or chicken (the focus of the study), the shape of their heads alter the pitch of the sound as it enters the other ear. This distinction in pitch is how they can determine direction, instead of by time delay humans use. One type of bird uses a technique that bats have mastered. The oilbird lives in caves and depends on echolocation to navigate the dark tunnels and chasms.

A few birds have an incredible range of hearing, though most only hear sounds between 200 Hz to 10KHz. The black-footed penguin found mostly in South Africa has a very wide listening range and can hear between 100 Hz and 15KHz. The bullfinch found in Europe and Asia has been documented to hear from 200 Hz all the way up to 25KHz, well above human hearing. The common pigeon can "hear" all the way down to .05 Hz. Detecting direction at infrasound levels this low is difficult for any species, so birds that sense these ultra low sounds often react differently to them than normal sounds.

Getting birds' attention can be challenging sometimes. Birds that live near airports, factories, and other sources of loud continuous sounds suffer hearing damage, as any other species would. They will often augment their calls appropriately. Airport officials that need to disperse birds from runways for aircraft safety often deploy audio loudspeaker devices that pierce through ambient noise levels in the frequency ranges that the resident birds are most sensitive to. Some of the sounds these devices emit are distress or predator calls. Others are simply electronic sounds that have been shown to disperse a particular species.

Of course we can't talk about birds without talking about how birds talk. Birds lack vocal chords, so they use muscles and membranes to generate calls. Many birds can mimic human language (the only other species that can), but the parrot is the master of our language. Why? Scientists have found parrots have two layers to their section of the brain devoted to song, unlike most other birds that have just one area. Another reason is that birds, especially parrots, are very social. Parrots are trying to fit into our "flock" and learn the language. Interestingly, parrots from different regions in the world have unique dialects. When transplanted, a parrot will pick up the new dialect. I guess it's kind of like moving to a different part of the country and trying to fit in. It's hard - even for a parrot. How do I know? A little bird told me.

Tags: Science, Birds, Avian, Hearing