Bird identification has typically revolved around plumage colours, structural features and behaviour. Field guides and birding journals tend to concentrate more on visible features than on vocalisations. It is not unusual for entire publications to contain no reference to calls or songs at all, and written descriptions in field guides tend to be of rather limited value. Consequently, the ways birders identify and describe bird sounds tend to be haphazard. If considered at all, the role of sounds in identification often goes little further than noting the general tone of voice. It is notoriously difficult to describe to someone else why a bird was what you heard it to be.
The following sections introduce sonagrams (the ‘structure’ of bird sounds) and how to read them, as well as how to approach different elements of bird sounds: tone and timbre, pitch and frequency, rhythm and timing, and loud and soft sounds.
Bird sound has structure but you can only see it in a sonagram. Sonagrams are simply graphic illustrations of sound, in the same way that graphs can illustrate a company’s share price. A sonagram traces the ups and downs of sounds across time. The higher on the page, the higher the pitch; the closer to the base line, the lower the pitch.
The Sound Approach uses sonagrams on a scale from 0 to 8 kHz on the vertical frequency axis, or from 0 to 12 kHz when including very high frequency sounds. We use a variety of time scales but ensure that sonagrams being compared are at exactly the same scale. We use colours to help in interpreting the sonagrams, and we can magnify some details to take a closer look.
With the right software, you can make sound recordings in the field, analyse them at home, and compare them with published recordings and sonagrams. Difficult identifications can be confirmed within hours. If you have a go, don’t forget to use the same scale when comparing any two sonagrams.
The general character or quality of a bird’s sounds is often described by the word tone, which might be thought of as a sound equivalent to visual jizz (general impression of size and shape). Timbre is a more specific word for the texture or spectral colour of a sound (squeaky, buzzing, shrill, etc.), independent of its pitch or rhythm. Describing tone or timbre, whether it is the general character of a bird’s voice, or just one particular note, is as hard as describing a colour, a smell or a taste.
Technically speaking, frequency is a measure of the number of sound waves occurring during a given stretch of time. High frequencies create sounds we hear as ‘high-pitched’, and low frequencies sounds we hear as ‘low-pitched’.
Whereas frequency is a measurement, in Hertz (Hz), of sound wave cycles per second (1,000 per second makes one kilohertz or 1 kHz), pitch is used to describe how you hear this. So, frequency and pitch are not quite the same. Bat sonar, for example, has measurable frequencies, but as long as they are beyond our range of hearing they have no pitch that is audible to a human. Despite these limitations, the human ear can receive a wide range of frequencies, and good hearing can register frequencies from as low as 20 Hz up to around 20 kHz. At high and low extremes, a sound must be very loud for us to hear it. Our hearing is at its most sensitive between 400 Hz and 3 kHz, a range roughly equivalent to the upper half of the piano. The majority of bird sounds fall within our most sensitive range. Even bird vocalisations that go above that do not actually go much higher than 8 kHz, and are well within most people’s upper hearing limit.
Most birds are thought to have a hearing range similar to that of humans. Few are supposed to be able to hear frequencies above 20 kHz, and most birds’ hearing is at its most sensitive between 1 kHz and 5 kHz (Dooling 1982). Owls are an exception, with greater sensitivity at higher frequencies (e.g., Long-eared Owl at 6 kHz) where squeaking and rustling can be heard. In general, sounds that are ultrasonic or above the range of hearing for us can be assumed to be beyond the hearing of birds too, although a few birds can hear sounds that are infrasonic (below our lowest limit of hearing).
When it comes to pitch perception in sound recordings, the equipment used is important to consider. Some basic microphones and recorders can’t record higher frequencies, so they can miss crucial harmonics.
The rhythmic pattern within individual songs or calls is of crucial importance in identifying bird sounds. Equally important is the rate at which a sound is delivered, and whether or not this is constant. This applies at different levels, including both the tempo of song or call delivery over a longer stretch of time, and how rapidly the notes of a particular chuckling or rattling sound are repeated. Bear in mind that the silent gaps between sounds can be as relevant as the sounds themselves. In everything from flight calls to complex songs, paying attention to rhythm can provide the key in recognising sounds.
Timing calls or songs in the field, or from recordings, can make it easier to recognise more subtle differences in rhythm. Sounds and the gaps between them can be timed with a stopwatch. Sometimes, just counting notes as fast as possible can give a useful, if rough, guide, although it can be difficult to count more than six or seven notes in a second. Most call notes are over in a fraction of a second, so beware of over-estimating the length of a sound.
The final measurement of bird sounds that we will consider is their loudness or quietness. In the same way that pitch relates to frequency, loudness is a word for how we perceive intensity, which is measured in decibels or dB. How loud a sound seems depends not only on the effort put into making it, but also on its pitch, and the prevalent acoustics, including any other sounds audible at the same time.
Loudness is shown in a sonagram by the depth of black in the tracing. When the sonagram is set to ‘greyscale’, a quiet whistle will show as a narrow, light grey line, whereas a loud one will be thicker and blacker. When judging loudness in a sonagram, it is wise to use the darkness of the tracing comparatively – and thus judge the louder parts of a song or call only in comparison with other parts of the same sonagram. A sonagram from another recording may not have been made from the same distance, or with the same equipment.
Birds produce strong and weak sounds depending on the distance their message is intended to travel and the meaning they want to convey. Most birds produce weak, short-distance sounds when subsinging, copulating or comforting young, for example. Stronger, long-distance sounds are used to advertise for a mate, defend territory, contact others or mob a predator.
How loud a given sound seems to be depends on the acoustic conditions and the distance from the bird, as well as the direction the bird is facing. Describe each sound separately and try to avoid unqualified use of the words ‘loud’ and ‘soft’. Powerful, moderate and weak are more meaningful words to describe the power at which sound is produced.
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