Part 1: ‘I don’t do calls’

Such examples could be seen as mildly entertaining if the combination of all these communication problems did not create such difficulty. The birds of the Western Palearctic (‘BWP’), in its first volume’s introduction (Cramp & Simmons 1977), has two and a half pages of frustrated comments on the vagaries of described bird sound. As one of the very few European publications to give so much space to bird vocalisations, it is sad when the editors sum it all up as “groping towards communication”, and go on, “even this has been frustrated by uncertainty over the intention of the writer of an original description, and whether his chosen words accurately conveyed his intention”. Even with the electronic versions of BWP, the bird recordings referred to in the text are not the ones provided for listening, although a few can be found in British bird sounds on CD (Kettle 1992).

Pitch and frequency

To understand hearing and the subtleties of identifying different bird sounds, one has to learn about sound frequencies. This is as fundamental to describing sounds as size assessment is to descriptions of appearance. Put simply, 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’.

Trying to assess the frequency of a sound often highlights the limitations of our individual hearing. Many of us experience some hearing-loss during our lifetime, just as many suffer from short- and long-sightedness, or colour-blindness. This most commonly occurs with age, as a loss of perception of high-pitched sounds. As a general rule, young girls have the best high frequency hearing, older women next, then young men and finally older men. It shouldn’t interfere with sound identification, although it is good to know what one’s strengths and weaknesses are. As I have grown older, I have found it gradually more difficult to hear Goldcrest Regulus regulus calls and the song of Common Grasshopper Warbler Locustella naevia at a distance. This loss of high frequency hearing is more noticeable in situations other than birding. For example, most people would probably notice that they are unable to understand speech clearly in a noisy pub before they find that they cannot hear a Goldcrest calling. Another of the reasons why bird sounds can be difficult to hear is that they are often not very powerful sounds. Personally, I find as compensation that I can often hear distant low-frequency sounds like a Great Bittern’s Botaurus stellaris boom or the low and relatively weak hoot of a Long-eared Owl Asio otus, long before my high frequency hearing birding colleagues. One of North America’s finest sound recordists, Lang Elliott, who recorded most of the sounds on the excellent Stokes field guide  (Elliott et al 1997), has also never been able to hear high frequencies.

Everyone’s ability to hear can be temporarily affected by, for example, a heavy cold, changes in air pressure during a recent airplane flight, exposure to loud noise at a club or loud concert and, more obviously, hats and ear coverings. Like our other senses, our hearing is best by mid-morning and is poor in the early morning and later in the day through tiredness or lack of concentration. For both birds and humans, the subtle differences in timing and loudness between the different versions of a sound reaching each ear allow us to pinpoint the direction a sound is coming from. Some people are partly deaf in one ear (one in 100), and this type of deafness will cause difficulty in determining the direction that a sound is coming from. 

It is more important for us birders to understand what we hear rather than focus on variations in physical hearing ability, as difficulties registering and describing what is heard have more to do with comprehension and vocabulary. These skills come with time, training and experience.

Whereas frequency is a measurement, in Hertz (Hz), of sound wave cycles per second (1000 per second makes one kilohertz or 1 kHz), pitch is a word used to describe how you hear this. So, frequency and pitch are not quite the same. A bat’s sonar and an elephant’s subsonic rumbles for example, have 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 a pair in good working order 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 be able 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, and we are less able to discriminate between sounds the further they are from the middle of our hearing range. Fortunately, our hearing is more than adequate for bird sounds, the majority of which actually 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. 

When musicians say they have ‘perfect pitch’, they are referring to the ability to identify or reproduce an exact pitch – say ‘middle C’ (262 Hz) – without recourse to a reference pitch such as a note on the piano. It is quite a rare ability, and by no means the only benchmark for a ‘good ear’. Many famous composers including Maurice Ravel, Igor Stravinsky and Richard Wagner did not have it, and you can certainly identify bird sounds without having this ability. 

Telling a listening companion which part of the sound spectrum they should concentrate on is as important a clue for picking out a sound as directions (“just left of the buoy”) are for a visual target. We have chosen a set of recordings to illustrate the broad range of different frequencies that birds use (CD1-02). A kind of doh ray me of bird sound, from the foghorn-like depths of the Great Bittern to some amazingly high, almost bat-like notes in the song of Lesser Whitethroat Sylvia curruca. 

Listening casually to the song of this male Lesser Whitethroat, you might wonder what it is doing at the top of our scale. Most of its song is pitched well below the Goldcrest, with some notes as low as 1.6 kHz, but the introductory notes are the highest complete notes we have found. Having tested your equipment to the limits to hear the stratospheric tittering notes of Lesser Whitethroat, the reeling song of a Common Grasshopper Warbler (CD1-03) should no longer seem quite so high-pitched. Here it is compared to the lower-pitched reeling of a Savi’s Warbler L luscinioides (CD1-04), a good example of pitch as the simplest means to distinguish the songs of two species. The song of Common Grasshopper is concentrated around 6 kHz, whereas Savi’s is around 4 kHz. 

Now, try a species-pair that illustrates rise and fall in pitch. These recordings of the typical year round calls of Common Ringed Plover Charadrius hiaticula (CD1-05) and Little Ringed Plover C dubius (CD1-06) illustrate upward and downward inflections, respectively. The Common Ringed is at a similar pitch to the Common Redshank in CD1-02e, but the Little Ringed is considerably higher.

Interestingly, those high notes of Lesser Whitethroat aren’t the only sound to test our hearing. Listen to the calls of a Red-troated Pipit Anthus cervinus in flight (CD1-07), the loudest parts of which are pitched as high as of a Goldcrest. A person who is less receptive to high frequency sound will hear and probably describe this call differently from someone with a wider range of hearing. Just as white light is composed of a rainbow of different colours, which can be separated by a prism, the sounds we hear are a blend of layers of sound. The lowest layer is called the ‘fundamental’. Typically, the layers above the fundamental are at exact multiples of its frequency, in which case they are called ‘harmonics’ (or ‘overtones’). Because the layers are related in this simple way, their sound waves melt together or harmonise, and we don’t hear them as separate sounds. Together, they are responsible for the timbre of the sound, a ‘nasal’ sound having lots of harmonics, and a pure tone having none. Because high frequencies don’t carry as far as low frequencies, the effect of the higher harmonics is reduced with distance, even for those with the best hearing.

Apart from the effects of distance on volume and harmonics, another reason why the Red-throated Pipit calls change slightly as the bird flies away is the ‘Doppler effect’. This phenomenon is the apparent shift in pitch as the source of a sound approaches (shorter wavelengths and higher frequency) or moves away (longer wavelengths and lower frequency). Think of a child describing a passing car: eeeuooow! The sound seems to change pitch, being higher as it approaches and lower as it moves away. The same can happen with a passing flock of geese, or a screaming flock of Common Swifts Apus apus or Pallid Swifts A pallidus flying past: as they get closer the pitch seems to rise, and as they fly away it seems to fall. Listen to the Doppler effect in the wingbeats of a passing pair of Mute Swans Cygnus olor (CD1-08). You can also hear their grunting calls.

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 (eg, 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). Western Capercaillies Tetrao urogallus have an infrasound component in their song (Moss & Lockie 1979), and the Rock Dove Columba livia is even thought to be able to hear sounds lower than 1 Hz. When 60 000 English homing pigeons were released in France in June 1997 and around a third of them were never seen again, it was suggested the birds lost their way after crossing the path of low frequency shock waves generated by the sonic boom from Concorde, which raises the question as to whether sound works for orientation instead of (or together with) earth magnetism.

When it comes to pitch perception in sound recordings, the equipment used is important to consider. Some basic microphones and recorders are incapable of recording higher frequencies, so they can miss crucial harmonics, or even those very high Lesser Whitethroat notes. At present something similar happens when you try playing bird sounds over the phone. Above a certain frequency (around 4 kHz) nothing at all is carried, so no matter how ear-splittingly loud you play the Goldcrest recording (CD1-02g) down the phone, it will not reach the listener at the other end. This will change as technology progresses into better phones. At the other extreme, really poor equipment might even add its own ‘artefacts’: distorting, harmonic-like sounds that were not present in the original.

Rhythm and timing

After pitch, the next consideration is the rhythm of the sound. 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; 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. 

Living in Dorset, England, I get to hear a fair bit of European Reed Warbler Acrocephalus scirpaceus song (CD1-09), but a singing Sedge Warbler A schoenobaenus is something I encounter much less often. Their songs are superficially similar, and there are several ways to tell them apart, of which tempo – the rate of delivery of the song – is the best. European Reed Warbler plods along with a constant regularity, pacing itself for a long run without stopping, and even when it impersonates other birds (eg, calls of Common Chiffchaff Phylloscopus collybita and European Goldfinch Carduelis carduelis in this recording), it does so within the confines of its regular tempo. Sedge Warbler can also sing for a long time without stopping, but its tempo is more varied and faster (CD1-10). The song is higher-pitched on average, because it likes to launch itself briefly into rather high frequencies, just as it sometimes takes off on brief songflights. 

There are other ways in which aspects of rhythm and timing can be important in identifying bird sounds. At the most detailed level there is the pulse rate of rattles, trills and reeling sounds. The denser reeling in the song of Savi’s Warbler (CD1-04) compared to Common Grasshopper Warbler (CD1-03) is actually due to a pulse which is twice as fast – about 48 per second in this particular Savi’s and 25 per second in the Common Grasshopper. So not only is Savi’s significantly lower-pitched; the individual pulses in its denser trill are much harder to discern.

Now try the two other Locustella warblers that breed in the Western Palearctic. Listen to the reeling of Lanceolated Warbler L lanceolata (CD1-11), which has a pulse rate slightly slower than Common Grasshopper Warbler, about 18 per second, resulting in a looser or weaker-sounding trill. Finally, the song of River Warbler L fluviatilis has a much slower pulse, in this recording only about 10 per second (CD1-12). It’s so slow you can hardly call it ‘reeling’, and the individual pulses are easy to hear. The exact pulse rates of these Locustella songs may vary a little but the rule always holds that Savi’s has the fastest pulse followed by Common Grasshopper, then Lanceolated, with River being the slowest.