A chorus of insects on the night air – it is one of those sounds that has lulled humans to sleep for millennia (in warmer latitudes anyway!). A nocturnal choir, singing us to rest.
But whilst it may seem to our ears like just a collection of bugs buzzing, a healthy nocturnal insect chorus is sonically complex, and shows evolution at work.
Insects choruses are among the most highly structured soundscapes on the planet. One of the best place to hear these rich and diverse symphonies is the dry savannahs of East Africa.
This is where we recorded our album; “An African Night“.
I would like to show you the structure of the insect symphony on this recording, because I find it fascinating. Here is a sonogram (a picture of the sound with time on the horizontal axis and frequency on the vertical) of a representative sequence.
You can see that nearly all the insect vocalisation fits into a band between 2kHz up to around 7kHz, with some other species intermittently calling higher, up to 20kHz at least. Within that primary 2-7kHz range, many species are vocalising simultaneously.
You can recognise clear bands. Each represents the voice of a particular species. By tailoring their vocalisations, each species has come to occupy a narrow frequency band. Its a bit like radio stations broadcasting on their own frequency.
Like radio stations, the reason insects do this is to be heard, and thus to survive.
In additional to frequency differentiation, some species employ temporal differentiation, where individual insects give shorter calls in antiphonal duets. To our ears, these are the ‘chirping’ or ‘chiming’ species, as opposed to the continuously zizzing ones. You can see this on the sonogram too. This strategy is similar to the way frogs vocalise, giving short calls with individuals alternating, sometimes creating complex polyrhythms.
These insect choruses are pervasive. They’re often loud and go all night (depending upon the locality and season). So it is interesting to see how other species in these habitats have developed calls that don’t waste vocal energy competing in that ‘occupied’ band of insect frequencies.
Large mammals – elephants, hyenas, impala, zebras – all call well below 2kHz, a factor of body size if nothing else. Elephants specialise in creating sounds of such low frequency that they are more felt than heard.
Diurnal birds often naturally sing in the same frequency range as nocturnal insects – but of course they don’t have to compete, they have a different time slot!
The night birds however, are different. Look at how the Slender-tailed Nightjar’s calls (a rapid chatter heard at around 30:12, and later, a quick series of calls on the wing at 39:34) are at a lower frequency range than the insects, not going higher than the lowest insect call. The Barn Owl (heard at 1:10:57) is similar.
Above the insects, the echo-locations of bats occupy the clear air of a frequency spectrum all their own. Occasionally they get a little interference from one or two very high frequency insects, which only call intermittently. Also a series of sharp ‘tick’s, which I think comes from a katydid, but may possibly come from a bat of a different species (although if it was a bat I think it would move around the stereo field with flight, which this doesn’t).
Established in 1993 by nature sound recordist Andrew Skeoch and photographer Sarah Koschak, Listening Earth offers a range of beautiful nature sound recordings from around the world.
"Our albums feature only the sounds of nature as you would hear in the wild - no music or other distractions. Recorded in often remote and pristine locations, they bring you the relaxing and beautiful sounds of our living planet. Listen, and let our recordings take you there."