Dear Readers, as you will have heard my garden has been completely smothered with fledgling starlings this week. From about 5 a.m. there is a wall of sound, which you would think would attract every predator from miles around. Strangely enough, I think that even the hunters find it all a bit baffling – a very handsome black cat appeared and was bombarded with such a barrage of screaming alarm calls that he gave up looking sneaky and just walked nonchalantly down the steps and past the pond, as if killing things was the very last thing on his mind. The magpie and the sparrowhawk have been notable by their absence, and in short, I think that this year there are so many fledglings that no one can make up their mind which one to eat.
To get an idea of the cacophony (and this is a very poor sample) have a look here.
Anyhow, all this got me thinking, and so I was glad to attend a very fine talk on Passive Acoustic Monitoring, given by Richard Beason for the Field Studies Council. You can watch the whole thing here. Passive Acoustic Monitoring is the name given to leaving a sound recording device somewhere in nature, and coming back later to download the results.
Increasingly, these devices are being used to record soundscapes – the whole range of sounds that can be heard in an environment, from the wind and rain through the calls of frogs and birds and crickets, to the sound of ancient timber being cut down by a chainsaw (oops). When you’re done, you can play the recording back, but many also come with spectrograms, which enable you to get a visual representation of all the racket.
One of the most interesting revelations, for me anyway, was that in a habitat, the inhabitants not only occupy geographical niches, but also aural niches. I’d never thought about it before, but if all the animals were calling at the same time, no one would ever hear anyone else. The way that it works is that different animals call at different frequencies, at different times, and often in different rhythms, so that everyone gets their share of the ‘sound stage’. Even in the UK we can see how this works, with soprano and common pipistrelle being distinguished mainly by the fact that their calls are at different frequencies. This way it’s unlikely that bats of different species will end up breeding together.
What PAM allows you to do is to see what’s living in a particular place, and compare it to similar habitat elsewhere. Firstly, of course, you have to identify all the sounds, which can be a very demanding task. From just looking at the sonogram, what bird do you think this is?
And if you’re stuck, you can have a listen to the bird below.
Once everything has been identified (and you’d better hope that there aren’t any starlings beat-boxing, or lyre birds pretending to be a camera on automatic ) it’s interesting to see how the spectrograms compare between, say, virgin rainforest and a palm oil plantation. And indeed spectrograms are being used in conservation in a variety of ways – in the Central African Republic, placing acoustic monitoring devices in the forest has led to the discovery of an important area where the rare African forest elephants gather to find minerals. In the diagram below, the blue dots are monitoring devices, and the red dots are the mineral licks. The discovery of the one in the south will make it easier to protect this area, and to think about how to create a corridor to the ones in the north.
Perhaps the most interesting thing, though, is how PAM can be used to both identify a problem, and suggest a solution. For example, in 2012 it was discovered that healthy coral reefs sound rather like popcorn, with a great chorus of fish chomping and calling and snapping- shrimps walloping things.
Other fish use these sounds to orientate themselves and to find a new reef, because a busy reef is a safer reef, and it is also a place where a fish can find something to eat and someone to mate with. But after a coral bleaching event, the reef is silent, and even after it starts to recover, the fish don’t return because they can’t hear it. So the answer appears to be what’s known as ‘acoustic enrichment’ – putting down underwater speakers which play the sounds of a healthy reef. Slowly but surely, the fish return, and make their own sounds, and after a while the speakers aren’t needed anymore. I find it fascinating how this whole cycle has been identified and how imaginative the solution is. In these times of crisis, I get a real lift from every piece of creativity that’s used to help preserve our beleaguered planet. Who would have thought that a couple of speakers would have increased the diversity and number of fish on a damaged reef by 50%? Furthermore, once the fish have found the reef they tend to stay, and their activities help to build the ecosystem of the reef. You can read the full story here.
PAM has been used in many other ways: to monitor the change in the population of Leach’s petrel in Northern Canada after the removal of Arctic foxes, and to estimate the number of anti-poaching patrols required in Central African Republic by recording the number of gunshots. It really is a area with endless applications, and a great addition to our environmental toolbox. In lockdown, the sounds of nature become much more prominent, a whole new way of appreciating the world. Let’s hope that we continue to appreciate them once things get back to whatever will pass for normal in the future.
And here, just to round off this very sound-based post, here are three of my favourite birdsongs. All of the singers used to be common, but none of them are now. See if you know what they are! Answers tomorrow.