Not silent yet: the changing sounds of spring

Cat Morrison uses citizen science data to explore changes in the quality of soundscapes across two continents.

Paper: Morrison et al. 2021 Bird population declines and species turnover are changing the acoustic properties of spring soundscapes. Nature Communications D.O.I. 10.1038/s41467-021-26488-1

During lockdown, across the globe, walking became a favourite pastime. Not one to be left out, every morning I walked from my house, in a village in the East of Scotland, into the surrounding countryside. At first, the fields and forests were relatively quiet, but as spring progressed, they became filled with bird song; the sounds of species such as yellowhammers, wrens and dunnocks were quickly joined by migrant chiffchaffs, whitethroats and eventually swallows. This formed the soundtrack of my spring, a great comfort in an otherwise upside-down year. Across the country, the sounds of our collective lockdown walks would have been as diverse as the places in which we live, from coastal paths filled with the cries of gulls and waders to starlings whistling in city centres. Imagine if lockdown had occurred ten or twenty years earlier; these soundtracks would have been different again. As our landscapes, climate and lifestyles have changed, our bird communities and our experiences of spring have also shifted. Understanding the extent of shifts in spring soundscapes and how they have been influenced by the changes in bird abundance and species diversity is the focus of our latest research.

Recreating historical soundscapes

For decades now, across much of North America and Europe, standardised annual counts of breeding birds have been carried out by a dedicated network of volunteer ornithologists. In North American, surveys of breeding bird numbers take place under the North American Breeding Bird Survey (NA-BBS), while in Europe, similar data are collated by the Pan-European Common Bird Survey (PECBMS). We can use these historical data to tell us about changes in species diversity and abundance across North America and Europe.  However, we have no similar record of how sites sounded when these surveys were taking place and hence no direct record of how changes in bird populations have influenced soundscapes. We needed a way of reconstructing historical soundscapes over large spatial and temporal scales and once again turned to data collected by others. Xeno Canto is an online database of sound recordings of wild birds from around the world and, by combining these recordings with our count data (1 individual in the count data = 1 song in the soundscape), we recreated the soundtracks of spring across hundreds of thousands of sites during the last few decades. You can listen to one of our soundscapes from a UK Breeding Bird Survey (BBS) site here.

Measuring how soundscapes have changed

The relationship between changes in the structure of the bird communities and soundscapes is not as clear as we might think. For example, the loss of a species such as willow warbler, which sings a rich and intricate song, is likely to have a greater impact on the complexity of the soundscape than the loss of a raucous corvid or gull species. How the soundscape changes will also depend on how many willow warblers occurred on the site and which other species are present. Predicting how changes in bird communities might affect soundscape characteristics is therefore not straightforward; to do so we first needed to turn our soundscapes into something we can measure.

Figure 1: A spectrogram of multiple species singing – the x-axis represents time, the y-axis represents frequency which can also be thought of as the pitch or tone of a song. The amplitude, which represents energy or loudness, is shown by colour, with dark blues corresponding to lower amplitudes (quieter sounds) and brighter colours up through to pink corresponding to higher amplitudes (louder sounds).

In Figure 1 you can see part of a spectrogram; this a way of visualising a soundscape, the darker pink areas indicate high acoustic energy (e.g., loudness or amplitude). You can see that the acoustic energy levels vary with time and occur across different frequencies. This could be driven by changes in the frequency of an individual’s song or in the number of individuals and species singing. We use 4 acoustic indices to capture these properties of our soundscapes: Acoustic Diversity Index (ADI), Acoustic Evenness Index (AEI), Bioacoustic Index (BI) and Acoustic Entropy (H). These acoustic indices describe the distribution of acoustic energy across frequencies and time (i.e., they quantify different patterns in the pink areas in Figure 1); lower values of ADI, BI and H, and higher values of AEI, reflect reduced acoustic diversity and intensity.

Are the sounds of spring shifting?

On over 200,000 sites, between 1996 and 2018, soundscapes have changed across both North America and Europe. In the past 20 years we have seen declines in ADI, BI and H and increases in AEI, making the soundtrack of our spring quieter and less varied (Figure 2).

Figure 2: Predicted annual variation in Acoustic Diversity Index, ADI (a,b), Acoustic Evenness Index, AEI (c,d), Bioacoustic Index, BI (e,f) and Acoustic Entropy, H (g,h) in North America (left column) between 1996 and 2017 and in Europe (right column) between 1998 and 2018.  Declines in ADI, BI and H and increases in AEI, mean that the soundtrack of our spring has become quieter and less varied.

Interestingly, these declines have not occurred to the same extent everywhere and the quality of soundscapes has in fact increased in some places (Figure 3). For example, reductions in acoustic diversity (signalled by decreases in ADI and increases in AEI) have been greatest in the North and West of both continents (Figure 3). This is likely to be driven by differences in the rate of change of local environmental conditions such as habitat degradation and fragmentation.

Figure 3: Mean site-level trend in Acoustic Diversity Index, ADI (a,b) and Acoustic Evenness Index, AEI (c,d) across North America between 1996 and 2017 and Europe between 1998 and 2018. Colours indicate the size and direction of trend in the acoustic metric (yellow – improving soundscape quality; blue – declining soundscape quality); note that the colour scheme is reversed for AEI, as positive trends are taken to represent a reduction in soundscape quality for this metric. The extent of changes in soundscape quality varies across both continents.

As you may expect, in general, sites that have experienced greater declines in either total abundance and/or species richness also tended to show greater declines in acoustic diversity and intensity, whilst sites where total abundance and/or species richness has increased tended to show increases in these acoustic characteristics (Figure 4). This relationship is shown in Figure 4 for acoustic diversity index (ADI); while you can clearly see the association described above, there are also many sites that don’t fit this general pattern. These inconsistencies suggest that, for any given site, initial community structure and how the call and song characteristics of the resident species complement each other, also play important roles in determining how soundscapes change. For example, if you look at the top right of both plots on Figure 4, you can see some blue dots; these are sites that have declined in Acoustic Diversity Index (ADI), despite increasing in the number of individuals and species overall. One possible reason for this is that the species gained have very similar and relatively simple songs and thus reduce the acoustic diversity of the site. Future work is needed to explore these patterns further.

Figure 4: The association between site-level trends in the total number of species and the total number of individuals in 202737 NA-BBS sites across North America (left column) and in 16524 PECCBMS sites across Europe (right column). Colours indicate site-level trends in Acoustic Diversity Index, ADI, (yellow – improving soundscape quality; blue – declining soundscape quality). Sites that have experienced great changes in their bird communities tended to have also seen greater changes in the soundscape quality, although this was not always the case.

Why are these findings important?

Unfortunately, we are living through a global environmental crisis, and we now know that the diminishing connection between people and nature may be contributing to this. As we collectively become less aware of our natural surroundings, we also start to notice or care less about their deterioration. A soundscape that is normal to me now would likely seem depauperate to my grandparents’ generation. Studies like ours aim to heighten awareness of these losses in a tangible, relatable way and demonstrate their potential impact on human well-being. 

Finally, and also unfortunately, we are living through a mental health crisis. Like many people I have an app on my phone that I listen to sometimes when it’s hard to sleep. This app is full of tracks that mimic the sounds of nature and, when I listen to it, I relax and nod off to sleep more easily. To me this is quite representative of our modern age: we must turn to an app to get what we don’t even know we are missing from our natural environment. Perhaps a good thing to come from the pandemic is a growing awareness and appreciation of our surroundings and for some, maybe, also a realisation of what we have lost. I hope that our study can help to highlight another aspect of these losses, which is not just fundamental to the health of ecosystems but also to the well-being of society.

Catriona Morrison is a post-doctoral researcher at the University of East Anglia. Her primary research interests lie in understanding the demographic and environmental processes influencing the population dynamics of birds. If you have any questions about the paper, you can easily find her on twitter @CatMorrison18.