Some kind of spectral representation would be a good starting point. That should show the spread of different bird sounds over time and energy giving an indication of density/intensity.

Thank you Stuart. Interesting ideas but not exactly what I was thinking of. Bird sounds have got louder and there is research on that. I was thinking more of how to represent bird populations with sound. eg greater population mapped onto higher frequency/volume/rhythmic intensity.

I see - sounds like a great idea, Chris. I think adding rhythmic 'layers' or textures would be a good strategy, also increasing the width of the spectrum that the sound occupies as the population grows, perhaps?

volume // population is not exactly the best option here as volume is logarithmic. Like Stuart, I would use filtering: for example you have a continuous noise and the higher the population, the higher the cutoff frequency. If you want to make the message clearer, you can also link this to rhythm: the bigger the population, the more complex the rhythm. For example: you deconstruct a rhythm so you have the bass drum, cymbals, toms, triangle. Low pop= only bass drum, higher pop= add toms, even higher= add cymbals, very high pop= add triangle