A lesson from the wild
The soundscape ecologist Bernie Krause, author of The Great Animal Orchestra (2012), noticed that in healthy habitats different creatures tend to call in different frequency bands and at different times, so they can all be heard. He called this the niche hypothesis, and he split natural soundscapes into biophony (sounds made by living things), geophony (wind, rain, water) and anthropophony (human noise). It is a perfect model for layering: give each sound its own niche and nothing has to shout.
Frequency space, simply
Frequency is how high or low a sound is, measured in hertz (Hz). Roughly speaking: lows (below about 250 Hz) are thuds and rumbles — a cajón, a water butt; mids (about 250 Hz to 4 kHz) hold most of a voice and many bird calls; highs (above about 4 kHz) carry the crispness of leaves, shakers and rain. When two sounds compete in the same band, the quieter one vanishes. This is called masking.
EQ (equalisation) is a set of tone controls that turns specific frequency bands up or down. Use it mostly to cut: trim the low rumble from a bird recording with a high-pass filter, or dip the upper mids of a drum loop slightly so a wren can cut through.
A layering process you can repeat
1Lay the ambience bed
Start with a long, steady garden ambience at low volume. It sets the "place" and hides tiny edit gaps. Loop it with a crossfade (below) so it never jumps.
2Add the rhythmic anchor
Bring in your main percussion loop and set its level so it sits comfortably — you should feel it more than hear it at first.
3Place the signal sounds
Drop in birds, drips or leaf crunches as single events. Place them in the rests of the rhythm, not on top of the loudest hits.
4Pan for width
Panning moves a sound left or right. Keep the anchor near the centre and spread signals to the sides, like plants around a central path.
5Mute test
Mute each layer in turn. If you do not miss it, remove it. Fewer, clearer layers almost always win.
Fades, crossfades and seamless loops
Every edit should start and end with a fade, even a very short one of 5 to 20 milliseconds, to prevent clicks. To loop an ambience smoothly, overlap the end of the clip with its own beginning and fade one out while the other fades in — a crossfade. Cutting at a zero crossing, where the waveform passes the centre line, also helps avoid pops.
Reverb and the sense of place
Reverb simulates the reflections of a space. Garden recordings already carry real outdoor space, so add reverb sparingly — usually only to dry, close-miked percussion so it sits in the same "air" as the birds. Send several layers to one shared reverb rather than giving each its own; that single shared space glues them together.
Levels and headroom
Gain staging means keeping each step of the signal at a sensible level. Keep individual tracks peaking comfortably below 0 dBFS and leave the master with a little headroom. If everything is too loud, turn everything down rather than adding a limiter as a first fix.
Ideas worth borrowing
Pierre Schaeffer's musique concrète experiments in Paris, starting with Étude aux chemins de fer in 1948, showed that recorded everyday sounds could be music in their own right. Brian Eno's Music for Airports (1978) demonstrated how loops of different lengths drifting against each other create endlessly shifting textures — try loops of 7, 11 and 13 seconds under a garden bed. Free software such as Audacity, or affordable tools like Reaper, is entirely capable of everything on this page.