The next stage is restoring the removed data below 1kHz, choosing

between wanted sounds (birdcalls, bill snaps, wingbeats and so on)
and unwanted noise (the louder components of aircraft, vehicle noises
etc).

Going back to the un-processed source recording, I set a noise floor
threshold in DINR that captures only sounds that are below 1kHz, and
ABOVE the actual noise (aircraft or vehicle noise) level. Attenuation
is now set to infinity. Thus the only sounds let through this active
filter are the louder sub-1kHz signals. By careful manipulation of
the threshold parameters, the above-threshold sounds can be trimmed
from the below-threshold noise floor. In practice, this may need to
be done almost sound-by sound, as a series of small edits, and can be
quite time-consuming. What you get is a series of disconnected tones,
grunts, bloops and plops. At the end you may choose to bounce all
these edits down to a single track.