setup:
zoom h4n in 4 channel 48000Hz 24bit mode with internal mics and usi xlr with +48V phantom power; quiet environment (in a quiet valley in the mountains, some birds and bees)

case 1, low gain:
rec gain on inputs (zoom mics and usi mics) set so that a close clap peaked around -9dB on the meters
amplifying/normalizing the recording of quiet birds in audacity showed noise floor around -35dBFS for both sets of mics (audible hiss)

case 2: high gain:
rec gain on inputs set much higher (eventually might have been at maximum? didn't note it down exactly)
amplifying showed noise floor about -15dbFS (entirely unusable roar)

now looking at better field recorders: sound devices mixpre 6 II (EUR1390 for 4 inputs) vs zoom f3 (EUR315 for 2 inputs)...

audio file:
zoom h4n low gain internal, then low gain usi, then high gain internal, then high gain usi