Interesting. Thanks for sharing. I'd never seen that ... that I remember ... but it is essentially what I do. I look at 0 - 1500 Hz. Usually clearly defined things are not what we're looking for. Look for gray foggy looking areas on the spectrogram. Things that make a single clear line on the graph usually can only make such a clear line ... in other words, they are only capable of making sound in a few narrow frequency ranges. We're looking for something with vocal flexibility. Those show up as less distinct / more fuzzly frequency ranges, thus more of a dull blob on the graph.
Insects usually produce 3 or more horizontal bands ... harmonics, I suppose. Airplanes do the same but usually those are rising pitches so there will be maybe 5 or more bands roughly in parallel but increasing in frequency. The heavy band at the bottom is generally wind noise. With different mics and recorders with appropriate sensitivity that's likely where infrasound would fall, but most of our readily available mics and recorders don't pick up "real" sounds under 80-100 Hz. If you're getting too much of that rumble from wind noise, screening the mic can help.
One of my friends who does more audio than I do uses Sonic Visualizer .. I've downloaded / installed it but haven't played with it yet.
MIB