Unlike musical instruments, birds don’t seem to bother to create a complex multi-layered harmonics pattern. Instead, they create a complex pattern with the base (fundamental) frequency alone. The “cloud” above the main drawing is what would be the 2nd harmonic. These sonograms are remarkably different from other sounds, as if birds “draw” with sound something that’s flying backwards in time.
Here's a good line ending a section about different prey:
"For a bird, there are only two sorts of bird: their own sort, and those that are dangerous. No others exist. The rest are just harmless objects, like stones, or trees, or men when they are dead."
What an odd thing to say at the very end there! What's he really saying huh?