I read somewhere that the Australian magpie has the most musically diverse call of any bird. After listening to them, and other Australian birds such as the kookaburra, I started to think about why this might be.
Drawing from some experience with high frequency automatic link establishment (ALE) radios, I believe the diversity and frequency agility of Australian birds is an evolutionary trait to ensure parts of the call can punch-out over the din of the bush. Rustling dry leaves, the roar of cicadas etc. can be almost deafening at times. Since it's impossible for a small bird to compete on a sound pressure level, the only alternative is to cleverly use the frequency spectrum to get warning and mating calls out.
The aboriginals seem to have adopted the same approach with the bullroarer, which uses frequency modulation as a way of generating sound above the din of the bush.
I have no idea if this is true... it was just a thought that came to me during my many bush walks in a country of unparalleled beauty.