[periheli0n]

When a bird sings to attract a mating partner, competing against other birds, does it not sing for aesthetics?

Also, couldn’t harmonics be used to improve decoding in noisy environments? Spreading the signal over a wide band is not unheard of in man-made electromagnetic communication either.

[karmakaze]

When using spread-spectrum transmission it would be counterproductive to utilize harmonics. Non-sinusoidal noise at frequency F would pollute frequencies k*F. Also the transmission band is likely less than an octave.