And this is just odd:
Mice are rarely the source of romantic inspiration, but it now appears that male mice may serenade potential mates with melodies approaching the complexity of bird song.
Mouse songs are sung at ultrasonic frequencies, which is why no one has noticed their complexity before, nor indeed been moved to celebrate the tunes in romantic poetry.
...They found that the songs included several syllable types (collections of notes) which were arranged into phrases and motifs, fulfilling the definition of song. Noises made by insects such as crickets, and calls by frogs, are far simpler. You can listen to a male mouse producing a snatch of song, here, and a more determined croon, here.