MiceSong
"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" (Hooper, 2005)

A paper in PLoS Biology by (Holy & Guo, 2005) demonstrated that male mice vocalise within a range of over 30 to 110 kHz (ultrasonic) in the presence of females, or female urine. The vocalisations have the characteristics of song with "several different syllable types, whose temporal sequencing includes the utterance of repeated phrases" the complexity of which, approaches "that of many songbirds".

Holy TE, Guo Z (2005), Ultrasonic Songs of Male Mice. PLoS Biol 3(12): e386
Hooper, R. (2005), Romantic rodents give secret serenades, New Scientist News Service, (Acc: 21/04/06)