Still sounds like squeaking to me...

Ultrasonic Songs of Male Mice: http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0030386

Scroll down the article (long and scientificly boring) and you can hear some audio files. It sounds suspiciously like squeaking to me. If it resembles a bird song, perhaps it is because birds squeak too.