Those sounds, while not nearly as musical to the human ear, allow those birds to prevail against the negative effects of their habitat. Musical birds are in the forest, where there isn't the heating effect of the sun to deal with. Those birds can use the long whistles and frequencies in the midrange that are pleasing to human ears. Frequency is so important to birds, in fact, that scientists can identify species of birds in a region simply by the frequencies of their calls. Songbirds hear relative pitch about as well as we do.
Most humans can hear relative pitch, that is, how far apart two notes are from each other.
But very few humans have the ability to identify perfect pitch, or the frequency of a note, even though humans have the biological equipment to do so. Musicians who have perfect pitch can, for example, hear a note played and can identify the name of the note unerringly. Diana Deutsch, professor of psychology at UCSD, is one of the few scientists who have studied perfect pitch in humans. People who speak tone languages, for example, are far more likely to develop perfect pitch because they are using pitch to communicate. Tone languages are those like Vietnamese that rely on specific tones or frequencies for meaning of words.