Duke University (11 Jul 2007). Songbirds Prefer The Latest Music: Golden Oldies Just Don't Cut It With The Chicks. ScienceDaily.
"When it's time to mate, female white-crowned sparrows are looking for a male who sings the latest version of the love song, not some 1979 relic. And territorial males simply find the golden oldie much less threatening."
...
"There were only subtle differences between the two versions of the song. The 1979 recording starts out with a higher pitched whistle and ends with trills that are more rapid. The newer song has headed toward Barry White territory, with a lower whistle and a more prolonged, broader-bandwidth trill at the end.
The California girls clearly preferred the newer one, arching their backs, raising their tails and beaks and doing a come-hither flutter of the wings..."
Bird songs are learned so younger birds or birds from smaller populations tend to sing less complex songs. Females prefer the complex songs. [Laiolo P., M. Vögeli, D. Serrano, J. L. Tella, 2008. Song Diversity Predicts the Viability of Fragmented Bird Populations. PLoS One 3(3): e1822. doi:10.1371/journal.pone.0001822]