Delayed maturation in passerine birds: An examination of plumage effects and some indications of a related effect in song

In some passerine species, 1st-year males are clearly distinguishable from adults; they have a different, typically dull, plumage. At least 15 explanations of the delayed plumage maturation of 1st-year males have been proposed. In the present study, we investigate if there is a similar widespread phenomenon regarding song, i.e. if males of some species have a distinct song in their first breeding season (delayed song maturation). A comparative analysis of 137 West Palearctic passerines showed that detailed data on how the song changes with age in breeding males is available for only a few species. Moreover, it is difficult to assess whether the song features continue to change considerably year after year or if, as we suggest, the main difference is between 1st-year vs 2-years and older males. We found that species with a suggested delay in song maturation were mostly those with delayed plumage maturation; a preliminary comparison suggests that these species have greater longevity. We also discuss the evolutionary significance of delayed song and plumage maturation and we suggest some possible directions for future research on these phenomenona.