Background As one of the most elaborate and diverse sexual signals,bird songs are prominent among mate choice criteria. Females generally prefer mates with larger repertoire size,which promotes the evolution of song complexity. However,there are also some songbirds that have far simpler and less diverse vocalizations,which have not been the focus of scientific scrutiny. Most Phylloscopus warblers are accomplished singers with complex songs. In contrast,Hume's Warbler (P. humei) has extremely simple songs. In order to explore the song's function,its evolutionary significance and particularly to assess its possible relationship with parental investment,we studied mate choice of the subspecies P. h. mandellii in Lianhuashan National Nature Reserve,Gansu,China.

Methods We recorded body measurements and songs of breeding males and then explored their relationships with the date of clutch initiation,reasoning that the characteristics of males that are involved with early nesting activities reflect female mate preferences. We also recorded egg size and body measurements of nestlings to assess the relationship between parental investment and mate choice.

Results We found that male wing and tail lengths were positively correlated with early clutch initiation as were songs characterized by short duration and rapid rise to maximum amplitude. We also found that early-breeding females did not lay large eggs,but produced more surviving young,which grew up faster.

Conclusions Female mate choice criteria in this bird include both visual signals and song characteristics. Our study supports the hypothesis that females may judge male quality from quite subtle differences. In order to reduce the risk of predation,a preference for such inconspicuous male characteristics may be partially driven by high vulnerability of this warbler to predators as a ground-nesting species.