Frequency-dependence may moderate fitness costs linked to reduced bird song complexity.

Animal cultures can undergo rapid changes associated with innovations, revolutions or population decline. Where a rapid shift results in reduced complexity of cultural behaviours, it may have fitness consequences for individuals. Here, we report a dramatic shift in the dominant song type of critically endangered wild regent honeyeaters Anthochaera phrygia. Between 2015 and 2019, most males in the Blue Mountains sang a typical regent honeyeater song (typical Blue Mountains song), but 5%-10% sang an abbreviated version of the song with half the number of syllables (the clipped Blue Mountains song), which was associated with lower pairing success. Since 2020, the proportion of males singing the clipped Blue Mountains song has increased to 50%-75% each year. The likelihood of successful pairing in these males showed a significant concomitant increase, suggesting that the fitness costs associated with the abbreviated song decreased as it became the dominant song type. Our results suggest that the fitness consequences of loss of song complexity in declining and fragmented populations may be ameliorated by frequency-dependent shifts in song type preference.