The effects of population decline on culturally transmitted behaviors in animals has rarely been described, but may have major implications to population viability. Learned vocal signals in birds are of critical importance to behaviors associated with reproduction, intrasexual interactions, and group cohesion, and the complexity of vocal signals such as song can serve as an honest signal of an individual’s quality as well as the viability of a population. In this study, we examined how rapid population declines recently experienced by Hawaiian honeycreepers on the island of Kaua‘i (USA) may have influenced the diversity, complexity, and similarity of learned honeycreeper songs. We analyzed the acoustic characteristics of songs recorded during 3 time periods over a 40-year time frame for three species of declining Kaua‘i honeycreepers. We detected a loss of song complexity and diversity over the 40-year time period that paralleled dramatic population declines. Concurrent with the loss of complexity we also found that the acoustic characteristics of the three honeycreepers’ songs became more similar to another. To our knowledge, this is the first documentation of convergence of acoustic characteristics among rapidly declining species. The reduction of song complexity and diversity and convergence of songs not only signals a loss of culturally transmitted behaviors in these endemic Hawaiian honeycreepers, but also potential challenges to recovery of these rapidly declining species. Moreover, the present study highlights that there is a “hidden” cost to declining populations beyond just the loss of individuals that is not often considered, the loss of culturally transmitted social behaviors.