Why do the females of many bird species sing in the tropics?

In the tropics female birds sing much more frequently than in the temperate north, and sometimes even join their mates to form duets. Why might this be? We discuss how this phenomenon may relate to differences in life history that have been proposed between these regions, highlight likely hypotheses and point to areas of ignorance where studies are needed. The two reasons that appear at present most likely to explain the greater prevalence of female song in the tropics are: (1) the need for mutual stimulation to achieve breeding synchrony in a relatively aseasonal environment and, (2) sex role convergence arising from more long-term relationships and greater fidelity than is usual among north temperate species.