We quantify the singing behaviour of eastern kingbirds (Tyrannus tyrannus) in the 'dawn song' period to describe variation in performance, identify sources of variation, and identify the main recipient(s) of the information conveyed in the display. All males (N = 87) participated in dawn song, but the probability that a male did not sing increased if he was relatively isolated, and it was late in the season. The earliest and longest song bouts were made by males with many neighbours and when population level female fertility was high. High song rates were typical of males in dense nesting assemblages, early in the season and nest cycle, and when social mates were infertile. However, among males with active nests, song rates varied only with neighbour density and either date or population level female fertility. Given the strong inverse correlation between population fertility and date, we suspect that population fertility drove the relationships. The association of start time and song rate with population fertility, continuation of dawn song after territory establishment, and depressed song rate when mates were fertile, are all consistent with the hypothesis that the availability of extra-pair females is the primary factor selecting for male participation in dawn song.