The functions of bird song are well described, but empirical studies examining the costs of singing are scarce. Two potential costs are a metabolic cost of singing, and lost feeding opportunities, but such energetic costs will only be biologically important if they have a significant effect on the bird's body reserves. Overnight loss of reserves has previously been found to increase with increasing song rates in nocturnally singing common nightingales Luscinia megarhynchos. However, it is not clear how such costs compare with those incurred by daytime-singing birds, which may forfeit foraging opportunities when they sing. In this paper we investigated the effect of variation in song rate on the body reserves of a typical daytimesinging bird, the European robin Erithacus rubecula, singing at different rates in three different circumstances: (i) Natural variation in song rate of free-living robins. (ii) Manipulations of the song rate of free-living robins using playbacks of conspecific song. (iii) Manipulations of the song rate of aviary-housed robins using playbacks of conspecific song. In all three parts of our study, birds gained less mass when they sang more. Our analyses suggested that this might have been due primarily to a reduction in food intake rate, rather than to the metabolic cost of the singing itself or a concurrent increase in locomotor costs. These results together demonstrate that the costs of singing, as measured by their overall net effects on body reserves, can have a significant impact on the energetic state of daytime-singing birds.