Female and male song rates but not female testosterone are elevated early in the breeding season in a temperate songbird ( <i>Troglodytes aedon</i> )

Abstract Male temperate breeding songbirds often show dramatic increases in song rate at the start of the breeding season, which corresponds with large increases in circulating plasma testosterone. Testosterone is a well-studied mediator of male birdsong. However, we know less about steroid hormone regulation of female birdsong, especially in free-living species with naturally produced female song. Here we investigated associations between male and female song rates and circulating plasma testosterone in nest box breeding Troglodytes aedon (Northern House Wren) throughout the breeding cycle. We found that both males and females sang most prior to egg-laying and that both of their song rates decreased throughout the breeding cycle. We also found that male plasma testosterone concentrations were highest prior to egg-laying and also decreased throughout the breeding cycle; however, male song rates and testosterone were not significantly correlated. By comparison, female circulating testosterone did not change significantly throughout the breeding cycle and was, on average, an order of magnitude lower than in males. Circulating testosterone concentrations were not indicative of individual song rates in either sex. We conclude that the concentration of testosterone does not directly correspond to the amount that an individual bird sings, consistent with testosterone having a threshold effect on the activation of song. Interestingly, this also suggests that female songs in this species are either mediated by very low testosterone concentrations, metabolites of testosterone, or alternative hormones or pathways. Our finding that females sing despite exceptionally low testosterone concentrations highlights the possibility that other mechanisms regulate female and potentially male song.