Testosterone and the trade-off between mating and paternal effort in extrapair-mating superb fairy-wrens

High investment in mating and parental effort in male birds are considered mutually exclusive, as testosterone suppresses paternal care and stimulates sexual and aggressive behaviours. Superb fairy-wrens, Malurus cyaneus, are unusual in that males concurrently engage in courtship and paternal care. Fairy-wrens live as stable socially monogamous pairs with up to four subordinate male helpers. The majority of offspring are not sired by any of the males in the group but all group males provide parental care. Throughout the period of care males visit and court neighbouring females. To examine the role of testosterone in this trade-off, I determined the effect of testosterone treatment on the males' time budget, in particular their investment in within- and extrapair sexual behaviours and paternal care. Testosterone did not affect time spent foraging and resting or the number of songs produced. However, the relative investment in sexual and parental behaviours changed, confirming proximate control of this trade-off by testosterone. While there was a large increase in within-pair courtship, which is always rare during the nestling period, testosterone had only a small effect on the most frequent sexual behaviour, extragroup courtship displays. Testosterone failed to induce the maximum frequency of courtship flights of which male fairy-wrens are capable, and I argue that this is because, at prevailing testosterone levels, males are always motivated to leave their territory and social circumstances largely dictate the actual departure rate.