Natural variation in circulating testosterone does not predict nestling provisioning rates in the northern cardinal, Cardinalis cardinalis

Behavioural transitions between territoriality and parental care are necessary for many seasonally breeding vertebrates. Among birds, such transitions can be mediated by the steroid hormone testosterone (T), resulting in a T-mediated behavioural trade-off. This theory is supported by many implantation studies with birds demonstrating that the administration of exogenous T during parental phases can negatively affect offspring care. However, little is known about relationships between naturally circulating levels of T and parental behaviour in wild bird populations. We examined covariation in circulating levels of T (before and after injection of gonadotropin-releasing hormone (GnRH)) and nestling provisioning rates in male and female northern cardinals, Cardinalis cardinalis, a highly aggressive, biparental resident songbird. Results indicate that both sexes had detectable levels of initial T when providing parental care, but only males produced significantly higher elevations of T following GnRH injections. Furthermore, T levels (both initial and following GnRH injections) and nestling provisioning rates did not covary for either sex, and these measures were not correlated between members of breeding pairs. When results of this study are considered with prior work indicating that elevations in T might not be necessary to support male or female aggression in this species, it appears that relationships between circulating T and reproductive behaviour could be more complex for the cardinal than for many others birds similarly examined. Such findings warrant additional examination of interrelationships between T, aggression and other forms of parental care to assess whether this species engages in T-mediated behavioural trade-offs. Possible alternative mechanisms influencing behaviour in the cardinal and future directions for research are discussed.

► We examined the relationship between circulating testosterone and nestling feeding rates in northern cardinals.
► Both male and female cardinals had detectable testosterone levels while caring for offspring.
► Testosterone was significantly elevated after GnRH injection in parental males but not in parental females.
► We found no relationship between nestling feeding rate and circulating testosterone level.