Interrelationships between testosterone, aggression, and parental care of a temperate-zone, resident songbird, the Northern Cardinal (Cardinalis cardinalis)
Interrelationships among testosterone (T), aggression, and parental care have received much investigative attention. Within birds, many studies have focused on examining such relationships using species characterized by relatively brief periods of territoriality and breeding. Few have investigated links between circulating T and reproductive behavior in birds that are year-round territorial residents and have lengthy breeding seasons, such as the Northern Cardinal ( Cardinalis cardinalis ). The following six chapters report and discuss findings from this dissertation's examination of aspects of T production and potential interconnections with circulating T, aggression, and parental care in male and female cardinals. This work suggests that male and female cardinals have the physiological capacity to significantly increase T levels during non-reproductive periods in response to standardized gonadotropin-releasing hormone (GnRH) injections. Male cardinals maintain the ability to significantly elevate T following GnRH injections across the pre-breeding and breeding seasons, yet, circulating T levels were not significantly higher following simulated aggressive encounters and no relationship existed between T concentrations and the degree of male nestling provisioning. Similarly, female cardinals are capable of significantly elevating T levels following GnRH injections administered in the non-breeding and pre-breeding periods, but significant relationships did not exist between circulating T levels and displays of maternal aggression or nestling provisioning rates of females. This lack of connection between relative circulating levels of T and behavioral performance suggests a complex association between T and reproductive behavior exhibited by this species. Whether this complicated relationship of circulating T and behavior is unique to the cardinal or characteristic of other temperate resident species exhibiting a similar behavioral ecology is unknown and deserves greater attention.