Seasonal Variation in Corticosterone, Stress Responses, and Physiology in the Northern Cardinal \u3ci\u3eCardinalis cardinalis\u3c/i\u3e

Seasonal regulation of the adrenocortical response (e.g., ‘stress response’) appears to be ubiquitous in mid- to high-latitude vertebrates. Northern Cardinals (Cardinalis cardinalis) are a Temperate dwelling passerine of Tropical-descent with a wide species range (tropics to Northern Temperate zones). This species encounters a wide variety of environmental conditions and strong seasonality, however corticosterone (CORT, the main adrenal stress hormone) has not been studied with regards to changes in seasonal levels. I initially analyzed samples from cardinals collected between 2007-2011 from the Lake Thoreau Environmental Research and Educational Center (Hattiesburg, Mississippi, USA). The data suggested seasonal differences of CORT, with higher plasma concentrations during the winter with decreases in pre-breeding, and even further decreases during breeding where the lowest average concentrations were observed. In 2012-2013 we used the same banded population of cardinals to monitor seasonal changes in stress responses. Using more stringent initial CORT (≤3min) samples we observed an even more pronounced variation in seasonal CORT modulation which mirrored our larger 2007-2011 dataset. The data confirmed preliminary results that significant seasonal variation in CORT ‘baseline’ levels exist and now stress responses vary, particularly during breeding. In cardinals dampened breeding HPA (hypothalamic-pituitary-adrenal) activity resembles that of species breeding in harsh environments (blunted response to stress), instead of typical hormonal patterns exhibited by Temperate-breeders (robust response to stress). Lastly, we tested whether seasonal CORT levels impacted testosterone (T) concentrations throughout the annual cycle. Results indicated a significant effect of stress responses on initial T levels during pre-breeding. Testosterone levels were seasonally highest at this time of year while initial CORT levels were low. During pre-breeding, T is important for the development of secondary sexual characteristics and behaviors associated with breeding success (e.g. singing rates, courtship, and territoriality). Since CORT and T are known to negatively correlate with each other, it could be adaptive to decrease CORT levels at this time to prevent any hormonal interference that could negatively impact overall reproductive success. This research has helped to improve our overall understanding in how cardinals seasonally modulate two hormones that are very important for survival and reproduction