Mercury Exposure And Chronic Food Stress Elevate Corticosterone In Blood But Not Feathers

Bird abundance in North America has declined by almost 30% in just the past 50 years. It is crucial that we understand the many factors contributing to declines, including the impacts of environmental contaminants. Mercury is a persistent and global neurotoxic contaminant of aquatic systems that will continue to increase in the environment as anthropogenic activities and climate change accelerate its emission. The impact of mercury on songbirds, a large and diverse but declining taxon, is not well understood. I sought to better understand the effects of mercury exposure on songbirds by studying its impact on the environmentally sensitive hypothalamic-pituitary-adrenal axis, which controls the stress response mediated by the hormone corticosterone. Using adult, captive-bred zebra finches as a model songbird, I tested the effect of an environmentally relevant level of dietary mercury on corticosterone in both blood and feathers. I also tested the effect of an additional stressor – chronic, unpredictable food deprivation – both on its own and in combination with mercury exposure, to better estimate how mercury might impact birds differently when another challenge occurred simultaneously. Baseline corticosterone was measured in the blood. Corticosterone was also measured from primary feathers grown during the treatment period, and mercury was measured in those same feathers, to test whether this minimally invasive technique might be useful in assessing stress due to mercury exposure in the field. The resultant baseline blood corticosterone levels were best explained by models including treatment, time elapsed between disturbance and sampling, sex, and an individual’s change in body mass over the study period. On average, birds under mercury stress alone or in combination with food stress had higher baseline blood corticosterone levels than controls. Interestingly, birds in the dual stress group did not exhibit the expected gradual increase in blood corticosterone levels in the first three minutes after disturbance. This was the opposite of all other groups, in which birds sampled longer after the initial disturbance of researchers entering the room had higher corticosterone levels. Feather corticosterone was not correlated with blood corticosterone. While treatment group alone was not related to feather corticosterone, the interaction between treatment group and pre-treatment feather corticosterone predicted corticosterone in replacement feathers. In individuals that were not exposed to mercury, there was a positive correlation between original and replacement feather corticosterone. However, mercury exposure disrupted this association. Chronic mercury exposure appears to elevate baseline corticosterone levels in adult songbirds, which could be detrimental to their health. Faced with combined mercury and food stressors, the stress response may have been suppressed, as suggested by the lack of an increase in corticosterone with time since disturbance. An effect of mercury on feather corticosterone was only observed when considering both treatment group and feather corticosterone before mercury exposure, so caution should be exercised in using feather corticosterone as a bioassay for contaminant or food stress in the field. Further study of stress-induced corticosterone is necessary to better understand the effects seen here, and the modulating roles of feather color and size, also examined here, must be considered