Mercury exposure alone, or combined with chronic food stress, elevates blood corticosterone in the zebra finch.

The abundance of songbirds has declined across North America over the last 50 years. Thus, it is crucial to understand drivers of population losses, including environmental contaminants, such as mercury. The effects of mercury on songbird health are still not fully understood. We studied the effect of mercury on the corticosterone-mediated stress response in captive zebra finches (Taeniopygia guttata). Birds were subjected to 45 days of sublethal (approximately 1 mg/kg) dietary mercury alone or in combination with another stressor, 25 consecutive days of unpredictable food availability, beginning 17 days into dietary mercury exposure. Mercury alone or in combination with unpredictable food resulted in elevated baseline corticosterone. As expected, birds sampled later within a three-minute capture-and-sampling window had higher circulating corticosterone, reflecting the beginning of an acute stress response due to handling. However, in birds that experienced both mercury exposure and unpredictable food availability, longer sampling latency was not associated with increased corticosterone, suggesting a different or delayed stress response. We also found that birds that lost weight over the study period had higher corticosterone levels, but only if they received mercury exposure alone, suggesting a complex relationship between mercury exposure, stress, and body condition. Our study demonstrates experimentally that exposure to an environmentally relevant concentration of mercury elevates baseline corticosterone in an adult songbird, and that the physiological response to mercury may be impacted differently dependent on whether it is combined with another stressor.