Effects of methylmercury and unpredictable food stress exposure on songbirds’ physiology and seasonal transition
Abstract
Organisms regularly adjust their physiology to respond to predictable seasonal or environmental variation. However, annual cycle transitions could be disrupted by contaminants or stressors. For example, methylmercury and stress exposure can independently disrupt birds’ neural and endocrine systems, energy balance, metabolism, or behaviour, all necessary for seasonal transitions. Although, the effects of combined exposure to stressors and methylmercury (MeHg), and how long they last after exposure ends, are poorly understood. The objective of my PhD was to evaluate the impact of MeHg exposure on songbirds’ physiology and its potential carry-over effects on seasonal transitions. I exposed song sparrows (Melospiza melodia) to environmentally relevant doses of MeHg in combination, or not, with unpredictable food stress. I observed birds’ physiological changes throughout two seasonal transitions: summer to fall and winter to spring. In Chapter 2, I demonstrated that MeHg is sequestered within feathers at the time of feather growth, making feathers an appropriate tool for bird monitoring under the condition that moult pattern is well characterised in the monitored species. In Chapter 3, I found that unpredictable food stress increased body condition but decreased basal metabolic rates, while MeHg exposure increased moult duration and feather mass/length ratio in fall. In Chapter 4, MeHg and stress differently affected nocturnal fall migratory activity and the combined treatment group had increased fecal corticosterone metabolites post-exposure; both measures were positively correlated. In contrast, in Chapter 5, MeHg exposure had no detectable effect on winter to spring changes in brain GnRH cells, testosterone levels or testis size. Thus, spring reproductive onset might not be affected by MeHg in birds. However, MeHg exposure did affect cloacal protuberance, fecal corticosterone metabolites and brain neurogenesis, suggesting that exposed birds’ mating success could be reduced later on. Overall, my thesis main findings were: i) except for corticosterone concentrations food stress did not exacerbate the effects of MeHg exposure, and ii) effects of MeHg on moult, migratory behaviour and secondary sexual signals may be a potential cause of concern for populations. My research highlights the importance of studying contaminant effects over multiple seasons and post-exposure periods when assessing risk for wildlife.