The Effects of Timing of Mercury-Induced Developmental Stress on Songbird Reproductive Success

This thesis explored two questions – Does developmental exposure to methyl-mercury affect the later reproductive success of songbirds? Does the timing of such developmental exposure affect the severity of any effects on reproductive success? The second question was formulated to test the Developmental Stress Hypothesis (DSH), which states that early developmental conditions incur later-life fitness consequences. Methyl-mercury (MeHg), a potent environmental toxicant, may induce stress in developing organisms, and have far-reaching negative effects on adult fitness. I tested the DSH by investigating whether the timing of MeHg-induced stress, early vs. late in development, affected later-life reproductive success (a strong measure of fitness) in captive zebra finches (Taeniopygia guttata). Groups of siblings were divided across four treatments – early exposure (conception + 64 days), late exposure (beginning at 64 days post-conception + 64 days), complete exposure (conception + 128 days), and control (unexposed) – in order to account for genetic effects, and therefore have a rigorous test of the DSH. Birds were paired among families and within treatments once they reached adulthood. Key reproductive metrics were assessed, including the overall number of independent offspring produced per pair in an 8-month continuous breeding period (mean reproductive output). Among-treatment comparisons showed that late-exposed pairs outperformed early-exposed pairs to varying degrees, in all metrics, with a 21% higher, but not significantly different, reproductive output. Evidence in support of the DSH was therefore mixed, as performances were biologically different, but statistically indistinguishable. Mean differences suggest that the impact of stress on developing systems is greater earlier in development, and that phenotype, and therefore fitness, can be explained not only by genotype and environment, but also by the timing of stressful events. The first question was addressed by combining all performances of pairs in mercury-exposed treatments, and comparing these to controls. Control pairs outperformed developmentally-exposed pairs in all metrics but one, with large differences in the hatching success (40% higher in controls) and pair survival metrics (52% higher). The final mean reproductive output of control pairs was 80% higher than that of exposed pairs (5.58 vs. 3.11 offspring). Results were similar to ones recorded in studies of zebra finches exposed to MeHg during the reproductive process, highlighting the severe and irreversible harm MeHg-induced developmental stress can have on developing songbirds, and, more broadly, on songbird population stability. I therefore suggest that conservation and management strategies give priority to the remediation and/or protection against mercury pollution of breeding grounds and migration routes, in order to achieve the greatest impact