Mercury reduces avian reproductive success through direct embryotoxicity rather than altered parental behavior
Mercury (Hg) is a heavy metal contaminant of major ecological concern that primarily enters the environment through anthropogenic sources such as coal combustion, and waste incineration. The effects of Hg on aquatic wildlife are well studied, given that Hg is a prevalent in these ecosystems. For aquatic avian species in particular, Hg has been documented to impair reproduction, specifically by reducing hatching success and fledging success. Developing embryos are considered to be the most sensitive stage to contaminant exposure, but altered parental behavior may also contribute to decreased reproductive success rather than direct embryotoxicity. We propose that Hg may impair reproduction by negatively affecting parental care that is essential for producing viable offspring. The objective of this study was to determine how dietary methylmercury exposure influences avian parental care and associated reproductive success, using zebra finches (Taeniopygia guttata) as a model species. Specifically, nest building, incubation behavior, incubation temperature, and provisioning behavior of parents were examined, as well as reproductive success and offspring growth. A factorial design was used, with parents cross-fostering eggs from another group. Four treatments were assigned: 1) control parents, not exposed to Hg, raising eggs from other control parents (n=15), 2) control parents raising eggs from exposed parents (n=14), 3) exposed (1.2ppm Hg via feed) parents raising control eggs (n=13), and 4) exposed parents raising exposed eggs from other exposed parents (n=13).
We expected reduced parental care ability in pairs exposed to Hg in comparison to control parents receiving 0.0ppm Hg, and that lower parental care measures correlate with decreased hatching and/or fledging success. Results indicate that Hg-exposed birds complete their nests faster than control birds, but build lighter nests and are less successful at bringing hay into their box. Incubation temperature and behavioral endpoints did not significantly differ between parental treatments. However, incubation period was shorter for treatments with control, but no significant difference was observed between parental treatments. Hatching success showed a similar pattern, where control eggs were more likely to hatch than Hg eggs. Overall, this suggests that embryotoxicity reduces reproductive success, rather than altered parental behavior. However, further research is warranted in field settings, where birds are exposed to higher-stress and limited resources. This novel study provides further insight into how Hg impacts avian reproductive success as well as informs future research directions, especially since the effects of Hg on terrestrial songbirds is still not well understood.