|Saltmarsh Sparrow / Photo by Wolfgang Wander|
Methylmercury has already been documented in multiple bird species, with particularly high concentrations in Wood Thrushes. Some birds such as raptors and kingfishers have fish as a mainstay of their diets. Songbirds consume mercury by eating invertebrates that spend the early stages of their life in the water (such as mosquitoes, dragonflies, and caddisflies) or by eating spiders that prey on those aquatic invertebrates. Mercury is associated with behavioral changes in songbirds and may skew sex ratios. Like in humans, mercury affects learning and memory in songbirds. It also hampers birds' ability to learn and sing songs.
A new study by Sheila Scoville and Oksana Lane adds to the evidence for mercury's harmful effects in songbirds. When a fledgling Saltmarsh Sparrow (Ammodramus caudacutus) was accidentally killed at a study site on Long Island, it was collected and analyzed for mercury poisoning. Feathers were collected and sent to the Biodiversity Research Institute to test for mercury exposure, and the sparrow's brain was analyzed at the Eastern Virginia Medical School. The fledgling showed some exposure to mercury, though not at the levels measured in adults at that study site (which overall were very high).
Based on feather samples taken from adult birds at the study site, it can be inferred that this fledgling's mother had high exposure to mercury at the time that she laid her eggs. This is analogous to in utero exposure in humans. In humans, in utero exposure can lead to brain abnormalities like Minamata Disease. This fledgling Saltmarsh Sparrow showed abnormalities in its cerebellum that would have similar effects on motor control and coordination. Birds with these sorts of defects would have trouble recognizing and escaping danger, thus making them more susceptible to predation and accidental deaths.
While this fledgling Saltmarsh Sparrow is only one data point, the findings have disturbing implications. Since mercury levels among adult birds were so high, many other young Saltmarsh Sparrows are presumably exposed to mercury at the time of egg formation. We cannot know how many of them have brain abnormalities like the one documented in this study, but it seems safe to assume that the problem is widespread.
Even without human interference, Saltmarsh Sparrows lead a precarious existence. At any time during the breeding season, their nests may be flooded by particularly high tides or destroyed by violent storms. Their breeding range is restricted to a narrow strip of saltmarshes along the Atlantic Coast of the United States. Their habitat is constantly under pressure from human encroachment, as more and more coastal areas are developed as beach resorts. Oil and chemical spills further degrade their remaining habitat. There is an additional threat from sea level rise due to climate change and subsidence. For these reasons, BirdLife rates the Saltmarsh Sparrow as a vulnerable species. Now we know that in addition to all these other problems, the species also faces a threat from ingestion of mercury.
The harm caused by mercury is not something conservationists can easily remedy. Shifting away from coal as a source of electricity would at least reduce the amount of mercury entering our waterways. What we can and should do is protect and maintain as much remaining saltmarsh as possible so that the sparrows that survive to adulthood will have enough habitat for breeding.
Thanks to Peter Doherty for alerting me to this study.
Scoville, S., & Lane, O. (2013). Cerebellar Abnormalities Typical of Methylmercury Poisoning in a Fledged Saltmarsh Sparrow, Ammodramus caudacutus Bulletin of Environmental Contamination and Toxicology, 90 (5), 616-620 DOI: 10.1007/s00128-013-0974-y