Mercury in Birds and Bats

I want to highlight this research report for a couple of reasons.   First, it’s a summary of a lot of research on birds and bats–and it is alarming.  Major findings include:

• Current environmental mercury loads have the ability to significantly reduce reproductive success in several songbird species of conservation concern in the northeastern U.S. including the saltmarsh sparrow and rusty blackbird.
• Bats also build up significant body burdens of mercury;  individuals from multiple species from all 10 areas sampled exceeded the subclinical threshold for changes to neurochemistry.
• Mercury loading in songbirds is not only restricted during the breeding season; some species, such as the northern waterthrush, build up high levels of mercury during migration and in tropical wintering areas

Basically, this expands what we know about the dangers of biomagnification out into song birds and bats.  I’ve written about some of the research this report is based on before.

From an interview with an author:

“It is a game-changing paradigm shift,’’ Evers said. “For years, we’ve understood the notion that birds like an eagle can obtain toxins by eating a bass, which has eaten a perch, and the perch has eaten a fly. Now we understand the same kind of analogy can be applied to a water thrush, which eats a spider, which has eaten a smaller spider, which has eaten a fly.’’

The other reason I want to point you at this is because it’s a great example of how to produce a report on complex research and make it really accessible.  They don’t just have data; they have information on how to interpret the graphs.

The PDF report itself is beautiful to look at, and focuses on specific actions/conclusions that can be drawn from the data.  It’s a report that I could hand to any of my non-scientist coworkers and be confident they could read it and understand it.  The PDF is presented within the context of a page with lots of supplemental info, including jpgs of some of the figures.  This makes it easy for journalists to build a story.

A thermometer is used to indicate risk to certain species–which cleverly uses something commonly associated with Mercury, but also something a lay-person knows how to interpret without a lot of special background knowledge.

Lastly, they cited their research through the report in ways that let you look up the original research, but that doesn’t detract from your reading.  It makes a powerful case that we need to really start paying attention to the mercury in our environment–because it’s not just the birds that are exposed.

Additional Reading:

• Aquatic mercury in food webs
• USGS: Mercury in the Environment
• Game Changers: a report on how global warming and toxins might change hunting and fishing.
• EPA advisories on which states have fish too contaminated to eat

Aquatic Mercury in Food webs

A new paper came out in Science a while ago that I’ve been meaning to write about:

Cristol, D.A. et al. 2008. The movement of aquatic mercury through terrestrial food webs. Vol. 320. no. 5874, p. 335;  DOI: 10.1126/science.1154082
Along a mercury-contaminated river in Virginia, United States, terrestrial birds had significantly elevated levels of mercury in their blood, similar to their aquatic-feeding counterparts. Diet analysis revealed that spiders delivered much of the dietary mercury….These results show that any risk from mercury faced by the river’s aquatic birds exists for birds in adjacent terrestrial habitats as well. By preying on predatory invertebrates such as spiders, songbirds increased the length of their food chains, increasing the opportunity for biomagnification.

A very interesting study that shows how little things add up–literally.

In the 1970s, a serious mercury contamination problem was discovered in a river near the site of a fiber production plant. The authors of this study looked at where that mercury ended up.
What makes this study different from other aquatic food web studies is that they sampled land birds– birds that were not fish eaters, or feeding on aquatic vegetation.  Most past studies have sampled aquatic birds as “sentinels” for contamination.

Spiders made up 20 to 30 percent of the birds’ diets, yet delivered about 75 percent of the mercury. In fact, the spiders had higher levels of mercury than kingfishers, aquatic birds that are quite a bit larger than a spider.

Studies of bioaccumulation and biomagnification have produced some classic ecological studies over the years. (Tracing DDT from runoff into lakes up to the Bald Eagle, for example.) This graphic is a visual example of how mercury can move out of the water and into living animal tissue.  The mercury accumulates in each level of this food chain (or, trophic level), which is called bioaccumulation.

Generally, each step in the food chain (plant-> plant eater -> predator –> bigger predator) multiplies contamination by a factor of 10.  We have known for some time that mercury was a problem for fish and fish-eating birds. For humans, there are mercury warnings about fish eating in over 40 states, including Michigan.

The new information from this study suggests that this graphic needs a new arrow–one that points off to the side to land arthropods and birds.

If spiders eat mercury-laden insects, the birds eating those spiders get a bigger dose of Hg than birds eating caterpillars feeding on “clean” land plants.

And that is a Very Bad Thing for songbirds that are arthropod predators.

What insects might the spiders be eating? That wasn’t quantified in this study, but the obvious suspects are the many insects that have aquatic larvae, but terrestrial, free-flying adults.

That would include mosquitoes and midges, damselflies and dragonflies, Caddisflies, mayflies, and many other aquatic insect groups.

Apparently we need mercury warnings for warblers, as well as people.

Additional Reading:

• USGS: Mercury in the Environment
• A lesson plan on mercury bioaccumulation
• NWF info on common household items that may contain Mercury
• Don’t worry too much about your fillings

Source of graphic