Don't eat the spiders!

Tree swallows feed on insects from dry and wet habitats and therefore are often used to detect pollutants like mercury.

Mercury in polluted rivers doesn’t just affect aquatic life. Scientists have discovered that spiders, moths and grasshoppers living near the water can deliver mercury to the food chain on land when they become tasty treats for birds.

These prey items were collected from the beaks of nestling songbirds. Spiders made up 25 percent of most birds' diets but delivered 75 percent of the mercury.

Daniel Cristol of the College of William and Mary and his colleagues studied a variety of birds, including screech owls, kingfishers, swallows, chickadees and wrens, near the Shenandoah River in Virginia.

They found that twelve bird species that eat spiders, moths and grasshoppers had potentially harmful mercury levels in their blood and feathers. These mercury levels were about the same as those of birds that eat aquatic insects.

When the authors analyzed what those twelve bird species ate, they found that insects, primarily spiders, provided much of the mercury.

The researchers don’t know yet why the spiders have so much mercury in their bodies. They may be eating insects that live in the polluted water or perhaps picking up mercury somehow by walking across polluted sediments on the riverbanks.

The authors, whose study appears in the 18 April issue of the journal Science, say that rivers contaminated with mercury may pose a threat to millions of birds that feed on predatory invertebrates in habitats near rivers.

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