In 2018, Researchers Banked on a Backyard Bird to Track Old Poisons. A Cardinal’s Egg Pointed Straight to One Contaminated Block.

Quick Take

• DDT was banned before most millennials were born, yet two Maryland bird eggs contained a form of it that chemistry says is fresh. How is that possible? See the fresh DDT evidence →
• A backyard bird's egg can pinpoint contamination to a single city block, and a cardinal proved it. How cardinals pinpoint pollution →
• The chemical that nearly wiped out the bald eagle doesn't truly disappear. It hides, and it can resurface somewhere you'd never expect. DDT's lasting legacy →
• Not all pollution spreads evenly. One finding from this study reveals what separates a routine old contaminant from something far more alarming. Discover the alarming outliers →

The United States banned DDT in 1972. Decades later, a study published in 2018 went into six neighborhoods around Washington, DC and collected Northern Cardinal eggs to test them for banned pesticides and PCBs. In two suburban Maryland nests, the researchers found DDT.

And most of it was fresh.

A backyard bird gets a lab test

The Northern Cardinal is about as familiar as a bird gets: bright red, loud, a regular at backyard feeders. That’s part of what made it a good test subject. The chemicals a mother bird takes in from her food end up in the eggs she lays, so an egg is a small chemical record of the place she lives.

Researchers gathered eggs from six neighborhoods near Washington, DC. They ranged from quiet, rural spots to older, more built-up suburbs. Then they tested the eggs for old, banned chemicals. The study, led by Christopher J. Schmitt of the USGS, came out in 2018 in the Bulletin of Environmental Contamination and Toxicology.

The chemical that nearly erased the eagle

To see why anyone would bother, you need to know what DDT did. It’s a bug killer that came into wide use after World War II. It turned out to make bird eggshells so thin they collapsed when the mother tried to keep them warm.

Bald eagles nearly vanished. [By 1963 only 417 nesting pairs were left, and their range had shrunk to Alaska and Florida]. Brown pelicans had it even worse. On one California island in 1970, 552 nests produced a single surviving chick.

The country banned DDT in 1972, largely because of what it was doing to birds. Even so, the eagle wasn’t removed from the endangered list until 2007.

Here’s the catch. [DDT doesn’t break down easily, and it builds up in an animal’s fat every time it eats contaminated prey]. It sits in soil and in bodies for decades. Nobody who studies this stuff is surprised to find a trace of it somewhere.

That’s what makes the next part strange.

More houses, more poison

Across the six neighborhoods, the pattern was clear. [Every chemical they looked for showed up more often, and at higher levels, in the more built-up neighborhoods].

DDT was detected in all six neighborhoods, mostly as DDE. DDE is a byproduct of DDT, what’s left behind as the original chemical breaks down. That’s exactly what you’d expect from a chemical sprayed everywhere decades ago and left to sit in the dirt.

Two nests that shouldn’t exist

This is the part I keep coming back to.

In most suburban neighborhoods, the original form of DDT, the version that hasn’t had time to break down, showed up only in tiny amounts: 0.5 to 16 nanograms per gram. A nanogram per gram is one part in a billion, so that’s a pinch so small it barely registers. In the rural areas, it wasn’t detected at all.

Then there were two suburban Maryland nests. There, DDT itself, the chemical called p,p’-DDT, measured 127 to 1130 nanograms per gram. The higher of those is roughly seventy times the most fresh DDT found in any ordinary suburban egg. In the most contaminated egg, the fresh form made up 65.7 percent of all the DDT in it.

Roughly two thirds.

That number is the whole story. Given years in the soil, DDT slowly turns into DDE, the way a cut apple slowly browns. If two thirds of what’s in an egg is still the fresh version, that DDT is new. The researchers wrote that it points to recent exposure to un-weathered DDT. Where it came from, the study doesn’t say, and I won’t guess. But the chemistry only points one direction: fresh DDT got into those yards long after the country stopped using it.

Why a red bird makes a good watchdog

The study’s bigger finding is that a cardinal’s egg can tell you which street a chemical is lingering on.

Compare the DDT with PCBs, another long-lasting industrial chemical banned long ago. PCB levels in the cardinal eggs stayed low everywhere, [under 5 to 21 nanograms per gram, and the mix of PCBs looked like old, evenly weathered leftovers in every neighborhood]. That’s the expected result: a thin, even layer of old pollution, the same everywhere you look.

The fresh DDT in those two Maryland nests was a hot spot. A couple of yards stood out, sharply different from their neighbors. That’s why the researchers concluded that cardinal eggs can be used to monitor localized contamination, meaning pollution centered on one particular place. A cardinal lays her eggs where she lives, so what’s in the egg came from close by. In this case, the bird found the block that mattered.

The study started with a pretty red bird and a routine test for old chemicals in a handful of neighborhoods. It ended with a warning: somewhere in suburban Maryland, a chemical the country banned before many of today’s homeowners were born turned up fresh. Eagles came back because people noticed thin shells and did something about it. This time a cardinal noticed first, and I’m glad someone thought to ask.