Some of Illinois' rarest and most imperiled birds return each spring to marshes on Chicago's Southeast Side, where they weave nests of loose reeds, start the season's courtship rituals and, scientists believe, resume ingesting a poison most people thought was gone a generation ago.

Researchers studying the state's endangered population of black-crowned night herons reported this year that the birds contain remnants of the insecticide DDT, a contaminant popularly imagined to have faded into America's hazy chemical past, but which scientists say has lingered in a persistent form almost everywhere in the world nonetheless.

The trace amounts of DDT byproducts found in the city's night herons do not threaten their survival, but the discovery shows the tenacity of the toxic compound and its propensity to work its way up the food chain.

It also draws attention to a disturbing trend for DDT and other organic pollutants such as PCBs: Though their levels fell quickly after they were banned in the 1970s, the drop reached a plateau in the 1990s and has declined much more slowly ever since. "It is striking and problematic that these chemicals are still available so long after having been banned," said Jeff Levengood of the Illinois Natural History Survey, lead author of a paper that found tissue from local night herons contained levels of DDE-a toxic byproduct of DDT that is stored in fat cells-in the range of one part per million.

Scientists say that is about one-eighth the amount that would cause the familiar and much-feared effects of DDT, such as egg-thinning and fertility problems. Levels of the contaminants have fallen significantly since the end of the chemicals' heyday in the 1950s and 1960s, when they were sprayed lavishly in urban areas to kill mosquitoes or on farmland to kill pests.

Though DDT is still popular in much of the developing world as a weapon against insects that spread malaria, it became the poster-chemical of the U.S. environmental movement in the 1960s when scientists learned how it was affecting people and birds, chiefly eagles and peregrine falcons.

Today, except for a few rare sites, the chemicals no longer exist in the dangerous concentrations that can threaten, say, North America's sensitive bald eagles.

But tests on the night herons and other species have helped scientists understand that the rapid decline of the toxic chemical has slowed. What is left is less likely to break down further and easily moves up the food chain if plankton and other bio-organisms find it.

Video and Full Story: http://www.chicagotribune.com/news/local/chi-poisonbirds_bd25may25,0,7187385.story

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