California condor egg shell thinning blamed on DDT — even though no significant correlation found

The AP reports:

In the coastal redwood forests of central California, scientists trying to unravel the mystery surrounding the reproductive problems of dozens of endangered condors think they have uncovered the culprit: the long-banned pesticide DDT.

The soaring scavengers with wingspans wider than NBA players are tall were reintroduced to the rugged coast of Big Sur in 1997 after a century-long absence. Upon arrival, the birds found plenty to eat, with dead California sea lions and other marine mammals littering the craggy shoreline.

While a good food source, sea lion blubber often has high levels of DDT, a pesticide banned in 1972 that has proven to be a persistent pollutant because it accumulates in bodies of creatures throughout the food chain when animals eat one another.

Once used widely in agriculture, DDT was banned because it is a human toxin considered a possible carcinogen by the U.S. Centers for Disease Control and Prevention.

Kelly Sorenson, executive director of Ventana Wildlife Society and a co-author of a new study on condors, said researchers who spent six years studying their reproductive problems have “established a strong link” to DDT in the birds’ food source…

Another former condor program manager, Noel Snyder, was critical of the DDT study, however.

He said it only looks closely at one potential cause of reproductive problems — DDT — and fails to properly evaluate the potential effects of other contaminants and factors that may be involved and more important.

“DDT is not the only thing that causes eggshell thinning, and the authors of the paper don’t present a significant correlation of DDT with the thinning found, and thus do not do a convincing job of linking DDT with the happenings in Big Sur.”

Read more…

13 responses to “California condor egg shell thinning blamed on DDT — even though no significant correlation found

  1. That settles it…DDT is now on double-secret probation!

  2. Why should they look any further when they have already found what they are looking for – a seed of hysteria they can plant and nurture into a full-grown research grant.

  3. Six years studying condor reproductive problems? Glad I got out of biology and into computers back in the 70s.

  4. What difference does it make? The birds are in the way of all that free, renewable energy and will be whacked by wind turbines. No need to worry about the eggs.

  5. While millions of PEOPLE have died in Africa from Malaria that DDT could prevented

  6. They just keep singing that old mantra. You can say it until you are blue in the face and it still does not make it right.

  7. Are they really trying to say that residual DDT, banned in 1972, is causing egg shell thinning? That’s not just a stretch, that’s a leap of faith.

  8. Willis Eschenbach

    The DDT/sea lion blubber study is odd. It’s at

    There’s a known DDT hotspot in the ocean off of Palos Verdes, in Southern California. However, the levels in the sea lions were higher in Northern California … to me, that means the findings are suspect, although they try to handwave the problem away.

    The real killer issue is that the DDT concentrations have dropped by a full order of magnitude since 1970 … but in 1970 no problems were reported in vultures, which also eat dead sea lions, or in the sea lions themselves..

    Finally, there is no evidence presented of any correlation between shell thinning and sea lion DDT concentration … and absent that, the whole claim becomes unsubstantiated.


    • Willis,

      If I remember correctly, the whole thinning eggshell study that spawned Silent Spring was not replicated. In my mind that leaves the DDT/eggshell link tenuous at best.

  9. Could it be lack of calcium in the diet? Or does it always have to be the fault of humans?

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