Effects of DDT on Birds

Does Dixy Know Something the Experts Do Not?


From the Environmental Review Newsletter Volume One Number Seven, July 1994

     In her book, Trashing the Planet, Dixy Lee Ray with coauthor Lou Guzzo, made several statements that contradict "conventional wisdom" about nuclear power, pesticides, acid rain, ozone depletion and global warming. Dr. Ray argued that these environmental concerns were blown out of proportion and were based on pseudoscience and hysteria. The late Dr. Ray was once a faculty member in the zoology department at the University of Washington, a one-term Governor of the State of Washington, a Chairman of the Atomic Energy Commission and Assistant Secretary of State in the U.S. Bureau of Oceans. With such credentials, Dr. Ray's opinions about the biological effects of pesticides could be persuasive to non-experts. However, when Dr. Ray's argument about DDT is examined, it is clear she selected only information that supported her opinions and ignored information that did not.  
     In this month's issue we examine the effects on birds of the pesticide DDT. Dr. Ray states that there is no valid evidence showing that DDT had adverse effects on birds. We discuss her argument with an expert on pesticide effects on birds, Professor Daniel W. Anderson of the University of California at Davis

ER: Dr. Anderson, what is your academic training?

DA: I have a bachelor's in zoology from North Dakota State University, a M.S. in wildlife ecology and Ph.D. in zoology/wildlife biology, both from the University of Wisconsin, Madison.

ER: Dr. Ray said DDT was not shown to be responsible for eggshell thinning in birds. She said eggshell thinning predates the use of DDT and has many other causes: diet poor in calcium or Vitamin D, fright, high nocturnal temperatures, toxic substances and diseases. Is that true?

DA: The basic issue is: is DDT linked to significant eggshell thinning in wild birds? Dr. Ray's argument here was the same argument that spokesmen for pesticides and the pesticide companies were using in the late 1950s. A lot of data have come in since then, especially on eggshell thinning, which she ignores. When she is talking about thin-shelled eggs and DDT she makes many statements that are not backed by the data. The second paragraph on page seventy-one says: "These charges have been repeated so often that they are widely believed, even though they are at best factoids, untrue in most instances."  Dr. Ray's statement that there are no data is untrue. Dr. Ray cites Gordon Edwards and Thomas Jukes and neither of these men did any research on eggshell thinning. And their arguments are thirty-five years old. Dr. Ray has gone back, dredged up those old arguments and used them again - ignoring twenty-five years of research on the problem.

ER: How did we become suspicious that DDT might be bad for birds?

DA: Rachel Carson and Bob Rudd wrote two books about widespread and direct mortality in birds caused by many pesticides, including DDT. Then it was discovered that something had happened to eggshells starting about 1946. Also the data from the museums showed widespread eggshell thinning in many, but not all species studied. There was a hypothesis that it might somehow be due to pesticide exposure. With controlled laboratory experiments it was eventually shown that DDE (a stable, widespread and common metabolite of DDT) was the strongest inducer of any of the known compounds. Other compounds can also induce some eggshell thinning to a lesser degree, but DDE was shown to be by far the most potent inducer among the environmental contaminants, and at levels comparable to those found all over the world in wild birds. Thinning was found in experiments where there was nothing but DDE added to the birds' diet.

ER: Dr. Ray said some bird populations increased when DDT use was highest. Is that true, or relevant?

DA: Dr. Ray is a little slippery when she says there is no supportive evidence of population declines and that bird populations actually increased. There are many cases, but let me mention one that I personally worked on. She does not mention one of the best case histories in North America of DDE-induced population effect which involves the decline and resurgence of the California brown pelican.

ER: California brown pelican populations were hurt by DDT?

DA: By DDE. But in this case, they were adversely affected not by DDT used as a spray but as DDT and metabolites were put into the sewage system in Los Angeles. It entered the Southern California Bight in large quantities. [A bight is a curve in the coast that forms a bay and also often entrains an oceanographic gyre. ed.] Some people in Professor Risebrough's group looked at the sediments in Southern California and saw where DDT and its metabolites first appeared.

ER: When did DDE start showing up in the sediments?

DA: About the 1950s. It was like a detective case and when the source was discovered, input stopped pretty quickly.

ER: What was the source?

DA: The major source was found to be a manufacturing plant in Torrance.

ER: When did they make the changeover to landfill disposal?

DA: It started in about 1970. And within a year the DDE residues began a long decline in the pelicans and other wildlife of the bight and the pelicans (and other species too) started to show signs of recovery.

ER: Was there experimental work with the pelicans that showed eggshell thinning after exposure to DDE?

DA: No experimental work like controlled feeding experiments was done in brown pelicans because that species was on the endangered species list. But there was a positive relationship between the amount of DDE residue female pelicans had in their bodies at the time they were laying eggs and the amount of eggshell thinning. Similar relationships have been shown over and over again, all over the world, for many bird species and by many, many investigators.

ER: That's what I meant by experimental work, they took measurements of DDE in the birds?

DA: Of course, many measurements were made in the field. The amount of DDE in the fat in the egg is positively related to the female's exposure when the eggshells are being formed. And DDE has a profound influence on the eggshell gland.

ER: So DDE has a specific effect on the shell gland.  It's not poisoning the chick.

DA: No, although DDE is toxic, it usually takes large amounts to kill birds (and even embryos) directly. It mainly affects the avian shell gland but there are other effects. There are also other residues in wild bird eggs - PCBs, dioxins, other pesticides and other chemicals which have toxic or physiological effects on embryos. But if you isolate out DDE, it has consistently been shown to have the best relationship to eggshell thinning.
     Even then, that is not direct evidence of eggshell thinning by DDE. It is correlative data. These kinds of field data illustrate how one starts in the field. A correlation provides a hypothesis upon which to base controlled laboratory experiments. The next step is to return to the lab or pen situations and find a species that is susceptible to this effect - because there is a lot of interspecific variation - and conduct controlled experiments. With DDE, all of these steps have been repeated many times and many places.
     It is interesting that Dixie Lee Ray talks mostly about quail and pheasants. "Quail fed 200 parts per million in all their food...hatched eighty percent of their chicks. No shell-thinning was reported." (page 72).
These two bird "species" (she doesn't say what pheasant or what quail) are somewhat resistant to eggshell thinning except at very high levels of DDE. They produce multiple and large clutches of eggs and their eggshell gland is more resistant to eggshell thinning.
     Birds that lay a limited number of eggs (determinate layers) tend to be much more susceptible to DDE. They only produce two or three eggs a season and they are very susceptible to any chemical that affects the shell gland.
     This is another step to the scientific process, the elucidation of the mechanisms of the toxin. There have been three or four mechanisms demonstrated experimentally that affect eggshell thinning. So, the physiological mechanism of how DDE affects the shell gland also does not involve just a single mechanism or explanation. A lot of work has been done on the physiology of DDE and the avian shell gland. Dixie Lee Ray did not talk about any of that work in her book - not one study!

ER: Was that information available to her in 1990?

DA: Oh, of course. By 1990 most of the many scientists who worked on these problems were off doing newer things - like looking more closely at PCBs and dioxins, getting into problems with oil spills, etc.

ER: Do pheasant and quail start out with a thicker shell than more susceptible species?

DA: No. Pelicans and their relatives (cormorants, boobies) for example, tend to start out with a relatively thicker shell than chickens.

ER: So they are just more sensitive to the DDE?

DA: Yes. These interspecific differences actually provide some of the more interesting questions to a scientist, such as looking at evolutionary solutions in different species. Many such questions would be interesting to explore further, but it's a subject that does not get a lot of financial support any more.

ER: Dr. Ray often refers to the fact that the amounts of pesticide residues to which people are exposed are small, parts per million to parts per trillion. What does such a 'low' concentration of chemical mean to a biochemist or physiologist?

DA: Endocrinologists show us that the binding sites for hormone receptors for example, do not require large concentrations, on the order of parts per million. So there are many natural physiological systems where parts per million concentrations of a chemical can be the difference between whether that system is working or not. [For instance cell reproduction or nerve function. ed.]
     Behaviorists and neurophysiologists show us that some animals are able to detect - to smell - extremely low concentrations of chemicals in their environment, parts per million (for example migrating salmon). If natural systems commonly function on this basis, there is no reason to think that contaminants at very low levels, contaminants in the sense of strange chemicals in the body of an animal, do not also have physiological effects at very low concentrations.

ER: On page seventy-two Dr. Ray wrote, "DDT rapidly breaks down harmlessly in the natural environment." Is that true?

DA: DDT itself breaks down pretty rapidly, but not harmlessly. It leaves metabolites. And when Dr. Ray talks about DDT not being responsible for eggshell thinning, she is talking about the parent compound which technically speaking, is true. DDE is not mentioned once in this book and that is very misleading. Dr. Ray as a professor, would certainly reprimand her students for such careless scholarship. This chapter (six) as a term paper in my class, would not pass.
ER: Dr. Ray also wrote "Actual counts of bird populations, conducted annually by the Audubon Society at Christmastime, have shown that many bird populations were in fact increasing throughout the years of heaviest DDT spraying." (page 71).

DA: Here Dr. Ray picked almost all pest species like starlings. That argument was also used in the early 1960s by some spokesmen for the chemical industry. It was brought up both in Great Britain and in the United States. It had been refuted then.

ER: How do you explain that? A non-expert would not know how to respond to that argument.

DA: If you consider starlings or blackbirds, they had increased in some areas. There were also some biases in Christmas bird count data. Out of curiosity we did an analysis for brown pelicans here in California and we corrected for observer biases and other things that we knew might well affect those bird counts, not knowing how the results were going to turn out. Yes, there was a definite trend of decline for brown pelicans, parallel to actual census data. We picked another species that we knew had not declined (the Heermann's gull), and its numbers showed a steady trend. That was our control group. One more small piece of data to support a larger story. [A control in an experiment is the untreated sample which is used for comparison. ed.]

ER: How could cowbirds, grackles and starlings increase in the face of an onslaught of DDT?

DA: First off, most of those birds, given their food habits do not accumulate high-enough residues to cause problems. There have been very few demonstrated cases of eggshell thinning in most passerine birds, although there were some interesting case histories back during maximum DDT use where some species did show eggshell thinning. [Passerines are the largest order of birds, those that sing and perch. ed.]
     Eggshell thinning is one phenomenon out of many that probably occurred due to DDT and other insecticides. Different insecticides played different roles. The concern during Rachel Carson's time and later into the early 1960s was the direct poisoning of birds. People saw though in these early studies that some bird populations could bounce back from direct mortality pretty fast after the use of insecticides was curtailed. And much of it was. But other populations were not bouncing back. There was serious concern at one time back in the 1960s for the woodcock. Do you know that bird?

ER: No.

DA: The "timberdoodle". It's a terrestrial bird of the East related to the shorebirds. The entire continental population was severely threatened and it was not related only to DDT. And nobody ever looked at eggshell thinning, which may or may not have occurred. There were at least two issues involved in the decline of the woodcock. Wright published a paper that showed that the amount of DDT used was related to the age-ratios of the woodcock population in the fall. The more DDT used, the less young these birds produced. Nobody ever knew what the mechanism was there. It was thought that there were some reproductive problems.
     Then in the winter the woodcocks would migrate to the southeastern United States. And in the wintering grounds they were being exposed to heptachlor and the "drin" compounds -- some of the real hard organochlorines -- which outright killed them. [Drins are chlorinated pesticides like dieldrin, aldrin, endrin. ed.] A lot of very careful painstaking research was done by the biologists at the Patuxent Wildlife Research Center (led by a remarkable couple, Lucille and Bill Stickel) that documented experimentally what doses of pesticides and what levels were found in the brain tissue for diagnostic poisoning. They did toxicological work to supplement field observations.

ER: None of this was mentioned in Ray's book.

DA: And a lot more of this kind of back-and-forth lab-field work. I use this case history with my students to show how field data needs to be supplemented with laboratory data. First you develop hypotheses, usually from the field. Then you go back to the lab, or you set up a field experiment to test a specific hypothesis under more controlled conditions.  Sometimes it's feeding animals in cages, different levels - realistic levels of chemicals. And in other cases it may involve going out and looking at sprayed versus unsprayed areas.
     The authors of this book insult Paul Erlich and they insult Joe Hickey by turning his data against what he really was saying. Dr. Ray uses one small bit of information about the peregrine falcon: that it had been declining since 1890. In general that is true, peregrines had begun declining since 1890 because of losses in habitat and persecution by shooting.

ER: She said that peregrines had been declining since 1890, and the decline was more related to the availability of prey and nesting sites than it is to pesticides. [page 71 ed.]

DA: But there were still viable populations of peregrines all over the world until DDE came along.

ER: DDT pushed them closer to extinction?

DA: Yes, Hickey did a survey back in the 1940s that showed there was a large population of eastern peregrines; and then all of the sudden, boom, they were gone. Nobody really studied the cause of the decline of the peregrines in the East, but peregrines began to disappear all over the world, in many cases. Tom Cade, his group from Cornell, and others studied peregrines all over the North American continent. And there were British scientists and others who studied peregrines throughout the world. In most cases where there was a contamination problem, it was related to a contamination by DDE. Not in all cases. In some cases there was direct poisoning by dieldrin and some of the other hardcore organochlorine pesticides.
     So the way we look at it is that contaminants attacked the reproductive rates and the survival rates of wild animals. And when the population crashes occurred the fastest, both of these things were happening.
     Dr. Ray's point that the large-scale indiscriminate use of DDT and other pesticides may have been part of the problem is one thing I agree with. Indiscriminate use came about from the idea: if a little bit is a good, a little more is better. Agriculture has come a long way from those kinds of early philosophies. There have been many improvements in attitudes about safer uses of pesticides in recent years - for some, perhaps not enough improvement - but encouraging. Pesticide-decimated wild bird populations are on the upswing in many cases.
     I don't think anybody ever claimed that all bird population declines were caused by DDT, or even by chemicals. Nobody I know ever said that insecticides were the only cause of decline for bird populations. What ecologists need to do or try to do, is sort out as many population-decimating factors as possible so that managers can start eliminating those threats one-by-one.  

Copyright 1994 Environmental Review