Revisiting eggshells, DDT, collections, and study design

On tours of our collections, we want to give visitors an appreciation for the important diversity of ways in which collections can and have been used to do important science and conservation. One of the best known and frequently recounted examples involves 1960's pesticide use in relation to declines in birds of prey including Peregrine Falcons (Falco peregrinus). This story is based on research that is often described with a focus on Peregrines. However, a re-examination shows the way we have told this story actually under-represents the value and breadth of what these collections provided scientists and therefore society (and biodiversity) in general with respect to eggshell thinning.


Field Museum Peregrine Falcon egg sets
In their important 1968 study linking DTT use to reduction in egg shell thickness, Hickey and Anderson used Field Museum Peregrine Falcon eggs from British Columbia, Canada as important "control" samples from areas where DDT was not heavily used.  In eggs from these these regions, eggshell thinning was not observed. 

In October 1968, Joseph Hickey and Daniel Anderson published a seminal paper in the journal Science entitled: "Chlorinated Hydrocarbons and Eggshell Changes in Raptorial and Fish-Eating Birds." The paper has been cited more than 500 times and it was a significant piece of the scientific argument used to convince the U.S. government to ban use of the pesticide DDT to protect declining populations of some birds of prey, thus initiating one on the most successful conservation success stories in the country's history: the recovery of several of our most iconic avian predatory birds.

Looking back on this paper is interesting with respect to specimen collections and how they can be used. These researchers examined egg sets in many different collections for more species than is generally remembered; however, they do not report which collections were actually examined (and the editor did not see that as an issue at the time). Through the years we, as curators and collections staff, have emphasized that our collections were used to document that eggshell thinning had happened in Peregrines (and other species). The basic story is that Hickey and Anderson used collections including ours at The Field Museum (FMNH) to demonstrate that eggs being laid by Peregrines in the 1960's were demonstrably thinner, as a result of pesticide use. This is what Hickey and Anderson demonstrated; however, a rereading of their paper made me appreciate that they did this with a more comprehensive scientific design than I had previously recognized, and they were able to do this because of the breadth of material in our collections, and others across the country. In short, the way we had been describing how our collections had been used really told only part of the important scientific argument these authors laid out in 1968 using eggs sets in museum collections.

An examination of their first table of data shows that Hickey and Anderson looked at temporal changes in eggshell thickness in multiple species, and they studied samples of these species from mutliple geographic regions. I could look at the information for species and geographic region they studied in relation to our egg holdings. I realized a number of things even though the data on exactly which eggs sets they studied are not included in their publication. First, I could ascertain was that none of our FMNH Peregrine Falcon eggs were part of their data sets showing eggshell thinning (because the authors focused on California samples in this species for this aspect of the study and we have no Peregrine egg sets from California). On the other hand, FMNH Osprey (Pandion haliaetus) egg sets from the Eastern seaboard (New Jersey) and Bald Eagle (Haliaeetus leucocephalus) sets from Florida were an important part of their data sets documenting eggshell thinning in these species in these geographic regions.

As I said, the study design Hickey and Anderson developed was much more involved than just looking for eggshell thinning in regions where they thought it was happening due to DTT. They also gathered control data from regions where DDT was thought to not be an environmental issue; they also sought data for species from regions affected by DDT, but that would not show signs of eggshell thinning because they were part of the food chains where DDT would not expected to accumulate to the same level. These were predatory birds eating terrestrial mammals.

Thus, FMNH Peregrine Falcon eggs from the Queen Charlotte Islands in British Colombia, Canada were part of their Peregrine Falcon sampling from a region where DDT was not expected to have caused eggshell thinning (and significant change was not observed). FMNH egg sets for "control" species that would have been measured include specific Osprey populations. Among the taxa measured as "controls" were Red-tailed Hawk (Buteo jamaicensis), Golden Eagle (Aquila chrysaetos), and Great Horned Owl (Bubo viginianus) from California. These samples did not show thinning when data from historical sampling were compared to samples taken in the 1960s. Thus, our excellent collections of egg sets made from 1890-1920 from different species and different regions provided critical data to demonstrate that certain species (Peregrine, Osprey, Bald Eagle) that were feeding more on aquatic prey were being affected, and this was further associated with regions where DDT was being more widely used. Thus, this study highlights well how valuable series of specimens from different time periods and from different geographical regions can provide critical data about ecological and evolutionary change well after they were originally collected. The original collectors of these eggs had no idea they would be used this way at the time they were collected, but by virtue of these collectors' efforts, their data collection, and the curation of these specimens in an accessible natural history collection, a number of iconic North American birds have recovered from the brink of extinction.