I just finished reading the Fall 2014 issue of the DNR’s Wildlife Diversity Newsletter in which biologist Bruce Ehresman wrote an article entitled “What’s Happening With Iowa’s Barn Owls?” It brought back nostalgic memories of my own encounter with Barn Owls more than 40 years ago. It was in the early years of my career as a research biologist with the U.S. Fish and Wildlife Service at the Patuxent Wildlife Research Center in Laurel, Maryland. I was conducting studies on the effects of DDT and other organochlorine pesticides on Barn Owls. The Chesapeake Bay was dotted with numerous off-shore blinds built by duck hunters. These were often elaborate structures consisting of a large wooden rectangular box mounted on stilts in shallow water. A partial roof projected over a bench for 3 to 4 hunters to sit inside. A rough lean-to on the blind provided a place to hide a boat. The outside of the structure was concealed with brushy vegetation. During the non-hunting season, Ospreys nested on the roof of the blind, Barn Owls nested inside the blind under the seat, and blackbirds nested in the vegetation on the outside.
Another biologist at the center where I worked was studying pesticide effects on the Osprey and I was studying the Barn Owl. We regularly visited the blinds to collect a sample of eggs of each of these species. The eggs were returned to the lab where we measured the thickness of the shells and removed the contents to analyze for pesticides. Statistical correlations showed that the presence of pesticides, especially DDT, was highly correlated with a reduction in the thickness of the eggshells and reduced reproductive success.
Barn Owls, like most owls, daily regurgitate hair, bone, feathers and other indigestible parts of their food in the form of pellets. I collected and dissected pellets from each of these sites and I was able to determine what each pair of owls had been eating and feeding to their young. I found that about 20 percent of the owls were feeding on small birds; the other 80 percent were feeding on mice. The owls that were feeding on birds had thin eggshells and elevated amounts of DDE, a metabolite of DDT. Other studies had shown that birds convert DDT in their food to DDE and store it in their fat tissue. During egg formation, the DDE in the fat is mobilized into the blood stream and interferes with calcium transfer to the eggshell. Mammals, on the other hand, convert most of the DDT to DDA, a water soluble form that can be excreted. Chemical analysis of the egg yolks from the thin-shelled eggs revealed that they contained several different pesticides including DDE and dieldrin, metabolites of DDT and Aldrin, two organochlorine pesticides in widespread use at the time.
So, which of these pesticides caused eggshell thinning and impaired reproduction? That question was worth answering but it required an experiment. But, that’s a story for another day.
Barn Owls in Iowa were once very common but they nearly disappeared from the state during the DDT-Dieldrin era of the 1950s and 1960s. Although we cannot know for sure that pesticides were responsible for the disappearance of this species in the state, I am pleased to learn from Bruce Ehresman’s article that the Barn Owl may be making a comeback. Shortly after I moved to Iowa in 1975, the Iowa DNR started raising barn owls in captivity to enable their re-introduction. The first releases were tagged with radio transmitters to follow their survival. It was quickly learned that the released Barn Owls were easy prey for Great Horned Owls. Thus, the recovery program was discontinued. Forty years later it seems Barn Owls are making slow progress on their own.