Written by our Board Vice President, Erv Klaas

My field study of Barn Owls on the Chesapeake Bay was published in Volume 1, Number 1 of the journal Estuaries in 1978. I found that eggshell thinning and residues of Dieldrin and DDE in the eggs were correlated. I then decided to find out whether it was Dieldrin, DDE or the two chemicals in combination that caused the eggshells to be thin.

It so happened that we had rescued several Barn Owls that had been injured by Ospreys over the course of studies. These owls were kept in outdoor aviaries at the research center where I worked and they began to produce young. The captive flock consisted of about 20 pairs when I became their care taker. In 1973, I took over a large outdoor aviary with 64 adjoining flight pens, and began to enlarge the flock of captive breeders. I mounted large wooden nest boxes in each pen with a small door that could be opened from the outside. A roost was placed on the end of the pen opposite the nest box. A food tray was placed next to the nest box. The pens were 30 feet long, 15 feet wide and 6 feet high. Given ample food, Barn Owls will produce two or three broods of 7-9 young in a season. By the second year, I had enough owls (60 pairs) to begin an experiment.owl

Putting small quantities (1 to 3 ppm) of chemicals into their food was the first problem that needed to be solved. I had determined that adult owls could be maintained on ground horse meat, sold commercially to zoos. However, the adults could not feed the horse meat to their young. In an attempt to simulate a mouse, I made the horse meat into sausages but that didn’t work. I decided to conduct the experiment by mixing the chemicals into the horse meat and feeding it to the adult owls. As soon as each clutch of eggs began to hatch, I changed their diet to laboratory mice. Hundreds of mice were obtained live each week from laboratories in the Washington, D. C. area, brought to the center, killed humanely in a gas chamber specially designed for this purpose, frozen and stored in a large walk-in freezer.

I learned something about owls feeding behavior. They preferred to eat mice rather than horse meat and they preferred to eat the heads of the mice and discard the bodies unless food was in short supply. The discarded bodies accumulated on the floor of the pen and became a stinking mess as well as a health hazard to the owls. It did produce a thriving colony of carrion beetles.   I reduced the number of mice fed each day to two mice per owl and that solved the problem.

The pens were constructed of wooden frames and chicken wire with dirt floors. During the growing season, vegetation grew inside the pens. Vines grew on the walls and roofs of the pens and each Fall, the owls had to be moved to temporary quarters while my technician and I mowed the weeds and removed the vines from the walls and roof. Removing the vines was especially important because this area often experienced freezing rain and occasional snow storms that could cause the pens to collapse.

The 60 pairs were divided into four groups of 15 pairs in each group. One group was fed 1 ppm Dieldrin, one group 3 ppm DDE, one group both DDE and Dieldrin, and a control group was fed no pesticide. Barn Owls lay an egg every other day until they have a complete clutch of 7 to 9 eggs. The female begins incubating immediately after the first egg is laid and the young hatch in the same order after about 28 days. I collected the third egg of each clutch for chemical analysis and measurement of the shell thickness.

After two years, the experiment was terminated and the results analyzed. The control group had no reduction in eggshell thickness. Eggshell thickness in the DDE group was reduced by 20 percent, enough to cause breakage of most of the eggs during incubation. Eggshell thickness in the Dieldrin group was reduced about 2 percent, and the combination group was reduced 22 percent. This was the first experiment to show that DDE alone was the primary cause of eggshell thinning in a wild bird.