A literature review of the effects of pesticides on owls in North America showed that relatively few studies have been undertaken. Owls used in experiments seem as sensitive to organochlorine pesticides (OCs) as other birds of prey, but wild owls experienced few serious problems, primarily because they were exposed to lower residues in their predominately mammalian or invertebrate prey. For example, the great horned owl (Bubo virginianus) and the common barn-owl (Tyto alba) neither experienced marked changes in mortality or recruitment rates nor was there any evidence of population decreases even during the maximum period of OC pesticide use. Also, eggshell thinning was not a widespread problem. There were adverse effects on individual owls including verified records of 74 owls of six species that died from secondary or tertiary poisoning related to strychnine, organochlorines, anticholinesterases (antiChEs) and anticoagulants in 16 states within the U.S. and one province in Canada. Most of the pesticide-related deaths occurred during the 1980s, although this probably does not represent a true temporal distribution. Verified mortalities of owls probably represent a small fraction of the actual number that died from pesticides. Incidence of mortality seems biased geographically toward areas such as New York that have active ecotoxicological programs. Burrowing owl (Speotyto cunicularia) populations currently are decreasing throughout much of the range in the U.S. and Canada. Studies in Canada indicate that antiChE pesticides, particularly carbofuran, were responsible for the declines there.
