Seventy percent of southern Ontario's original wetlands have been converted to other uses, particularly agriculture. At Montezuma N.W.R. in New York State, one thousand pairs of black terns nested in the late 1950s, but less than 10 pairs are now present. Several studies have detected pesticides in black tern eggs and organochlorines are linked to low reproductive success. It is suspected that many of the contaminants may be accumulated while terns are migrating on their wintering ground. Weather, predation, diseases, and parasites affect black tern populations, but are not new threats.
Human disturbance is a factor.
As with all tern species, incubating or brooding black terns will leave the nest at the approach of people. Repeated or prolonged disturbance may increase the likelihood of abandonment, predation, or exposure to inclement weather. People viewing black terns or travelling near a nesting colony should keep at a distance so the birds are not disturbed. Terns are birds of great energy and mystery. They make great journeys to southern continents.