The gray squirrel, introduced into local woodlands ten years ago, threatens the indigenous population of an endangered owl species, because the squirrels' habitual stripping of tree bark destroys the trees in which the owls nest. Some local officials have advocated setting out poison for the gray squirrels. The officials argue that this measure, while eliminating the squirrels, would pose no threat to the owl population, since the poison would be placed in containers accessible only to squirrels and other rodents. Which one of the following, if true, most calls into question the officials' argument? ['The owls whose nesting sites are currently being destroyed by the gray squirrels feed primarily on rodents.', "The officials' plan entails adding the poison to food sources that are usually eaten by rodents but not by other animals.", 'No indigenous population of any other bird species apart from the endangered owls is threatened by the gray squirrels.', 'The owls that are threatened build their nests in the tops of trees, but the gray squirrels strip away bark from the trunks.']