Frozen is a recent Disney flick.....................Norway maples.............Japanese knotgrasses.............the invasive illuminati..........

So, yes, the birds made the sanctuary a stop on their way north in the spring or south in the fall. But the trees and plants that settled in were the wrong kind: Norway maples, black cherries, Japanese knotgrasses — invasives all.

“Wisteria was a huge problem, too,” Mr. Blonsky said. “In a woodland, it will strangle everything, and that’s what was happening here.” The sanctuary, on a big boulder made of Manhattan schist, was covered with wisteria, he said. Leading the way to the sanctuary on Wednesday, he said some root systems remain embedded in the creases in the rock, despite parks workers’ efforts to weed them out.