Nine monarch butterflies released by the Toledo Zoo have been found in central Mexico.
The zoo says the discovery supports its belief that its captive-breeding program works.
Monarch researchers have said mass releases may lead to disease and accelerate the butterfly's decline. But zoo officials say dozens of monarchs released last fall likely survived.
Experts said last month the butterflies have made a big comeback in their Mexico wintering grounds after suffering serious declines.
The area covered by the orange-and-black insects in the mountains west of Mexico City this season was more than three and a half times greater than last winter.
The number of monarchs making the 3,400-mile migration from the United States and Canada steadily declined in recent years before recovering in 2014.