Mexico City - The monarch butterflies that winter in the mountains west of Mexico City survived the severe cold snap that hit the area last week, authorities said Friday.

Mexico's environmental protection agency released photos of patches of frost, snow and ice in parts of the butterfly reserve. Activists had expressed concern about the unusual cold snap because driving rain and bitter cold in 2001 killed millions of monarchs wintering at the reserve.

But the agency also released photos showing clumps of butterflies still hanging from trees. They tend to drop off trees when they're frozen.

The agency said its employees observed "branches and trunks covered with butterflies in good condition."

"The monarchs held up under the snowfall," it said.

The butterflies are starting the process of returning on their annual migration to the U.S. and Canada.

Authorities said in February that the monarchs have made a big comeback after suffering serious declines. The area covered by the orange-and-black insects this season was more than three and a half times greater than the previous winter. The butterflies clump so densely in the pine and fir forests they are counted by the area they cover rather than by individual insects.

The number of monarchs making the 3,400-mile (5,500-kilometer) migration from the United States and Canada declined steadily in recent years before recovering in 2014. This winter was even better. This December, the butterflies covered 10 acres (about 4 hectares), compared to 2.8 acres (1.13 hectares) in 2014 and a record low of 1.66 acres (0.67 hectares) in 2013.

Illegal logging remains a problem Mexico's monarch wintering grounds. It more than tripled in 2014, reversing several years of steady improvements. Illegal logging had fallen to almost zero in 2012. Authorities said the reserve's buffer area lost more than 22 acres (9 hectares) in 2015 due to illegal logging in one area, but said the tree cutting was detected and several arrests were made.

The forest canopy acts as a blanket against the cold for butterflies.