The head of Mexico's nature reserves, Luis Fueyo, said there are still some problem to be solved at the wintering grounds in Mexico, including some scale-logging and water availability. The Monarchs don't drink any water throughout their long migration until the reach Mexico, and the mountain streams in the area have been affected by drought and human use.
The migration is an inherited trait. No butterfly lives to make the round-trip. The millions of Monarchs cluster so densely on tree boughs in the reserve that researchers don't count their individual numbers but rather measure the amount of forest they cover.
This winter, the butterflies covered just 2.93 acres (1.19 hectares), down from 7.14 acres (2.89 hectares) last year.
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