The number of Monarch butterflies that reached wintering grounds in Mexico rebounded 69 percent this year from last year’s disastrous levels, the World Wildlife Fund said, but remained extremely low. Last year, the Monarchs covered only 1.65 acres, the smallest area since record-keeping began in 1993. At their peak in 1996, they covered more than 44.5 acres. This year, the butterflies rebounded to 2.79 acres, according to a formal census by Mexican environmental authorities and scientists released Tuesday. The Monarchs are suffering from the loss of milkweed habitat in the United States, illegal logging in Mexico and climate change. Each year, the butterflies migrate from Canada to Mexico and spend the winter in the same forests.