Cold mornings limit the ability of the monarch to fly to sources of water and water became increasingly difficult for the butterflies to find as the winter progressed. At El Rosario, the lack of water contributed to an unusual redistribution of the monarchs late in the winter. In late February and March, a large portion of the colony moved downhill to a source of moisture and trees on the property of the adjacent Angangueo ejido. Much of the tourist traffic destined for El Rosario was diverted to Angangueo to the consternation of many of the El Rosario guides and vendors. This appears to be the first time in memory that the colony has resettled in Angangueo.
Winter can be a stressful period and the monarchs that survive are often in poor condition.
The condition of the Monarchs at the end of winter probably determines their ability to remigrate in the spring. We don't know. When I visited El Rosario in mid February, the Monarchs seemed to be in good condition. The wings did not seem to be especially worn and the butterflies did not appear to be thin as they do when the fat body is depleted. David Marriott of the Monarch Program, who had a longer opportunity to evaluate the condition of the population in late February and early March, reported that the monarchs appeared to be in poor condition.