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. After leaving the overwintering sites, the Monarchs zigzag 600 miles of more through the mountains of Mexico before they reach Texas. Although there is some egg laying on the milkweeds they encounter in the mountains, it is not clear whether offspring reared in Mexico subsequently move northward.
Monarchs are rare or absent from most of northern Mexico during the summer months.
This spring the mountains were unusually dry but whether this limited the availability of nectar for the northward migrants is not clear. Even under dry conditions many herbaceous and shrubby plants bloom during the last two weeks of March when the monarchs are moving through the area.