(Credit: www. Monarch-butterfly.info)
Last September, Sanctuary visitors witnessed orange clouds of monarch butterflies fluttering along the Railroad Bed Trail near the Patuxent River. In the late afternoon, monarchs rested on twigs, leaves, and tree branches on the southern, shaded side of the trail. If disturbed, they would fly, then light again, open and close their wings several times, and return to stillness.
(Credit: www.wunderground.com)
A line of trees jutting out into the marsh provides a perfect resting place for monarchs on their migratory path. The butterflies often migrate on cloudy days, stopping along the way for an average of two days. Maybe the monarchs here were waiting for another cloudy day to continue their migration, while feeding on nectar plants to build up their energy reserves. Red-winged Blackbirds winging over the marsh presented no danger, because monarchs are a highly toxic meal. The fat, yellow and black-striped caterpillars consume plants in the milkweed family, which contain the toxin cardenolide. How free it must be to migrate without the threat of being eaten. But there are other challenges to survival. Monarchs only have to survive flying thousands of miles through uncertain weather.
Credit: Monarch Watch
Sensing shorter days, fluctuating temperatures, and host plants becoming dry and brittle, the last monarchs to metamorphose into adults in the summer go into reproductive diapause—a state of suspended reproductive development—and prepare to migrate. By winter, adult monarchs hailing from a 100-million hectare area in the Midwest and eastern United States have converged on a tiny 20-hectare area in the mountains of Central Mexico (One hectare = 2.5 acres). Flying as many as 2,500 miles, they land in the oyamel fir forests on steep mountain slopes. The oyamel forest is a relic from a time when the earth was cooler and wetter. As the earth warmed, the forest retreated up the mountainsides, to an elevation of nearly 3,000 meters to retain the cool (3 to 18 degrees C), moist climate to which the trees are adapted.
How do they navigate? Theories abound. Monarch movement is likened to that of other animals, who may depend upon celestial cues, earth’s magnetic field, polarized light, infrared perception, landscape features such as mountain ranges and river valleys.
A monarch butterfly feeds
on nectar plants in the butterfly garden
at the Glendening Nature Preserve
Monarchs eat little during the winter. Many mate before the spring migration. The migration takes place in two stages, in a leap-frog fashion. Monarchs overwintering in Mexico re-populate the southern U.S., and their offspring then complete the journey to the northern U.S. and southern Canada.
From the Railroad Bed Trail, we could hear country music twanging from a fisherman’s radio on the river. We evolutionary latecomers have little over these butterflies. Though we’ve designed jetliners that can fly to Mexico in a few hours, none of us could make it to that destination on our own fuel, by flapping our appendages, unprotected by an armor of steel.