As darkness fell over the mountains of central Mexico on Jan. 11 earlier this year, a huge, moist weather system moved into the area.
Clear skies and low temperatures followed. The overnight lows Jan. 14 through 16 dropped into the low 20s. This unprecedented weather killed tens of millions of monarch butterflies wintering among the oyamel fir trees in a few isolated forest preserves.
The monarchs that you may have seen flitting through your back yard last September were almost certainly among the victims. As were the monarchs that arose from the caterpillars on your milkweed plants late last summer.
Normally the oyamel forests provide a cool, moist environment that protects wintering monarchs from severe winter weather.
But extreme variations in temperature and precipitation, though rare, do occur. And monarch butterflies, relatively small and fragile insects, can only withstand so much freezing rain.
What makes this story so compelling is that monarchs are one of nature's few migratory insects, and we've only known their winter whereabouts since the mid-1970s.
Each spring monarchs appear suddenly in south Texas and work their way northward all the way to Canada. Along the way, they mate, and females lay eggs on young, vigorous milkweeds. After laying a few hundred eggs, the females die.
When mature, the next generation continues the northward journey until a third or fourth generation appears. Those are the adult monarchs we are seeing right now.
And those are the monarchs you might see fluttering southward through a football stadium on a crisp autumn afternoon.
How these butterflies find their way nearly 2,000 miles to the wintering grounds in Mexico remains a tantalizing mystery.
Whether they use a sun compass, geomagnetic forces, features of the landscape, or somehow smell their way south, it's a most remarkable journey because they've never been there before.
So when word of the January die-off spread, the big concern was how it would affect this fall's population. The good news is that its impact seems to have been minimal.
In fact, according to Monarch Watch, a group dedicated to monarch conservation, monarch numbers this spring exceeded the numbers reported during the same period in each of the previous four years.
But just as important as winter survival and early spring returns are the environmental conditions for reproduction that greet northbound monarchs.
A surging population of fire ants, which eat monarch eggs and caterpillars, met the first wave of monarchs that arrived in Texas.
Then much of the Midwest was cold and rainy in May, and later, drought conditions developed. As a result, monarchs were as much as three weeks late arriving in northern states and Canadian provinces.
It's difficult to predict how many monarchs will return to the wintering grounds in Mexico, but Dr. Chip Taylor of MonarchWatch suggests that the population will recover to about half its nine-year average.
Though this might sound discouraging, such a rebound would constitute an impressive recovery from the January die-off.
My own limited observations offer additional encouragement. As I've walked the woods, fields and country roads in recent weeks, I've searched for monarch caterpillars in every patch of milkweed.
And more often than not, I've found two, three, or six distinctive fleshy green, white, and black-ringed caterpillars. Maybe I've never paid so much attention before, but I've never seen so many monarch caterpillars at the end of summer.
For more information about monarchs, visit MonarchWatch at www.monarchwatch.org. And for detailed information on a variety of migratory wildlife, visit www.learner.org/jnorth/.
Though any curious naturalist will enjoy these sites, teachers incorporating any aspect of migration into their lesson plans will find them invaluable.
NOTE: Though the electronic media seem bent on "commemorating" the anniversary of Sept. 11 with formal ceremonies, perhaps we should use that day to rethink our own priorities. In the aftermath of that terrible day, we learned together the intrinsic psychic value of nature. We sought refuge in the sanctuary of the outdoors, and it soothed our souls. Maybe that's how we should remember 9/11 -- take a hike, catch a fish, watch a butterfly.