Four Wings and a Prayer Read Free Page B

caught up with the monarchs, but we couldn’t follow them. There was no road where they were going, so we moved on, scanning the sky, focusing on the foreground and the middle distance, but we saw none. We had lost them.
    More than that sense of loss, I don’t remember what I was thinking. We were coming to a town, Tula, where I hoped to find a phone to call home, so maybe it was that. But suddenly the brakes were on and we were making a dusty U-turn, and Bill Calvert was pointing to a stand of willows ten yards from the road, encircling a muddy pond. “Whoa,” he said. “There’s a roost. Probably two thousand monarchs in there.” We picked our way over a barbed-wire fence for a better look, passing a great blue heron arrayed in its winter whites, and paused to admire a vermilion flycatcher, a bird so radiant I had to fight the urge to squint. Compared to this, the monarchs’ orange and black markings, and especially their dried-leaf appearance when their wings were folded as they roosted, might have seemed drab. But the monarch itself did not. I had sometimes heard lepidopterists refer to the monarch’s charisma, to its character, and as I stood in that swamp, looking up at the monarchs resting in the branches overhead, I knew exactly what they meant. These less-than-a-gram creatures had flown, most of them, nearly two thousand miles. Theyhad almost made it. They seemed … admirable. Bill Calvert got out his equipment: a ruler, glassine envelopes, a digital balance, a tattered net, extension poles, duct tape. His subjects had arrived: he was going to do science.
    W HEN BIRDS MIGRATE , they do so primarily because of food. Winter comes, and mosquitoes and berries and other food sources dwindle or become less accessible. Birds fly south, and the landscape becomes one big commissary. This is oversimplified, of course, but even schematically, what birds do is nothing like what butterflies do. Monarchs do not leave their northern breeding grounds because the flowers have withered. They leave for the same reason the flowers wither: the climate changes. The monarch butterfly, which is, genetically, a tropical species, cannot survive sub-freezing temperatures. And when monarchs are wet, they are even more vulnerable. If they are going to reproduce, they have to move to a more hospitable place—or, as is really the case, a less inhospitable place. At ten thousand feet, the Neovolcanics are not the Bahamas for butterflies; the overwintering sites are not warm. Rather, they have the right microclimate for monarch survival, warm enough so the monarchs don’t freeze and cool enough so they don’t drain their finite supply of energy, the lipids stored in their bellies. Monarchs spend an average of 135 days at the overwintering colonies, days of entropy when food may be sought but is not much available.
    But they need food; they need energy, both to fly long distances and to survive the winter. Intuitively one might expect the butterflies to bulk up in the north, the way we might fillup the gas tank before driving cross-country. The problem is that a loaded gullet may actually require more energy to transport. And it may cause drag. So the question of when a monarch obtains its winter food supply is an important one, both because it may suggest how the butterflies find their way to Mexico (do they, for instance, follow the asters and the black-eyed Susans?) and because it may have implications for conservation (what happens if wildflowers are replaced by roads or subdivisions or wheat fields?). Besides, it is just plain interesting: science for science’ sake.
    This last, more than the others, appealed to Bill Calvert: the questions, one begetting another—no; begetting many others. We would drive, and I would ask Calvert, who has devoted his life to studying monarch butterflies, how high monarchs flew, and if they followed corridors of wildflowers when they headed south, and if predation was greater during migration or