Threads from the Web of Life: Stories in Natural History

Living on the Edge of Springtime

100 Living on the Edge of Springtime THE Aztec Hummingbirds spend their lives confined behind an impassable boundary. It is not an obvious feature of the terrain—a bottomless canyon or the margin of a wide desert or the treeline drawn across the top of a mountain range. Nor is it a virtual grid of longitude and latitude lines or isolines of elevation like those that circumscribe topo maps. The hummers cross all of those with ease. The boundary that restrains the hummingbirds is not fixed but moves freely before them across the landscape with the ebb and flow of the season. It cannot be drawn as black grids or green contour lines—it is traced instead in white and gold arcs of flowers. It flows northward across the fields as the days lengthen—rising in elevation over the slopes to the east and west—following the advance of the springtide. The Aztec Hummingbirds are fair-weather creatures, governed by the seasons. They flee before the approach of fall across the Pacific Northwest, toward the milder winters of the Mexican Sonora. As they fly, they show the same battle colors, the red plumage, the burnished gold reflecting the sun’s highlight, as did the Aztec warriors who once shared their winter range. With the return of the spring, the hummingbirds’ reproductive strategy compels them to abandon the warmer lands and escape the snakes, ants, and birds there that prey on small nests. Parting company with the local tropical hummers, the Aztec Hummingbirds return thousands of miles to the north. Their destination is a very different place—a habitat that suffers a winter bereft of flowers but one that also supports far fewer resident dangers to newborn hummingbirds. The Aztec Hummers are no bigger than the hawk moths that co-occupy their territory. The two similar animals are separated Living on the Edge of Springtime 101 only by the boundary between night and day. But while the moth’s body temperature rises and falls with dawn and dusk, the hummingbirds maintain a constant temperature of 108º F from noon to midnight and back. Their hearts beat a thousand times a minute. They can sustain day-long activity all year, allowing them to pursue seasonal advantages across both winter and summer ranges. However, the metabolism rate that makes this sustained activity possible, the highest of any creature, combined with their diminutive size, leaves them unable to carry fuel enough to support them over the length of their seasonal trek. They live every minute of their lives a few hours from starvation, so they are constrained to migrate along a route that offers nectar every hour of every day. Thus, they follow the sweet scent of the vernal highway. They leave the Sonora long before the distant northern sea coast feels any signs of winter’s departure. They begin by flying unerringly through the narrow, unpredictable window in time when the southwestern desert briefly softens under carpets of wildflowers. Their iridescence awakens valleys sheltered in the lee of buttes and mesas whose summits are still in the grip of March’s arctic winds. Overall, the bird’s route is as much sideways as north. The discovery of a glade crimson with Penstemon may persuade an Aztec Hummer to tarry for days and pollinate the whole lot. Solitary migrants, they can afford to rest among branches thick with redbud blossoms until the flying weather improves; it may be a month or more before the edge of springtime arrives at their ultimate destination. The resting hummers bathe on the wing, dipping from hover into shallow rocky pools beside seasonal spring-fed streams. They bide their time, waiting for the floral flyway to open, sitting out the chill or the late storm before resuming their northerly advance. THE spring cross-quarter day, halfway between the equinox and the solstice, finds the boundary line of flowers gliding rapidly north, an indigo band of Ceanothus and Brodiaea surging across the valley floors, creeping up the mountain walls to the east and west. As it undulates out of California across the divide between the Sierra and the Cascades, the moving line 102 Threads from the Web of Life smoothly fades from the blues of lupines to the pinks of phlox and twinflower. Not only the hummers but many families of migrant fliers move along just behind this floral front across the terrain. Each is constrained by its...