The sandbar itself snugs in against the cliffside with greenery growing in three distinct tiers. River cane and mature salt cedar stand 4.5 meters (15 feet) tall against the flagstones. And stairstepped in front of these are seepwillows—not a willow at all, but a kind of sunflower that pioneers sandbars—and a younger, shorter stand of salt cedar. The canebrakes fairly crackle with wintering birds: black phoebes, cardinals, brown-headed cowbirds, and a migrating yellow warbler, perhaps. There is plenty for them to eat in that thicket. You yourself discover a nursery of orange true bugs beautifully crossed with olive green, all crowded together in every stage of development on two or three willow leaves. But the most intriguing thing about that sandbar is the record left by its visitors: a lizard’s five-toed track with the long unbroken mark made by its tail, and the great blue heron’s left-and-right footprints striding along almost in a straight line. You find the cat-like tracks of the ringtail, the dog-like tracks of the gray fox, and the flat-footed print of the hog-nosed skunk, pear-shaped as a bear’s. At the water’s edge honey bees are collecting moisture to water-cool their hive. One by one they sip and lift off, making a beeline for a cliff. In the canyons where water flows from wall to wall, you find shore life restricted to those few plants and animals that can make a home on a cliff face. A spindly tamarisk has established a roothold in a thimble-sized deposit of soil just above waterline, and in cracks and crevices higher up, ocotillo and pricklypear are working down from the desert that tops the wall. Empty cliff swallow nests cluster on the undersides of overhangs. In spring you might see baby birds poking their heads from the colony’s doorways. Along the border, people call the Rio Grande by its Mexican name, _Rio Bravo del Norte_. Nowhere does the river seem more wild, more powerful than inside Santa Elena, Mariscal, and Boquillas Canyons. To enter one of these mighty limestone vaults is to understand why mankind has always had to skirt canyon country, and why to this day, except for its historic fords, the river is all but impassable. If you go in by boat the only way out is through. The adventure calls for preparation, knowledge, hardiness, and considerable skill. [Illustration: Cliff swallows colonize with as many as 50 nests in close order. How the adult birds pick out their own condominium from among such clusters remains a mystery.]