The Shark and the Jellyfish: More Stories in Natural History

Focal Point

154 Focal Point Two birds approach each other along the chill, shaded bole of a great old Douglas fir—one walking straight up, one walking straight down. The white-breasted nuthatch grapples noisily with the bark, arresting his headlong plunge with every step down the cylindrical surface. His talons and upturned bill dislodge a small shower of duff and bark chips as he goes. He watches these particles float away before him, his quizzical eyes hidden in black stripes set off from his black cap by white eyebrow lines. The brown creeper is less obvious—camouflaged in dark, stippled plumage—quietly making her way upward. With her stiff tail pressed against the bark behind her, she scuttles from side to side, avoiding the largest of the particles falling from above. Then she stops to search for food—the spiders, mites, and pupae that can best be seen from underneath . From his opposing perspective, the nuthatch encounters a menu of different insect fare—prey species with alternate concealment strategies. Thus, the two birds do not compete directly but coexist in the vertical dimension they share. They pass each other on opposite sides of what appears to be a slim, raised flake of bark. Each bird glances at the low wedge but then quickly looks over it to regard the other bird. They pass closer than either would allow a member of its own species to approach, then continue on their ways—one up, one down. For an instant, both birds had considered whether this raised flake might be breakfast. It was textured in the concentric whorls of annual growth layers in bark—a pattern commonly employed as camouflage by moths and other insects that rest exposed on tree trunks by day. First light in the boreal forest finds these insects paralyzed by the cold, unable to fly, dependent for their survival on escaping notice behind their mock-wood texturing. But neither bird had observed the aerodynamic curve of a lepidopteran wing—there was none there to be seen. Instead, each bird’s quick inspection had revealed a silver crescent standing out clearly on the irregular dark background. Their eyes were drawn to this shiny focal point, which had the reflective appearance of a slime mold or other viscid, shapeless white fun- 155 gus conforming to the bottom of a recess in dead bark. Such wet, inedible spots are commonly encountered by these birds, who have become inured to the sight of them. Both foragers quickly scanned the white shape standing out against the darker surface, and then their searches moved on. As the two prospectors diverge, their roles invert. Anything dislodged by the creeper would now fall on the nuthatch, who is facing the wrong direction to see things coming toward him from above. He bends his neck until his bill points straight out from the trunk and cocks his head to look back up. But the creeper is light footed—too careful a climber to create a rain of debris. When the nuthatch reaches the base of this tree, he flies to the crown of the next. There he pauses to sing—his call sounding like notes blown on a White C. A shiny mark punctuates the cryptic coloration of the anglewing’s underwing. 156 toy soldier’s tiny tin bugle. But the forest is still; no other nuthatch returns his call, so he begins his next descent. The creeper continues less obtrusively to the treetops. From there she plunges through the strata of limbs to the base of another great trunk to continue her perpetually uphill trek. The little bird will walk almost a mile straight up before the morning is over. The two feathered hunters move off through the forest, and quiet returns to the trees. After a while the first slanting rays of sunlight find the slim, raised flake of bark decorated with the silver crescent. Responding to the warmth, the dark form begins to expand, antennae slowly erecting from their stowed position. Paired forewings rise from between the folded hind wings, unveiling wing margins deeply indented with irregular notches. This jagged edge, which gives butterflies in this genus the name anglewing , shows none of the smooth profile that birds associate with the wings of butterflies and moths. Anglewings are found in all the world’s conifer forests. Each species bears a distinctive silver chevron on the lower hind wing. In some, the imprinted emblem...