The Shark and the Jellyfish: More Stories in Natural History

Life in the Sky

125 Life in the Sky The sea is tinted green with plankton, streaked vermilion with floating krill, crossed beneath the surface with silver flashes of herring, and shot through with the plunging forms of blue-footed boobies—a feeding frenzy is churning the surface. Above it all, currents of warm air rise off the tropical ocean. The commotion summons an apparition—a black silhouette from the haze on the far horizon. It is the quintessential embodiment of this water world—the magnificent frigatebird. Its effortless transition across the seascape is proof to sailors that they have reached the tropics. Distant shores move past it, but this scissor-tailed kite approaches motionless—a shadow changing only in perspective while the open ocean slides away behind. While the winds aloft buffet everything else, the frigatebirds sail serene, spinning the same spirals through the sky that the most accomplished high-flying eagles and condors follow above the coastal mountains. But the signature bent-wing figure of the frigate is not found over mountains, or far inland, or anywhere far from the low latitudes—the frigatebirds fly only over tropical oceans, where the waves are warm enough to generate the thermal updrafts on which they soar. Only the albatrosses have longer sea-gliding wings, but their flight paths tend toward the stormy latitudes. They do not soar on thermals—thermals do not rise from their colder oceans. They slice across tempestuous seas only a few feet above the water, cutting through troughs, slipping over the whitecaps, slope-soaring along the faces of the waves while nothing soars farther above them. The instantly recognizable figure of the frigatebird is unique to the tropics , sculpted by the convergence of circumstances there. These gliders lock their wings in position and hold them steady for hours; they use little energy and generate little muscular heat. The tropical warmth helps maintain their body temperature when their lightweight, high surface-to-volume form loses heat to an afternoon thundershower. But the warm tropical water works against them—precluding the production of the abundance of fish found at the albatross latitudes, where oxygen dissolves to greater 126 levels. The frigates’ gliding way of life is an adaptation to this shortage of food—it optimizes the bird’s capacity to wander far and wide. Frigates climb thousands of feet to scan the curved oceanic horizon from the vantage of their lazy circles—looking for concentrations of prey that rise into view from tropical seas only rarely. These birds have taken their specialization in soaring to its aerodynamic extremes. They have the lowest wing-loading ratio of any gliding bird—a surfeit of surface area in proportion to their weight. Their feathers are dry and light—not oiled enough to repel seawater and allow them to float—so they never dive into the waves or even alight upon the surface. The exaggerated aspect ratio of their wings—narrow yet more than seven feet long—maximizes the speed they can maintain when gliding at their minimum sink rate. Their sharp wingtips direct the drag generated by their motion far out and way from the main portion of the wing. They slide through the sky straight and flat, yet they bank easily into updrafts. They are one of the few birds that can survive a hurricane by riding it out on the wing. Frigates may follow schools of porpoise or tuna, watching for smaller fish to be driven to the surface. These birds cannot enter the feeding frenzy directly—they live along the margins. A needlefish that shoots from the water might confound its pursuers below, but where it skips along the surface , sculling with its tail, it exposes itself to the frigatebird. The black raptor drops from above on half-closed wings to pick off any airborne morsel that enters its dominion. With the grace of a kestrel, the frigate times the trajectory of the flying squid, or the glide path of the flying fish, and snatches away its prey just before it reenters the water. Only its hooked beak gets wet when the bird skims the waves to pluck a catch from close beneath the waterline. Thus, the herring that feed on floating krill will dash up for an attack, strike, and jackknife to dash back down all in the same motion, minimizing their exposure to the skimming fisher. The boobies are the aerodynamic opposites of the frigatebirds. Their compact bodies are stout enough to endure the pounding of repeated headlong crashes into...