The Double-Crested Cormorant: Symbol of Ecological Conflict

4. A Time to Sink and Swim

4  A Time to Sink and Swim when a species manages to occupy a new ecological niche the adjustment signi‹es that the organism has undergone a number of physical and behavioral adaptations to permit it to do so. But when a species makes the quantum leap to inhabit an entirely new medium, it’s a sign that huge changes have taken place in the species’ behavior patterns and throughout its body. Simple plants in the sea adapted to conditions ashore and gained a foothold on land as the result of long-term mutations. When marine animals ‹rst stepped onto sandy beaches or climbed onto rocky ledges they changed the way they lived, how they moved, how they breathed, how they gathered food, and what they ate. These transformations took ages to accumulate and continued to build on each other through the immense power of natural selection. As in the cormorant’s ancestor’s leap into the air, its plunge beneath the water’s surface built into the bird’s constitution a greater genetic propensity for ›exibility. It con‹rmed the cormorant’s presence as an active predator, not a scavenger . But in some ways this same ›exibility forced the cormorant into a set of rigidly ‹xed behavior patterns: with very few exceptions it eats only ‹sh. To feed on ‹sh it must live near water. And as we’ll see, the double-crested cormorant’s diving range is limited, so it must hunt along shallow shores, preferably those of large bodies of water to ensure a constant variety and supply of prey. Adding the subsurface world to its dining options changed the cormorant into the bird we see today. Cormorants catch their prey using a method known as pursuit div41 ing. Pursuit-dive foraging is a widespread practice in the avian world, and as many as 150 species across the globe feed in this manner. The small forage ‹sh they feed on thrive in schools, usually swimming adjacent to shorelines, near submerged structures, or close to the bottom to help avoid being ambushed by predators. As expert predators, doublecrested cormorants have become adept at shallow-water feeding maneuvers and rarely forage far from shore; a behavior that many times puts the birds in direct con›ict with commercial and recreational ‹shermen. Any animal that dives from the water’s surface to the depths faces a number of challenges from the moment it dives, including a complete change in the very medium through which it travels. The physiological demands that allow a bird to dive are far different than the ones required for ›ight, walking, or swimming on the surface. In the evolution of ›ight many of the adaptations that support one type of locomotion may limit the other. It is this con›ict of advantages that adds still another layer to the interesting complexity of the double-crested cormorant. Water, as an environment in which to live even on a part-time basis, has its own unique set of physical characteristics. It is far heavier than atmospheric air and thus produces greater pressures with increased compression of biological tissues and air spaces. Being heavier, water is also denser than air, in fact eight hundred times denser, making the effort to overcome resistance more dif‹cult and requiring more energy to move through it. Water also re›ects, absorbs, and bends light waves entering it from the atmosphere, greatly reducing visibility and distorting underwater images. Even the molecular structure of water works against an animal diving from the surface. It is the same feature that keeps ocean temperatures relatively stable and more slowly affected by temperature changes than air. The hydrogen bonds between oxygen and hydrogen atoms in the H2O molecule allow a body of water to absorb a great deal of energy before changing temperature itself. Water also has a property that lets it draw heat from a warmer body many times faster than air at the same temperature. In other words, the thermal conductivity of water is about twenty-‹ve times greater than that of air, meaning that an unprotected submerged animal loses its body heat twenty-‹ve times more quickly in water than it does in air, possibly faster than the animal can generate heat by consuming and digesting food. Cormorants, as diving 42 • the double-crested cormorant birds, therefore play a tricky balancing game of energy production versus energy conservation in every pursuit dive...