Hen’s Teeth and Horse’s Toes by Stephen Jay Gould Read Free Book Online

its “nest.” Galápagos, North Seymour Island. PHOTO BY DUNCAN M. PORTER .
    Three species of sulids inhabit the Galápagos Islands: the red-footed, the blue-footed, and the masked booby. The red-footed booby lays a single egg in a conventional nest built near the tops and edges of trees and bushes. By contrast, its cousin of markedly different natural pedicure, the blue-footed booby, lays its eggs on the ground and builds no true nest at all. Instead, it delimits the nesting area in a remarkable and efficient way: it squirts guano (birdshit to nonornithologists who have not read Doctor No ) in all directions around itself, thus producing a symmetrical white ring as a symbolic marker of its nest.
    Within this ring, the female blue-foot lays, not one (as in many boobies), but from one to three eggs. In his most impressive discovery, Nelson has explained much about the breeding behavior and general ecology of boobies by linking the production of eggs and young to the quality and style of feeding in parents. Boobies that travel long distances (up to 300 miles) to locate scarce sources of food, tend to lay but a single large egg, hatching into a resilient chick that can survive long intervals between parental feedings. On the other hand, when food sources are rich, dependable, and near, more eggs are laid and more young reared. At the extreme of this tendency lies the Peruvian booby, with its clutch of two to four eggs (averaging three) and its ability to raise all chicks to adulthood. Peruvian boobies feed on the teeming anchovies of their local waters, fish that may be almost as densely packed in the ocean as in the sardine cans that may become their posthumous home.
    The blue-foot lies between these two tendencies. It is a nearshore feeder, but its sources have neither the richness nor the predictability of swarming anchovies. Consequently, conditions vary drastically from generation to generation. The blue-foot has therefore evolved a flexible strategy based on the exploitation by older siblings of their parents’ intellectual style: yes-no decisions triggered by simple signals. In good times, parents may lay up to three eggs and successfully fledge all three chicks; in poor years, they may still lay two or three eggs and hatch all their chicks, but only one can survive. The death of nest (or, rather, ring) mates is not the haphazard result of a losing struggle to feed all chicks with insufficient food, but a highly systematic affair based on indirect murder by the oldest sibling.
    I was reminded of the quip about painting and saluting while observing blue-footed boobies on Hood Island in the Galápagos. Their guano rings cover the volcanic surface in many places, often blocking the narrow paths that visitors must tread in these well protected islands. Parents sit on their eggs and young chicks, apparently oblivious to groups of visitors who gawk, gesticulate, and point cameras within inches of their territory. Yet I noticed, at first by accident, that any intrusion into a guano ring would alter the behavior of adult birds from blissful ignorance to directed aggression. A single toe across the ring elicited an immediate barrage of squawking, posturing, and pecking. A few casual experiments led me to the tentative conclusion that the boundary is an invisible circle right in the middle of the ring. I could cautiously advance my toe across the outer part of the ring with no effect; but as I moved it forward, as slowly and as unobtrusively as possible, I invariably passed a central point that brought on the pronounced parental reaction all at once.
    Three hours later, I learned from our excellent guides and from Bryan Nelson’s popular book ( Galápagos: Islands of Birds ), how older siblings exploit this parental behavior. And, anthropomorphic as we all must be, it sent a shiver of wonder and disgust up my spine. (Science, to a large extent, consists of enhancing the first reaction and suppressing the second.) The female