Pick from the Past
Penguins under the Sun
Now! came the signal from Phil, and I ran out across the water-worn beach boulders, rapidly closing on the squat bird, itself intent on gaining the safety of the sea. Launching myself into a flying tackle, I managed to hook my fingers around its webbed feet but landed badly, my knee colliding with a protruding rock. I lay momentarily stunned, with the two-foot-tall, seven-and-a-half-pound bird clasped beneath one arm. With one baleful brown eye fixed on me, the penguin was quick to seize an opportunity. In a flash, it thrust its head forward, butted its bill into my cheek, and tore off a neat strip of flesh. Score settled and honor satisfied, the penguin sat placidly, a thread of my bloody skin dangling from its bill.
Such was our introduction to the jackass penguin. Named for its donkeylike bray, the jackass penguin lives far from the icy regions favored by its Antarctic relatives. The only penguin found in Africa, it comes ashore along the coast of Namibia and the west and southeast coasts of South Africa. Because the mainland is home to jackals, leopards, and other predators, jackass penguins have long sought the safety of offshore islands on which to breed. We had come to one of these islands to study the birds, in particular to investigate their breeding behavior. During our five-month stay on Dassen Island, we were to learn just how ingeniously their breeding system allows jackass penguins to survive natural challenges. The 700-year history of the island also provided an insight into how much this subtropical species has suffered at the hands of humans.
Dassen Island lies about fifty miles northwest of Cape Town and four miles off the coast. At eighty-nine acres, it is one of the largest of the South African seabird islands and is the breeding ground of white pelicans, Cape cormorants, kelp gulls, and swift terns, as well as 6,000 penguins. The penguin numbers, however, are a mere vestige of the pre-1900 population, which is estimated to have ranged as high as one million. The level of noise that must have been generated by the braying of a million penguins can only be imagined, as we were driven to distraction by the lone pair that chose to nest outside our Dassen Island bedroom window. Yet a little peace and quiet is scant compensation for the loss of so many penguins.
Penguins spend most of their lives at sea and have always fallen prey to sharks and seals. Injured birds stagger ashore and carcasses wash up on the beaches of all the penguin-breeding islands. The wounds can be terrible; the penguins are seized by the stomach and literally shaken apart. But sharks and seals kill only small numbers of penguins each year. Human activities, however, have long threatened, disturbed, and foiled the jackass penguins attempts to lay eggs and raise healthy chicks and are largely responsible for the dramatic reduction in jackass penguin numbers throughout their range.
Although for centuries penguins had regularly been killed on Dassen for food, fuel, and fun, large-scale exploitation of these birds began in earnest with egg collecting in the early twentieth century. Staggering quantities of seabird eggs were collected for an eager market on the mainland. Almost 13 million penguin eggs were taken between 1900 and 1930, with nearly 600,000 collected in 1919 alone. Even in the early 1960s, an average of 35,000 eggs were removed annually, before the practice was halted in 1967. Penguin eggs, three to four times the volume of hens eggs, are apparently an acquired taste. Their whites are faintly green and the flavor slightly fishy. To insure that the eggs they gathered were the freshest, egg collectors first cleared hundreds of thousands of eggs from nests and tossed them into the sea. The new eggs that the female penguins would then lay in the emptied burrows were of guaranteed quality and were rapidly gathered for sale. Egg collectors left a permanent mark on Dassen in the form of a three-foothigh wall, which encircles the entire island. It was built to force penguins to breed on the fringes of the island, concentrating them and thereby facilitating egg collection. Today, rocks and accumulations of guanobird excretaform ramps by which the penguins can easily cross the old wall.
The relatively meager penguin population on Dassen today has not been able to replenish the mother lode of guano that once covered the island. For a short time during the 1800s, this white gold was in hot demand as a fertilizer for exhausted soils. While guano scrapers did not completely strip Dassen of its guano crust, some smaller islands were denuded of their guano cap, clear down to the bedrock. This episode, though brief, created side effects still felt today. Apart from the general disturbance that guano scraping must have caused the birds, the practice robbed the penguins of an ideal material for their burrows, on which they depend for both nest sites and cool retreats from the African sun.
With their thick, insulating layer of fat, penguins are superbly adapted to the cold Benguela Current that flows along the west coast of southern Africa. But penguins must come ashore annually to breed and then, after regaining weight at sea, spend three weeks landbound to molt and regrow their sleek covering of feathers. Most penguins breed during the southern African winter, when temperatures are less torrid, and when ashore, the birds tend to be most active in the less intense temperatures of evening. But still we often saw them panting in the near-inescapable heat. Adults can dispel some heat by flushing blood to a bare patch of skin behind their bill, but the rotund chicks, encased in folds of fat and dense, woolly down, do not have this option. We saw some penguins shading their chicks with outspread flippers. By far, the best way to beat the heat is to escape into the cool cellar of a guano burrow, where adult penguins can incubate eggsand where the hatchlings can wait for parents to deliver foodin relative comfort. Where the guano has been removed, the penguins are forced to make their nests in shallow, exposed hollows. The many penguins breeding in winter face other problems arising from the scarcity of guano: Burrows excavated in guano drain freely and stay dry, but shallow nests close to bedrock fill with water in heavy rain. Birds incubating eggs sit for days in water up to their thighs, the eggs bobbing to the surface whenever the adult stands up.
More recent threats victimize adult birds at sea. Shipwrecks, spills, and the illegal cleaning of oil tanks at sea all contribute to oil slicks. Even if they avoid swallowing the oil, penguins with a coat of tar over their insulating feathers become cold, waterlogged, and uncharacteristically slow in the water. Such birds have little chance of catching fish and soon die.
Even more likely to undermine the chances of long-term survival of the penguins is competition with humans for fish, a clash that may pose the greatest threat to penguin populations today. The collapse of the commercial pilchard fishery in the 1960s caused both jackass penguins and humans to switch to catching anchovies. Unfortunately, anchovies are far from idea penguin fare, being lower in fat and protein and less easily caught.
Penguins have probably always been faced with unpredictable food supplies; like all fishermen, they face good seasons and bad as the fish come and go. We believed that aspects of the jackass penguins breeding system actually help them to cope with this variability, insuring that even in a year when fish are scarce, pairs have a chance to raise at least one healthy chick and keep the species going. So we moved to Dassen to find out.
In general, birds try to produce as many healthy offspring during their life span as possible, without taking risks in any one breeding season that might compromise their ability to produce more young in the future. Small species, such as sparrows, have short life spans. With fewer years in which to raise young, they tend to take more risks. But a penguin can have a full ten years to breed. Like many other species of seabirds, penguins do not sacrifice themselves for their young. A parent will stop feeding a chick rather than go hungry itself.
Jackass penguins lay two eggs, three days apart. Because they begin incubating as soon as the first is laid, that egg will hatch two to three days earlier than the second. (If birds wait until the entire clutch is laid before beginning to incubate, the young will hatch at about the same time.) In jackass penguins, the first chick is larger and will dominate the smaller during the intensely competitive feeding sessions. The smaller chick, if it survives, will have to work hard to get enough food throughout the ninety days of chick growth. But these small chicks often perish, particularly in years when fish are in short supply. This system has evolved not only in penguins but also in bird species as diverse as eagles, boobies, egrets, and blackbirds. Our work was designed to calculate just what advantages the strategy of having different-sized chicks confers on jackass penguins.
During our first month on the island, we created nests with equal-sized chicks by switching hatchlings among nests so that they were matched in size and age. Because the ability of the parents to recognize their own young, usually by voice, doesnt develop until chicks are more than two weeks old, all our foster chicks were accepted into their new homes. For the next four months, we monitored two hundred neststhose with same-sized chicks and control nests with unequal sized youngchecking each daily and measuring chick growth every five days.
Nest checks could be an arduous affair, as most of the penguins, on noting our approach, dived either directly into existing burrows in the soil or into hollow niches under boulders. In areas of sandy soil and guanoprime turf where penguins were packed togetherwe had to tread carefully among burrows to avoid breaking through the roofs and exposing the nests to the elements. Some chicks camped deep within rock piles and could be caught only by our lying full length in the mud and guano and jamming one shoulder against the entrance to afford us the longest possible reach. After heavy rain, this was a particularly unsavory chore, made worse by the soggy remains of old chick corpses.
While the chicks were young, we had to run the gantlet of parents defending the nest with bill and flippers. The razor sharp edges and hooked upper mandibles of the beaks were formidable weapons that frequently drew blood and left permanent scars. (Seabird researchers can usually be identified by the fine, long scars on their hands, although few sport such scars on their cheeks. Nor do we exaggerate: A court case in Cape Town describes how an elderly gentleman pleaded not guilty to stabbing a jackass penguin to death. He contended he had to beat the penguin off with a steel pipe and stab it with a penknife after it attacked him.)
When the chicks grew older, they were often left unguarded as both adults responded to the extra demand for food by fishing at sea every day We found the young most appealing at about forty days of age, with dense, chocolate-brown down, white fronts and faces, long rubbery flippers, and outsized leathery feet, which they spent the next fifty days growing into. Some chicks discovered a love of digging, so we had to carefully probe for them with cane poles and dig them out of their long, spiraling tunnels with shovels. Sadly, because the season was a poor one for the fish and therefore for the penguins as well, we had to watch many of the chicks in our study group waste away and die. These were the conditions under which we would have expected to see an orderly reduction in the size of the brood; that is, we anticipated that in nests of different-sized chicks, one would perish and one survive. However, we found no difference in the survival rates of different-sized and same-sized chicks. This being the case, we hypothesized that any advantage in having unequal-sized chicks might lie not simply in chicks surviving at the nestling stage but also in the overall health and adaptability of the young when they become independent and take to the sea. To test this theory, we monitored the relative health of our chicks by watching them feed.
Both male and female parents feed the young. Parent penguins fish offshore and later regurgitate a partly digested fish dinner into the mouths of their chicks. After a day of fishing, adults return to the breeding colony in the late afternoon. They spend some time splashing in the shallows or congregating in large groups on the beaches to preen. Then they waddle inland through undulating carpets of roosting terns to the nest sites, where the chicks eagerly await their arrival. Consequently, most feeding takes place at dusk or after dark, and we had to watch the frantic process by full Moon or flashlight. Both chicks plead for food from the adult, peeping shrilly and tapping the adults face and bill with their bills. If the adult leans toward one chick and starts to bring up food from its stomach, the other chick grows desperate in its efforts to intercept the broth, scrambling between its sibling and the parent or over the adults back. The parent seems to feed randomly to its left or right, so the chicks continually dart from one side to the other, weaving over and under, creating a writhing mass of feathers, down, feet, and bills. The adult merely tries to remain upright during the onslaught.
The larger chick has a distinct advantage. It uses its superior weight to maneuver into the best position and to shove its sibling aside; it eats first and more, and thus maintains its weight advantage. After the most urgent hunger pangs abate, the heavier chick stops pressing its advantage, and the younger chick can feed, if there is anything left.
Some younger, thin, and hungry chicks were emboldened to beg from adults other than their parents, although their efforts were almost always foiled by the adults ability to recognize their own offsprings voices. The interlopers were then driven away by vicious jabs to the head. At one of our nests, designated 45W, only one adult cared for the off spring (the other adult had most likely died), and both chicks were skinny and wily. These two resorted to stealing and were fast and sneaky One patrolled from early evening onward through the various nest sites to check whether other broods had started feeding, and in between, it hurried back to make sure its own parent had not yet returned. When a brood started to eat, always a noisy business, this chick would arrive within seconds and hover behind the adult and feeding chicks, waiting for an opportune moment. While on its mission, it did not vocalize, as that would have alerted the adult to its identity. Instead, it waited until the adult bent over to begin to bring up food for one of its own chicks, and then it would whip around to the front, shoulder the feeding chick aside, and neatly insert its own bill into that of the adult, gulping convulsively. Speed, timing, and desperation seemed to allow the young thieves to occasionally displace heavier chicks. In comparison with the fat, placid chicks from single chick nests, life on the edge seemed to confer a glimmer of intelligence on the 45Ws. Once, while catching chicks for weighing, when most of the chicks in the group were cowering away from me in dumb fear, I noticed the leanest of the 45Ws gazing up at me speculatively, a greedy little gleam in its eyes, probably wondering if I would vomit if it begged me to.
By observing feeding, we found the answer to why having chicks of unequal size was indeed advantageous. We found that first-hatched chicks with a smaller sibling weigh more and reach independence about two weeks earlier than either of the chicks from equal-sized-sibling nests. In good years, when food is plentiful, the early departure of the first hatched chick enables its smaller sibling to then feed uninterruptedly, gain weight rapidly, and reach independence not long afterward. In contrast, when both chicks are the same size, neither is able to dominate feeding. Both have to work harder to get fed, the food is often spilled or wasted in the frenzy, and neither gets a full meal. These chicks gain weight slowly, are in constant competition, and reach independence later and with lower body weights than chicks emerging from different-sized broods. Even in nests of same-sized chicks, one often dies, while the other remains thin and ill-prepared to face independence at sea.
Body weight at independence may be an important indicator of how well a chick will do in its first year. Juvenile jackass penguins (recognizable by their gray heads) have a less than 10 percent chance of surviving their first year at sea. Suddenly, dinner is not delivered as a hot rush of fish porridge. In a new and challenging aquatic environment, they must find and catch fish on their own. The better the condition of a chick when it goes to sea, the greater the likelihood it will survive those crucial first weeks when it is learning how to fish.
The breeding strategy of jackass penguins seemingly a biological puzzle that creates competition between siblings and often results in the death of one of themis ultimately practical and efficient. The first-hatched chick always uses its size advantage to make sure it gets fed first, so that at least one chick is well fed and fledges at a high body weight. The smaller chick either expires quickly or manages to hang on until the older sibling leaves the nest, and then it too feeds freely and well. If one or both chicks go to sea with a healthy and timely start, parent jackass penguins also enjoy an advantage, not having to work so hard for quite as long. Then they too can leave Dassen Island and return to the sea and its many challenges.