EARLY BIRD LISTING
I wonder how many of the people who go out making lists of spring birds know that bird listing goes back to ancient times. It's a modern sport, but earlier bird watching was serious, and a competitive listing of birds played a part in as important an event as the selection of the site of the city of Rome.
The story, as Plutarch tells it, is that Romulus wanted the city on what became known as Roma Quadrata; Remus wanted it on the Aventine Mount. As was the custom in those days, they concluded at last to decide by a divination from a flight of birds. The twins placed themselves apart at some distance and watched. Remus, they say, saw six vultures, a truly notable flight; Romulus saw twelve and from this rare and unusual occurrence Romulus' choice of the site for the city was accepted.
VULTURES HIGHLY REGARDED Partly from this the vulture became chiefly regarded by the Romans in their divinations from birds. But even before this the vulture was highly regarded. Hercules, it was said, was always very joyful when a vulture appeared to him upon any occasion. He considered it the least harmful of creatures; not pernicious to corn, fruit tree, or cattle, it never killed or hurt any living thing. It was also thought not to eat other birds, a weighty point in its favor, as Plutarch quotes from Aeschylus, "What bird is clean that preys on fellow bird?" And apparently its deciding claim to esteem was its rarity and infrequency, which gave rise to the opinion in some that it came from another world, an opinion foisted by the soothsayers of the day.
Earlier yet, birds played a part in Rome's history. Plutarch warns that some give you mere fables of the origin of Rome, but it is widely current that Remus and Romulus, fathered by Mars, the God of War, were exposed in a remote place to perish. This would have taken place, but for a she-wolf that nursed them, and birds of various sorts that brought little morsels of food which they put into their mouths. Some, however, hold the belief that not birds of various sorts but a woodpecker was the bird that constantly fed and watched the twins, and even in Plutarch's time the Romans still worshiped and honored the woodpecker for this service to the founder of the city.
BATTLE OF THE SEXES AND ITS EVOLUTIONARY SIGNIFICANCE [Ref]
I used to think that the battle of the sexes so ably portrayed by James Thurber was artificial, a man- and/or woman-made thing. But recently I've come to see it as old—probably as old as sex itself in the animal world.
Under the severe tide, "Secondary Sexual Characters and Ecological Competition," in a paper from the Bird Division of the Chicago Museum, I've outlined the possibility of competition for food, between the sexes, being a factor in evolution, responsible in part for characteristics of structure and traits that distinguish them.
In circles that discuss evolution the idea is current that food competition is important between species. It may even be stated as a rule: two species with the same food habits cannot live in the same place. Competition drives one out, unless they have different food habits. These differences seem especially evident when you look at closely related species, and they are accomplished in a variety of ways. A habitat difference is very common. The long-eared owl hunts in the woods—its cousin, the short-eared owl, hunts the meadows; the song sparrow favors the drier shrubbery while its cousin, the swamp sparrow, lives in wetter shrubbery.
THE SIZE FACTOR Sometimes the difference is accomplished by size; take the downy and hairy woodpeckers of our wood lots, very similar except that one is larger and is adapted for larger prey, the other smaller and adapted for smaller food items. Sometimes they feed differently, as the Baltimore oriole, which picks flowers and pecks through their sides, while the orchard oriole probes into flowers as they hang on the branches. Thus more individuals of several species live in an area.
When a pair of birds "sets up housekeeping" and starts "raising a family" they can no longer drift about, looking for easy living and places where food is plentiful. Their wanderings are restricted by having a fixed point, the nest, as their center of interest. Two individuals must draw on the food supply from an area about the nest. Competition would be extreme, and, if there were a scarcity, perhaps critical.
We know how different the sexes may be; how different the rooster is from the hen in our domestic fowl, or the drake and the duck in the mallard, or the red male and the green female of the scarlet tanager. These sexual differences have mostly correlated with display and mating. But logically there should be differences in feeding behavior and adaptations between the sexes.
The basic idea is contained in the old nursery rhyme:
Jack Sprat could eat no fat,
His wife could eat no lean;
And so between them both,
They licked the platter clean.
The two birds of a mated pair, limited to a single area, could be expected to have different food preferences or adaptations for getting it. And we find that there are cases of this. The most striking is that of the huia from New Zealand, of which I've written in a Chicago Museum bulletin. Both sexes have similar food preferences, especially wood-inhabiting insects, but they get them in different ways. The male has a short, straight, stout bill for digging out the wood-boring grubs, woodpecker fashion; the female has a much longer, slender, and curved bill for probing into holes for them, creeper fashion. The female may get grubs in wood too hard for the male to chisel. They supplement each other.
DIET VARIATION BY SEX It is possible that further study may show more sexual differences to have a feeding advantage; the larger size of female hawks fitting them to take larger prey; the smaller size of certain female songbirds fitting them for smaller prey, the smaller bills of female hornbills, the straight bill of the male western grebe, and the upturned bill of the female. Perhaps all are of advantage to the species in giving each sex slightly different advantages in getting food.
Selection could have its effect in the populations with most sexual difference in feeding habits being most successful in raising and leaving progeny. Thus, slowly, differences between the sexes would accumulate. However, it must be kept in mind that this sort of evolution would be limited. The drifting apart of the sexes would be checked by the necessity for their coming together periodically for at least a short period, at nesting time.
WATER IN THE DESERT [Ref]
Water is a precious thing in the desert. Without it no life is possible. When rains come plants spring into vigorous growth. During the long stretches without rain they rest, some as seed, while some plants store water in root systems, or in large trunks. Animals have developed a number of ways of surviving long dry spells in arid country.
Among mammals the kangaroo rat of our Southwestern desert seems able to get along without water. This is caused by an arrangement within the body whereby the necessary water is manufactured within the animal from other foodstuffs: metabolic water.
The accessibility of drinking water in a desert may be the determining factor in whether or not some birds can survive there. The nests of Gambel's quail must be close enough to drinking water for the newly hatched young to walk there, else they perish of thirst. It has been said that newly hatched chicks of the related valley quail of California cannot travel more than a few hundred yards from their hatching places without water. Broods hatched farther away are doomed to die.
Sand grouse, relatives of the pigeons that have adopted the general appearance and habits of quail, live in the Old World, primarily in arid or even desert areas. Where they occur their daily traveling to water is a well-marked phenomenon. Their flight is swift and powerful, and though they may traverse long distances of barren, inhospitable country to watering places, their punctuality in arriving at water, morning and evening in some species, is remarkable.
But what of the young of these desert dwellers that need water? A most unusual situation exists; indeed it seems to be unique. The old birds bring water to the young! This has long been recorded, but as recently as 1921 it has been questioned. However, Mr. Meade-Waldo's observations on birds in captivity seem to definitely establish the custom, and its methods.
PARENTS CARRY WATER Both birds incubate the eggs, the male by night, the female by day, and both parents care for the young. But it is the male only that brings water to the young. He rubs his breast violently up and down on the ground, and then, his feathers awry, he gets into his drinking water and saturates the feathers of his under parts. Then, in captivity, he would run to the hen, make a demonstration, whereupon the young would run out from under her, get under him, and suck the water from his feathers. This they did by passing the feathers through their bills, continuing and changing about until the supply was exhausted. It was found that until the young can fly they take water in no other way.
This was in captivity. Presumably in the wild the process is the same, the adult flying with wet under parts from the water hole to the resting place where the young are under the care of the female.
The similarity of the young sucking water from the feathers to young mammals suckling their mother has been pointed out. But another and a truer similarity exists: that of the young sand grouse getting water from the feathers, and young quail getting water from dew-wet leaves in areas where dew is heavy and there is but little surface water.
BIRD GRAVEYARDS [Ref]
The best-known stories of animal graveyards are those of elephants. But when I asked the curator of mammals about them the answer I got was little better than a snort. Apparently the evidence for them is so vague that it's little better than a myth.
But in birds we have a few bits of evidence from far-scattered places that occasionally such things as graveyards exist.
In the antarctic Dr. Robert Cushman Murphy found on the island of South Georgia a place where Johnny penguins went to die. It was in a lake in a coastal range of hills. The lake bottom was thickly strewn with scores of penguin bodies, all of which had apparently died a natural death. The icy water, Murphy thought, might preserve them for years. The hills, away from the sea, seem a surprising place for the graveyard of such aquatic birds as the penguins, but it correlates with another peculiarity of their mental makeup. They like to nest on high land, or at least far from the sea. The blind instinctiveness of much penguin behavior is well shown by these birds when there is no high land on their nesting island. Then they may nest so far from the beach on which they land that they are close to the water on the other side. Yet they always returned to the sea by the long route, never taking the shorter route.
Another aspect of this preference for land distant from the sea is shown by their behavior when threatened with danger from man or dog. They flee away from the sea, back onto the land, when safety for them actually lies in the sea. Presumably this fixed behavior dates back to the time when the seal that is called the sea leopard was the penguin's main enemy. Then the sea held their only danger. With man's arrival the situation changed, but only after considerable experience with man do the birds change this behavior.
Apparently, when the time came for the penguins in this South Georgia graveyard to die, they followed their age-old pattern, climbing to the high country and away from the sea.
IN A HOLLOW TREE In a hole about eighteen inches in diameter and twelve feet deep in the trunk of a wych elm in Hants, England, Ursula M. Grigg reports finding the bones of at least ninety jackdaws, thirteen starlings, six green woodpeckers, and twenty-five stock doves. All the remains were clean, and not much broken or decomposed. The idea that these bones were the remains of owls' or other predators' feasts was discarded for a number of reasons; as was the idea that this had been a natural trap, the birds entering to roost or nest and being unable to escape. The most tenable idea seems to be that this was a favorite roosting place in winter, and that during the severe weather old and weakened birds, roosting there, succumbed and added their bodies to this communal grave.
ON AN ISLAND Another instance comes from the little Cape Verde Isle of Cima in the South Atlantic. A photograph in the National Geographic magazine for 1927, Vol. 52, P. 27, has the caption that this island is unique and uninhabited and covered with the tiny bones of millions of petrels which in ages past have come here to die. Certainly the plate shows an amazing litter of bird bones on the tiny plateau of this islet.
Petrels are mostly pelagic birds, coming to the land only to nest on isolated islets. Can this "graveyard" be merely the normal accumulation of the bones of the nesting season mortality, or can it be that the birds actually come here to die?
ANIMAL GARDENS [Ref]
Best known of the "gardens" and "animal husbandry" of the lower animals are those of the ants; the aphis kept by the ants for the sake of a sweetish secretion, and the underground fungus garden of the ants. In the vertebrates I know nothing comparable to this, but we do get a number of cases where there is a definite relation between the animals and the growth of vegetation.
It has been said that in the antarctic the nesting colonies of some penguins are detrimental to the vegetation. The constant passing and standing of the birds on the limited areas of soil preclude the growing of vegetation over sufficiently large areas to be an important factor in hindering plant growth. But the reverse is true of the Johnny penguin in the Falklands, where it is sometimes known as the best farmer in the country. The Falkland Islands, off southern South America, are cold, wet, and windy. Sheep raising is one of the main industries. And the Johnny penguin helps to provide better pasture for the sheep. The birds nest in colonies and their droppings help to enrich the land so that the grass grows taller and richer. Rather than using the same area for their breeding colony each year the birds select a new, clean area at the beginning of each breeding season, so that they improve the ground over a larger area.
From the arctic comes another example of a relationship between bird and plant. On the arctic barrens, here and there, are large boulders, erratics left by the glacier that covered the land in times past. And on these boulders, and here only, one finds patches of bright yellow or reddish lichen known to scientists as Xantheria or Xanthoria. Apparently its presence is owed to the fact that these boulders are the lookout places of snowy owls, hawks, and other birds. Their droppings, falling on the rocks, provide the nutrient layer necessary for the growth of the lichens. It is probable that these lichens are transported from place to place by the birds carrying the soredia on their feet. In recognition of the close relationship between these lichens and birds an ecologist has coined the rather formidable term "ornithocoprophilous" to express the relationship.
Also in the arctic are the arctic-fox gardens. The arctic fox often makes its burrows in sandy places, and about the entrance to the burrow accumulate remains of former meals, fox droppings, and suchlike animal debris. This in time enriches the soil and the vegetation there grows taller and more lush than elsewhere on the barrens. This lush vegetation attracts the small, mouselike arctic rodents, the lemmings, that feed on green, succulent vegetation. There is of course one further step in this chain. One of the important foods of the arctic fox is the lemming, which he thus brings to his door by the richer vegetation he unwittingly causes to occur there. A charming arrangement, one of the old naturalists called it.
DROPPING THINGS [Ref]
The story is well known, being recorded by Pliny, of how the poet Aeschylus came to his death through a bird mistaking his bald head for a rock and dropping a turtle on it. The bird was evidently the lammergeier or "lamb vulture," one of the largest and most magnificent of the Old World birds of prey; nearly four feet long. In the Atlas Mountains of North Africa its normal food is turtles, and these it cracks open, so that it can get at the meat, by carrying them up into the air and dropping them on a rock. Now it lives in the Himalayas and in Africa, having been almost if not completely exterminated from Europe because of its alleged predation on sheep. Not only turtles but bones are treated in the same manner, to get at the marrow. Though the habit is well known, it is surprising how difficult it is to find a firsthand description of it. So far I know of only one description written by an eyewitness. And yet, in East Africa recently a stony mountaintop was found littered with broken bones that seemed to be the result of the lammergeier's habit.
GULLS DO IT As I have mentioned, gulls open clams and mussels in this way; and crows, which are among the most intelligent of birds, do it also. They pick up the mussels left exposed by the falling tide, fly up above a hard stretch of beach, a big rock, or a stretch of nearby paved road, and drop the shellfish there. While in general this practice is restricted to a few groups of birds, it is practiced by them in many far parts of the world. The Pacific gull of Australia, widely separated from its near relatives, has the same maneuver for opening shellfish as has our herring gull.
It's hard to understand just how this habit came about. One can imagine that some birds found it out by accident when flying about with a stubborn "nut" they were unable to crack. Or perhaps it was in play they found it. The raven is known to fly about carrying and dropping things in play.
SPARROWS DO IT TOO Often, to find a background if not an explanation of a habit, we look about to see if it's used in some other connection. I've already mentioned the play of some of the crows. Only one other "dropping" habit has come to my attention, and that is a single record for the very common house sparrow. Edmund Jaeger writes that in Nebraska, and again in Riverside, California, he saw house sparrows on gravel roofs, dropping small stones over the edge. The pebbles, or small bits of crushed stone, were carried to the edge of the building by the sparrows, dropped, and as each pebble was dropped the sparrow turned its head, apparently the better to watch or listen to the pebble fall and strike. No obvious utility appeared in these actions. It, too, looked like pastime. Perhaps there was no better reason behind them than that behind small children dropping stones down a well.
LEARNING BY BIRDS [Ref]
Of course birds can learn. Indeed there's a trite saying that no animal has been discovered so low that it cannot learn. One of the simplest cases of learning is shown by parts of some experiments I carried out years ago on the curve-billed thrasher. I had raised a number of these thrashers by hand, and in connection with finding out about their tasting abilities I first fed them on the white of egg, hard-boiled and cut into little squares. They liked it. Then I soaked more squares of boiled egg white in evil tasting (to me) formalin. The birds came to the dish, and also ate them. But after that for a week they refused to eat such egg white. They had quickly learned to avoid the ill-tasting food.
BAD-TASTING FOOD Once I hand-raised a barred owl from a nestling to adulthood. Sometimes getting food for it was a problem, and upon occasion I fed it frogs, which it seemed to like well enough. But then came a day when I fed it a toad. The owl seized the toad at once. Now toad skin, presumably as a defense weapon of the toad, secretes a substance irritating to the mucous membranes of some animals. And this was evidently irritating to the owl, for it did not hold the toad long in its bill. It spat it out, and the owl's face gave evidence of disgust. After that the owl not only refused to take toads, but it also refused to take frogs such as it had found palatable before. Evidently frogs looked too much like toads. The learning was effective, and extended not only to the original object, but also to other, similar objects.
BUCKET-DRAWING When in Florida with the Archbold Expeditions I was studying blue jays. A very simple but amusing thing that chickadees learn is to sit on a perch and pull up a little container of food that dangles far below the perch on a string. Jays, along with crows, are among the most clever of birds, as I've said before, and I gave two jays in one cage a chance to learn the trick. In three days one of the jays was regularly and quickly pulling up the little bucketlike container and getting its food from it. The process was simple: the jay reached down, seized the string in its beak, secured the slack under its foot, and reached down again for another pull. Sometimes five separate pulls were needed to raise the food bucket the eight inches to the perch.
The jays were regularly fed in this manner. Soon I noticed that only one of the two birds pulled up the bucket, though the other also fed from it. In effect one was depending on the work of the other. After this had gone on for a month, I wondered if the second jay, which had never done any of the work, would be able to pull up the bucket if left alone. Certainly it had had lots of opportunity to learn by seeing its cage mate go through the motion. So I left it alone in the cage. This second jay, despite its chances to learn by observation, took one day longer to learn how to pull up the bucket of food than had the first jay. The jays certainly learned the trick quickly through a trial-and-error process, but simply watching the process seemed to be of little help in learning it.
CAN BIRDS COUNT? [Ref]
If birds can count, it's a rather rudimentary thing—perhaps no more than impressions of the size of groups. The widely known example showing that birds don't seem to distinguish between one and two persons is the ruse used by bird photographers and students of birds who are using blinds from which to watch the birds at close range.
The hide, or blind, is a little hut built perhaps a few feet from the nest to be photographed. If the photographer enters the blind in the sight of the parent birds, and conceals himself there, the birds who saw him go in will be a long time in coming to the nest and in resuming their normal activities. But if the photographer takes a companion with him, both go into the blind and conceal themselves, and then one of them goes away leaving the other concealed, the bird quickly disregards the intrusion and goes about its activities as though no one were left in the blind. This subterfuge has long been used and is very successful. Apparently the bird is unable to distinguish between the two people that arrive at the nest and the one only that leaves, and behaves as if both had gone away.
In my duck-hunting days a duck hunter who used wooden decoys told me he was sure that there was a certain number of decoys necessary before they were effective.
The decoys were wooden blocks, carved and painted to resemble life-sized ducks, weighted to float like them, and anchored in shallow water in a flock within gunshot range of the blind in which the duck shooter sat. The idea was that ducks flying by would see the flock on the water, assume that here was a safe resting place, and fly in and light, or attempt to light among them, giving the wild-fowl gunner an opportunity to shoot the wild birds.
The duck shooter claimed that if less than twenty-five or thirty decoys were put out in the flock, the setup was much less effective than if more than twenty-five to thirty decoys were used. He thought that the ducks could distinguish between less than twenty-five or thirty and more than twenty-five to thirty, and favored the latter. Though this is distinguishing between greater and lesser amounts, it hardly comes in the category of counting.
DISTINGUISH "MORE" FROM "LESS" However, a series of experiments summarized on Page 121 in the periodical Bird Banding for 1940 seem to indicate that birds can distinguish between different numbers of things, such as peas and numbers of dots. The birds, including pigeons, parakeets, and jackdaws, were trained either to choose a certain number of objects under certain circumstances, or to choose between two quantities of objects with reward and punishment motivation. It was found that these birds were able to distinguish up to a maximum of six. That this is really counting in the human sense of the term, which is linked with speech or written symbols, is improbable, but it does indicate, as one would expect, that birds do at times distinguish between different quantities, and sometimes with considerable precision.
COURTSHIP FEEDING [Ref]
A young man, giving his best girl a box of chocolates, and a bird, giving his prospective mate a worm or a berry, have this in common: they are both practicing courtship feeding. Further, humans and birds are the only vertebrate animals that do this.
With birds, during courtship, the female often begs to be fed by acting like a young bird—with fluttering wings and widely gaping mouth. The male normally places the food he has collected directly in the open mouth of the female.
The significance of this courtship feeding has been discussed especially by David Lack, in the scientific journal, the Auk. It seems that in courtship feeding the food as such is not of primary importance. The female does not need the food she is begging for; indeed she may have had a full meal since her mate, whom she is soliciting, had last eaten. Perhaps it is of help in maintaining the bond between the pair during the period that exists before they have a nest and young to look after. In this connection it is interesting that with waxwings during courtship feeding the fruit that the male gives the female may be "handed" back (by beak) and the food exchanged back and forth.
In looking for significance and correlations in courtship feeding we find that some species practice courtship feeding and some do not. And the birds that do practice it are usually those in which both sexes care for the young. It might be considered an early, useless appearance of a habit that later becomes useful when the male feeds the incubating female and helps feed the young.
This type of behavior, in which an act used elsewhere is introduced into courtship, is sometimes called "symbolic." Other such symbolic acts are the preening that sometimes takes place between a pair of mating birds, and the passing or the manipulation of nesting material long before there is a nest to be built.
Some species during courtship go through actions that resemble courtship feeding except that no food is passed; the bill touching of the mourning dove and of the waxwing falls in this category. Perhaps it is incipient courtship feeding on its way in the long course of evolution, either upward, to include food, or downward, away from courtship feeding.
Their functions seem to be to give something for the pair to do; something they can share. It helps fill up the pair's day and keep them together. It is something that helps strengthen the bond between them, against the day when both will be working together raising a brood.
THEY TURNED THE TABLES [Ref]
Most birds prey on animals enough weaker than themselves to be in no danger from their prey; their hunting is more like that of the gunner after rabbits than that of the hunter after lions. But there are exceptions.
The great blue heron, armed with a spearheadlike bill, lives largely on fish. These it spears in the water, stalking about after them on its long legs, or waiting like a bird on a Japanese screen, as patient as any fisherman, for its prey to come within striking distance. The heron's size and its great bill render it safe from most enemies. But it sometimes overestimates its ability. Audubon recorded one on the Florida coast that, standing in deep water, up to its belly, struck a fish too large for it. The fish dragged the bird for several yards, now on the surface, now underwater. Finally, after a severe struggle, the heron freed itself. It was exhausted, and stood near the shore, head turned away from the sea. As if, Audubon said, it was afraid to make another attempt at fish catching.
A more serious encounter for the bird was recorded in Field and Stream magazine. The heron had caught a shad about a foot long. He tried to swallow it, but it was too big to go down. He tried to disgorge it, but the fins of the fish, acting as barbs, kept it from slipping backwards and out. The result was death for both bird and fish.
CAUGHT BY A CLAM The oyster catcher, a large black and white relative of the sandpiper, feeds on, among other things, shellfish. Mussels and oysters look like hard nuts to crack, even with a stout, wedgelike bill such as that of the oyster catchers. The oyster catcher's favorite feeding times are when the tide has fallen and the shellfish are first exposed to the air and before they have closed up their shells, and again when the tide is rising and the shells are just beginning to open. The oyster catcher stabs into the shell, and with its bill cuts the strong adductor muscles that hold the shells of the bivalves together. The rest is easy. But a danger lurks here: what about stabbing into too big a shellfish, or making an inept stab? And this very thing has happened. On the South Carolina coast Mr. W. P. Baldwin found a trapped, drowned bird. It was held, with the tip of its bill caught in the shell of a hard-shell clam, as if in a trap. Apparently the rising tide had flooded and drowned the bird.
The raven eats most anything, living or dead, and except for man has little to fear in the northern forests where it lives. Yet from Wisconsin comes a record of one that met his death through a porcupine. The porcupine's quills are a dreadfully prickly covering that one would think would protect it from most encounters. Yet one animal, the fisher, kills and eats it as a matter of course, and wolves and bears sometimes eat them without too many ill-effects from the spines. The slow-moving porcupine has little regard for automobiles, and many are run over on country roads. A porcupine is too big and tough for a raven to kill and the Wisconsin raven had probably fed on a dead porcupine. Stuck through its gizzard was a quill, and another, which had apparently caused its death, was stuck in its heart, having apparently worked there from the digestive tract.
Many small insectivorous birds eat spiders as well as insects. This they do almost with impunity in temperate latitudes, where only occasionally do spiders make webs strong enough to trap a bird. But in the tropics, where there are more large spiders, their webs must be a greater hazard to birds. That the hazard exists in both climes, however, is shown by a goldfinch reported caught in a spider's web in Cape Cod, Massachusetts, and a dusky flycatcher caught in a spider's web in Cameroons, West Africa.
SURVIVAL OF THE UNFIT [Ref]
To care for the weak, the unfit, and the cripple is usually considered an extremely highly developed altruism in our society. As our society progresses, more and more provisions are made for the unfit.
In nature the unfit usually is soon weeded out. If an animal is unable to feed itself it is doomed; or if it is less successful than its fellows it has less chance of leaving progeny. That is natural selection.
Hence on both counts it comes as a surprise to find two well-authenticated cases of crippled birds, unable to search for food for themselves, surviving for long periods.
The first is T. R. Peale's record in 1848 of a brown booby on Enderby Island in the Pacific. An adult bird whose plumage indicated it was several years old was found on the island, and it had only one wing, the other having been lost by some accident and the wound completely healed. The bird was unable to go to sea and get its own food, and was being fed by its fellows.
The second was a frigate bird, found on the Revillagigedo Islands, reported by A. W. Anthony in 1898. This bird, too, was fat, and had one wing withered and useless, evidently from hatching. It had never flown. Frigate birds are masters of the air that snatch their food on the wing from the surface of the water, and a flightless frigate bird would be as badly off as a flightless swallow. The cripple had been fed all its life by its neighbors.
At first the uncritical might think, What altruism, what charity, for the healthy to feed these two cripples. But an explanation involving less advanced principles, principles more in keeping with what we know about bird behavior, is possible. Remember that young birds that are unable to begin feeding themselves at the proper time may continue to beg for food, and be dependent for a long time, as I have shown with young shrikes under the chapter, "Conditioning in Birds." Remember that a young bird begging for food may be fed by adults, not its parents, and even by other young birds (shown in "Bird Helpers at Nesting Time"); and we have the clue.
The cripples, hungry, begged for food; the healthy birds responded by feeding, as they might do to other begging young, and owing to the unusual circumstances both were continued.
These certainly are cases where the unfit survived. Natural selection has not operated. But such cases are rare exceptions.