What is a Bird?
In 1861, workmen splitting slate in a quarry in Bavaria came across a fossil which they passed on to the local museum. The fossilized creature had obviously been reptile-like, and it had a long tail. Quite large numbers of this sort of fossil were being found, but one thing saved this one from being left to collect dust on the museum shelf: clearly outlined around the fossil in the fine texture of the slate were the unmistakable imprints of feathers! As a result, this small creature instantly aroused the greatest interest and has since become one of the most famous and important fossils offal time. To this day it remains the earliest known fossil bird: Archaeopteryx lithographical (literally, “ancient wings in slate”).
The slate in which Archaeopteryx was found dates from the Jurassic period some 50 million years ago. The species of which several other specimens have sub-sequent been found provides a remarkable “missing link” between the birds and the dinosaur stock from which they are believed to have descended. Archaeopteryx was about the size of a large pigeon. Apart from its long tail like a lizard’s (but feathered). Its most obvious differences from modern birds are that it had toothed jaws. Front limbs that were modified as wings but still retained claws, and a relatively small breastbone. The feathers the most obvious characteristic of birds seem to have been very similar to those of modern birds. The positioning of the tail feathers was odd because the animal still possessed a” birdlike” tail, but the number and arrangement of wing feathers were more or less identical to those in modern birds.
Archaeopteryx is thought to have evolved from one of the small dinosaurs of the order Saurischia. Indeed the line of descent is sufficiently clear for some people to claim that the dinosaurs did not become extinct since the birds are their living representatives! Feathers probably first evolved to keep the animal warm Archaeopteryx, like the early mammals of the same period, would have been warm-blooded. Although warm-blooded animals need more food to survive they are at a great advantage over cold-blooded reptiles in that they can be more active in cold conditions at night, dawn or dusk, and in temperate climates. It is probable that several groups of dinosaurs developed the ability at least partly to main-taint their body temperatures in cold conditions. Other reptiles, probably close to the likely line of evolution of Archaeopteryx, had greatly extended, overlapping scales, again probably to help keep the animal warm. Viewed in this light, feathers are best regarded as highly complex derivations of reptilian scales.
Probably as Archaeopteryx’s ancestors clambered about in the trees, using the claws on both pairs of limbs, and started to leap from branch to branch, the gradual extension of the scales at the rear of the fore-limb and on the tail would have provided an expanded surface area helping the creature to extend its leaps into glides. Natural selection would have favored the evolution of progressively longer and lighter scales for this purpose. Although the wing feathers were long enough for Archaeopteryx to have been able to glide a reasonable distance, the lack of a well-developed breastbone suggests that the large muscles required for powered flight may have been missing. In addition, the long tail would have made it somewhat poor at maneuver-in in flight compared with the more compact modern birds.
In this one species lies all that we know about the early evolution of birds. No older fossils of animals intermediate in structure between Archaeopteryx and its ancestors have yet been found. Most bird carcasses were probably taken as carrion or disintegrated before they could become fossils, and because of their relatively small size and thin bones, birds fossilize poorly. It is no coincidence that Archaeopteryx and most Bird fossils from the later Cretaceous are those of water birds or others that fell into fine moods, for this gives the best chance of them being adequately preserved.
The next fossil birds appear after a gap of some 3o million years in the fossil record, early in the Cretaceous period. They are all obviously similar to modern birds, and include a diver-like bird, Hesperornis, whose sewing structure shows that its ancestors had been flying birds, although it had already reverted to lightlessness! Hence within a 3-million-year period, there must have been a considerable evolution of the birds, but we know next to nothing about it. By the end of the Cretaceous period 65 million years ago, a few birds were beginning to show the characteristics of modern families. However, it was some 65-38 million years ago during the first half of the Tertiary period that the great radiation of birds took place. From the Eocene (54-38 million years ago) we know fossils of at least modern families, including some birds that probably belong to the Passeriformes the perching birds–that huge order of small birds that dominates our avifauna today. Byte the end of the Eocene, the birds had truly “arrived.”
Size and Energy
Compared with some other classes of animals, birds are a very uniform group, in both structure and size. The mammals, for instance, include horses, lemurs, whales, bats, and tigers, to name but a few examples of the range of mammalian representatives. Furthermore, mammals range from tiny bats and shrews to great whales, an aweigh ratio of some I to 7 00 million. Whereas flying birds range only from some 2.5 g (o.o9oz) to 15kg (331b), a ratio of Ito a mere 6,000.
The reason for this more limited range of size and form in birds may be found in the requirements for flight. (Flightless birds, evolved from the same ancestors, are freed to some extent from these weight restrictions, only to confront other dangers and, often, extinction sees pro.) Flying is, inters of energy required, an exceedingly expensive method of locomotion, and so is of paramount importance to do it as economically as possible. Virtually every distinctive characteristic of a bird’s anatomy has evolved as an adaptation for flying.
The size of birds has been constrained for different reasons at the small and large ends of the spectrum. Birds need to be able to maintain a constant warm body temperature between 41°C (r o6°F), and43.5°C (I rib of) depending on species in order to function efficiently. However, with decreasing size, the volume (or weight) of the abode is reduced in far greater proportion than its surface area. This is important because a body loses heat at a rate that can be related to the ratio of surface area to volume. As the surface-to-volume ratio increases (i.e. as objects become smaller) threat of heat loss increases. So a small bird loses heat relatively faster than a large one. Since the heat which is lost has to be made good by obtaining more food, small birds must eat more, relative to their body size, than large ones. Below a certain size, the time and effort required for energy replace-mint are so “uneconomic” that survival is not possible.
It is no coincidence that the smallest birds For example, the Verlaine hummingbird (Mellisuga minima) of Jamaica which weighs in at about 2.4g (o.oXoz) live inward parts of the world. Even in the tropics, many hummingbirds go torpid at night to save energy. They have to warm themselves up again at dawn before they can start their day’s activities, which include taking up to half their body weight in food. The upper limit to the size of flying birds also results from problems associated with size and scale. One bird, which has linear dimensions that are twice those of another, to ixia has a surface area which is four times and a volume 1 and weight) eight times greater. Hence large birds have a heavier weight-to-wing-area ratio than smaller ones: wing loading increases with size. Compared with a small bird. A large one must have relatively bigger wings and/or flight muscles, which in their turn will add further weight.
There is also anatomical evidence that large birds may be more constrained by weight than smaller ones. In the smaller species, only the largest bones may be Hollow (pneumatized), but in larger species, more of the bones may be hollow. For example, the Marabou stork not only has hollow leg bones; most of the toe bones are also hollow. The actual act of taking off is the most energy-demanding moment of flight; the bird must accelerate rapidly to pass stalling speed. This is not a problem for small birds, which just leap into the air and fly away. A large vulture, however, especially one with a full crop, may have to run along the ground to gain sufficient speed to become airborne a swan may have to do the same on water, and an albatross may have great difficulty taking off at all unless it can face into a strong headwind.
The upper weight limit in modern flying birds seems to be of the order of r 3 kg (331b). It is perhaps no coincidence that the largest birds of a number of different groups approach this weight. For example, the Great bustard may weigh I5kg (331b), exceptionally even a little more, the largest swans may be aboutr5kg (331b), the largest condors about 14kg (3 ribs), the largest pail-cans about i5kg (331b) and the Wandering albatross about r2kg (26.51b). However, even for these species such weights are exceptional; most adult individuals are smaller.
This line of argument has one serious flaw: some fossil-flying birds were considerably larger! Until quite recently the largest fossils known were mostly birds of the genusTeratornis, which are usually thought of as giant condors, although there is considerable doubt about how they really lived. One Of these, Territories incredibly had a wingspan of the order of 5m (I6ft) and probably weighed well in excess of 20kg (Ales well-known species of marine bird, Osteodontornis out, had a similar wing span. Both pale into insignificance against the remains of another species recently discovered in Argentina. Argent avis magnificent, which probably belonged to the same family as Territories, had a wing span of some 7-7.6m (23-25ft)! It has been suggested that these giants specialized in riding the up currents off hot, an open country as do the vultures in East Africa today, but this is speculation. For the biologist attempting to explain how they flew, the giant birds and the gigantic reptilian pterodactyls of an earlier age pose similar problems.
A Body Plan for Flying
The skeleton and musculature incorporate most of the birds’ major adaptations for flight (that is, apart from feathers). These adaptations achieve two main aims. First, because a flight is so expensive in energy, weight has been reduced as far as possible; and second, the need for maneuverability in flight has required the bird to become a very compact unit with as much weight as possible placed close to the center of gravity.
To begin with, the skull has been greatly lightened. The eyes are large and their sock-est. takes up a lot of space in the forepart of the skull, virtually meeting in the middle. The major feature is the reduction of the heavy jaws which other vertebrates possess and the complete loss of teeth. The shape and size of the bill vary enormously, enabling dif-ferment types of birds to obtain and “handle “a very wide range of foods. At the other end of the skeleton, the tail’s bony elements have been greatly shortened and the bones are fused so that all the tail feathers start at more or less the same place. This has resulted in a structure that, is invaluable for helping the bird to steer and which is much more effective than the long “floppy” tail of Archaeopteryx. The size and form of the tail are varied, matching the flying needs of the bird concerned, even in a few species (woodpeckers, woodcreepers, tree-creepers) being stiffened so as to act as a prop during climbing.
The main skeleton has been greatly reduced in weight in many places, by evolution. Of bones that are hollow (pneumatized). Including the major limb bones, parts of the skull, and pelvis. The light ribs have rearward projections (uncinate processes) which overlap the next rib. And give extra rigidity. In some diving birds. Such as the guillemots, which are very long. Overlapping two ribs and providing extra support prevent the body cavity from being compressed during dives. Many bones have fused, making a rigid frame without the need for large muscles and ligaments to hold the separate hones together.
The forelimbs, and to a lesser extent the hindlimbs, incorporate some of the greatest changes. The forelimbs have become the wings, and associated areas of the body are adapted to provide the attachment areas forth massive flight muscles. The hand has lost two of its digits while the other three arcs are much reduced. The main bulk of thawing muscles is at the base of the wing (closet the center of gravity). Although the downbeat results from the direct pull of the muscle, the upbeat (or recovery stroke) require the “pulley” action of tendons over the shoulder joint. The joints of the wing are so shaped that there is very little movement possible except in the plane of opening outland and closing the wing, so saving the need for muscles and ligaments to prevent “unwanted” • movements.
At the base of the upper arm (hummers), there is a broad area for the attachment of the pectoral muscles. These enormous muscles are attached at their other end to thievery large keel-like breastbone (sternum). When these muscles are contracted to beat the wings downward, they produce a force great enough to crush the bird’s body between the sternum and the wing were these not kept apart on either side by strong strut-like bone, the coracoids, supported by the wishbone (fused clavicles, furcula) and the shoulder blade (scapula), the ends of which join together and provide a point of articulation for the wing.
Birds are an unusual class of animal in that they use two methods of locomotion, flying (using the forelimbs) and walking and/or swimming (using the hind limbs). Balance in flight is not a great problem, since the large flight muscles lie close to the center of gravity just below the wings. However, partly because of the presence of these muscles. It would be difficult for the legs also the positioned close to the center of gravity; in fact, the cup-shaped is acceptable in the pelvic girdle, which receives the top ends of the upper legs (femurs). Lie some way behind the center of gravity. The balance would therefore him difficult for a walking bird supported directly at this point.
Birds have overcome this problem in a unique way. The femur is inserted at the acetabulum in the normal vertebrate manner, but projects forward along the side of the bird’s body and has rather a little movement, being bound to the body by muscles. In a sense, the lower end of this bone (the knee) acts as a new “hip” joint to which the lower leg is attached and which is quite well-positioned with respect to the center of gravity. The two sections of the leg which are clearly visible are not comparable with ours: the upper section is the equivalent of our lower leg whereas the lower section or false shin (technically called the taros-metatarsus, or tarsus for short) is formed from parts of the lower leg and sections of the foot bones and has no human equiv.-lent. This explains why the leg appears to bend in the opposite way to the human one the visible joint is not the knee, but more closely equivalent to our ankle. As with the wing, the leg joints are so shaped that movement in unwanted directions is restricted. Leg movements are controlled via tendons by muscles placed near the top of the leg and so close to the center of gravity.
Feathers are by far the most characteristic feature of birds, and a major factor in their habits, lifestyles, and distribution. Keratin, the main constituent of feathers, is a pro-teen aceotiS4 ‘1.1bSt111) which is widespread in vertebrates the hair and fingernails of mammals are made of it, as are the scales of reptiles. The ancestors of Archaeopteryx evolved the basic feather for insulation, and this purpose is well served by the evolution in modern birds of feathers that are light and waterproof, and trap quantities of air so as to slow down heat loss. The principal body feathers consist of a central quill (or rachis) from which the main side projections of the barbs spread out on either side. These are locked together by barbules (see the previous page).
However, feathers have also evolved to serve a number of other functions important to birds. The feathers along the trailing edge of the wing and on the tail have become greatly enlarged, strengthened, and specially shaped so that they form the surfaces which provide the lift for flight and for maneuver-in. The rest of the visible feathers (contour feathers) which cover the surface of the body add to the efficiency of flying by streamlining the body.
Down feathers, found on young birds and also as an insulating underlayer on many full-grown birds, lack the interlocking bar blues and are not organized in one plane, and so look more like shaving brushes. Most Simple of all is the single shafts of the bristles often found around the eyes or at the base of the bill. In many cases, these are thought to have a sensory function. The wide range of colors in feathers also performs a number of valuable functions. On the one hand, feathers may camouflage a bird such as a nightjar so well that it’s difficult for a predator to find it. At the other extreme, in the peacock, hummingbirds, quetzals, and other species, feathers provide some of the most dazzling colors found in nature.
Feather colors are produced in one of two ways or a combination of both. The com-monist of the pigments in feathers is melanin, which is responsible for the browns and black. Some pigments are very rare, such as the green turacoverdin found only in some traces. The other type of colorist is caused by the physical structure of the feather reflecting only a part of the visible wavelengths of (white) light. Such colors include the metallic blue-green of the star-ling and most iridescent colors. Feathers reflecting all light wavelengths look white!
Feathers are not just distributed at random but grow in clearly defined tracts. Each feather grows from a papilla a special ring of cells. As these cells multiply they produce a series of cells that form into a tube. On one side of this tube is a thickened sec-tin, the rachis, and on the opposite side alien of weakness. As the feather grows, it breaks along the line of weakness and spreads out. The individual barbs of the feather also “break apart” at lines of weakness.
Feathers are replaced at intervals. They may be molted because they have become worn and need replacing. Some birds put one thicker covering of feathers for the winter. Feathers are also changed in order to produce different colored plumage. Many birds put on bright plumage for the breeding season and change to a duller one for winter. It is thought that the birds need brighter colors in order to display to one other during courtship; duller, often disruptive. Camouflage plumage provides better defense from predators and so birds revert to this at other times of the year.
The ptarmigan’s white winter and brown summer plumages help the bird to merge into the background and be more difficult for predators to see. The drakes of many species of duck remain in bright plumage almost the whole year round but acquire camouflaged brown plumage for about a month during the summer while they are in full molt and more vulnerable. The mature feather is a dead structure. Its replacement requires energy, and while building the new feathers a bird is less well-insulated and may be able to fly less well. Some species, such as ducks and most auks, lose the power of flight altogether during the molt. On the other hand, molting allows damaged flight feathers to be replaced (an advantage over, say, a bat, which cannot mend a badly damaged wing).
Other Adaptations for Flight
In order to be able to fly, birds have to beagle to mobilize a large amount of energy very quickly. This requires a very efficient respiratory system to supply the necessary large amounts of oxygen. At first sight, the lungs of birds do not seem likely to perform this function: they are actually smaller than those of a mammal of similar size. However, the air capillaries (alveoli) of the avian lung are very small compared with those of mammals, and thus there is a greater surface area for gaseous exchange in a bird than in a mammal.
The bird lung is very efficient, though not more so than that of mammals at sea level. Its particular advantage is its efficiency at altitude. If mice and sparrows are placed in a chamber containing air at the reduced pressure found at the top of Mount Everest, the mice are soon almost totally exhausted and can barely move around, while the sparrows hop about quite happily their breathing is not noticeably impaired. In fact, many birds migrate in a fairly rarefied atmosphere cranes, ducks, and geese such as the Bar-headed goose. Often cross high above the Himalayas on their journeys between northern Russia and their Migrating Snow geese on their way south from summer breeding grounds in the north of Canada. They winter along the Pacific and Mexican Gulf coasts of the southern USA.
Current migration routes probably date from the end of the ice ages, when birds came to exploit the abundant fast-growing food sources in higher latitudes made accessible by the receding ice. About half the bird species of the world (over 4,000 species) spend their summers and winters in different locations. Like other migratory birds, the Snow goose is stimulated twice yearly to undertake its journey by changes in day length. These trigger hormonal changes which result in increased restlessness culminating in departure for the first stage of its 5, (3, Ioom) flight. For fuel migrating small birds use up fat reserves, up to half the total body weight, that is laid down through feeding on the summer or wintering grounds or on feeding grounds on the route.
Winter quarters in India. Although not many have to fly as high as the summit of Everest (8,848m/29, o28ft) there have been reports of large birds of prey seen from aero-planes at this height or even higher.
The respiratory system of birds differs from that of mammals in several important ways. To begin with, birds possess a large number of air sacs throughout the body spa-cues, some even penetrating into the hollow hones. No gaseous exchange takes place through the thin membranous walls of their sacs themselves, though they may be important in preventing a bird from overheating. The importance of air sacs to breathing lies in the fact that the inspired air passes first through the posterior air sacs, then into the lungs proper, and finally out of the bird via the anterior air sacs. Air flows in one direction through the lungs instead of the “ebb-and-flow” system found in mammals.
Hence the whole volume of air in the lung can be replaced with each breath, while humans, for example, only change perhaps three-quarters of the volume with each breath, even when breathing deeply.
The blood vessels of the avian lung are very efficient in their uptake of oxygen and disposal of carbon dioxide. Since the airflows always in one direction. The blood vessels can be arranged so that blood continuously flows in the opposite direction to the flow of air. The blood which is just reaching the lungs, and which is low in oxygen, meets air that has flowed some way through the lungs and has also had its concentration of oxygen lowered; however, there is still enough oxygen there for the blood, with its low concentrations, to be able to take it up. As the blood flows along against the lung wall, it takes up progressively more oxygen and meets air that contains progressively more oxygen. This system helps to maximize oxygen uptake in a way that is impossible in a mammalian “ebb-and-flow” lung. The same thing happens, in reverse, for the disposal of carbon dioxide.
The digestive tract of birds. Too, is adapted for flight. The large, weighty jaws, jaw muscles, and teeth of reptilian ancestry have been lost (though some birds still have remarkably powerful jaws). Their function of grinding up food is largely taken over byte the muscular portion of their stomach, the gizzard. To get food to the gizzard, some birds may use their bill to tear it into small pieces which are then ingested through the wide gap and swallowed. Once in the gizzard, food is ground down often with the help of grit which the birds take in for the purpose, though this is only necessary in certain species, for example, grain eaters such as sparrows. Fish. – and meat-eating birds such as kingfishers and eagles, and insectivores such as swallows and flycatchers do not need grit; their food is comparatively soft, so they manage with. Their strong digestive juices.
Although many birds eat seeds and fruit, few specialize in leaves, as do grouse, or grass (egg geese and some ducks), at least compared with mammals. Such foods are rather difficult to break down. (In the case of many mammals, for instance, the cow, this involves the action of symbiotic bacteria in a very large, very heavy stomach.) Since the gut of birds does not contain these bacteria those that are herbivores have to con-some large quantities of material in order to extract their nutritional requirements.
Above the gizzard, many birds particularly seed caters also have a rather thin, extensible side wall to the esophagus, the crop. Into it the bird can cram a large quantity of food in a very short time so that it can retire to a place of safety to digest it. Many seed-eating birds, including finches and pigeons, also take quantities of food to roost in this way, effectively reducing the length of the night’s fast. Many species use the crop to carry food to their nestlings. Birds reduce the amount of water carried in the waste products that are to be excreted, again a weight-saving adaptation. The water is withdrawn from the contents of the hindgut. Urinary products are highly concentrated and are formed primarily of uric acid. The latter becomes mixed with the feces in the cloacae before excretion (birds have no urinary bladder). Some carnivorous species (egg owls) do not digest parts of their prey.
The reproductive system of birds also keeps weight to a minimum. For most of the year, sex organs and associated ducts are greatly reduced in size, most markedly in females. As the breeding season approaches, they rapidly develop as gametes are pro-diced. All birds lay eggs, and even the great majority that has a clutch of several eggs only lay one per day; some lay only every other day or less frequently. Belaying in this way most bird species are able to produce several relatively large eggs one after the other.
While clutch size ranges from one up to an average of 19 eggs in Gray partridges in Finland, the laying bird only carries one fully developed egg in the oviduct at a time(though one or smaller, developing eggs may be present in the ovary). If the females were to carry all the eggs at the same stage of development (as does a mama-mal her young) then the size of the individual egg would have to be much smaller or the bird would have to have fewer young.
Egg size as a proportion of adult weight varies from about T.3 percent in the ostrich to 25 percent in the kiwi and some storm petrels. One advantage of having relatively large eggs is that it shortens the development time in the nest and enables the young bird to be able to fly at an early age (I 2-14days in many small species); this, of course, tends to reduce the period of threat from predators when the young are helpless in the nest. (Large species such as swans and eagles may take 4-5 months to reach the flying stage.) Most birds lay their eggs in the early morning. Hence the female does not carry a fully-developed egg during the morning when she needs to be most active while feeding.
Most animals rely particularly on just one or two of their senses. Most mammals, in particular night-active ones, rely chiefly on their powers of smell and hearing. Even when sight is important, most mammals seem to rely less on color vision. In birds, however, the power of vision is almost always the most important sense; with hear-in second and smell a very poor third; indeed in many birds smell may be hardly used at all. In this respect humans are an exception among mammals; our senses rank in importance in the same sequence as those of birds, and furthermore we, like the birds, have good color vision.
This parallel may explain why birds are so “popular” with people. We rely mainly on On the same senses, and also a pattern of day-time activity, so can enjoy and appreciate their colors and songs, and twosome extent can “share” their lives. By con-trust we have a poor idea of what information even such a well-known mammal as the domestic dog or cat obtains from smell-in objects; we have little chance of sharing their world in this respect. If we go out into pieces of woodland, we may see lots of birds and almost no mammals, yet there may be more mammals than birds present; they are just less easy for us to perceive, for many are nocturnal, or live underground, or do both.
A bird is a high-speed aerial life. Sight and hearing are obviously more useful than smell in such conditions. The importance to birds of their eyes is reflected in their size; they fill very large portions of the skull; indeed the eyes of an eagle approach those of a human in size, although the eagle is much smaller than a human.
The eyes of birds are relatively immobile: the large eye leaves little room in the skull for muscles. However, birds have very mobile necks which enable them to turn their heads easily (egg owls), and their actual field of vision is very wide some birds may be able to see the whole 36o degrees. A bird such as a woodcock, which has eyes placed very high on the sides of its head, may be able to see both all around and over the top of its head! (To some extent, most birds pay a penalty in that the fields of vision of the two eyes only overlap a small amount, so they have only a small amount of binocular vision. Birds with forward-facing eyes, such as owls, have good binocular vision.) Birds also have a large part of this field sharply in focus at one moment, perhaps about 20 degrees compared with the 2-3 degrees across which people can focus sharply.
Most birds have good color vision, including those species of owls where it has been tested, though they may see slightly less well at the blue end of the spectrum than we do. The visual acuity of birds of prey and certain other species is perhaps two to three times greater than that of humans, but not more. Some birds, such as the nocturnal owls, have exceptionally good night vision. However, even owls probably depend largely on unhearing to locate and catch their prey at night.
Hearing is employed in communication between individual birds, and is particularly valuable in wooded areas where it is difficult to keep in visual contact hence the striking songs and far-carrying calls of many forest birds, such as the Musician wren and the bellbirds. As with vision, birds and people perceive sounds over a range that is roughly similar, though possibly birds ageless good at hearing sounds at the lower frequencies. However, the hearing of birds differs from ours in one important respect. They can distinguish sounds which are very much closer in time than we can. For example, what to us may sound like a single note may be a bird be heard as up to ion separate notes? A snatch of “simple” bird song the main fact conveys to a bird a lot more information than appears possible to our ear.
Many birds seem to lack almost entirely the power of smell, though this is not true of certain groups, such as the nocturnal kiwis, which probe for food on the forest floor and have their nostrils close to the tipoff the bill. The smell is also known to be used by the vultures of the New World (though Not those of the Old World) as they search. For carrion on the forest floor. In some other groups, for example, some of the petrels, the olfactory lobes of the brain are well developed, indicating that they too are able to use the power of smell. Taste is by no means strongly developed in all species; the sense of “taste” actually involves olfactory information in birds as in man, and, as we’ve seen, birds have poor powers of smell.
The tongue of many birds is very horn and would not easily accommodate taste-receptive cells. What taste buds do occur are found toward the back of the mouth, and birds probably only taste a food item when it is well into the mouth. Nonetheless, birds are capable of distinguishing the four tastes: salt, sweet, bitter, and sour. The sense of touch is well developed in the tongues of many birds and also in the bill tips of many species, especially those such as snipes, godwits, and curlews which probe deep in mud for their prey, and birds such as avocets, spoonbills, and ibises which “scythe” through water and soft mud with their bill open.
Patterns of Breeding
The two main senses of birds’ vision and hearing are used strikingly in courtship and breeding. Many birds establish breeding territories by the use of song. Which functions both to repel would-be intruding males and to attract potential mates? Although not all birds have songs, some of those that do produce, to the human ear the most beautiful sounds in the natural world. The nightingale and European skylark are among the most famous, but others such as the South American Musician wren are also master singers. The songs of the sonic of the non-passerine species, such as the kookaburra, are also remarkable. A few, such as the lyrebirds and the Marsh warbler, show great versatility by being able to mimic the calls and songs of many other species the cage birds best at “talking” are mynahs and parrots.
The display of the peacock is legendary, though in some ways the Argus pheasant can put on an even more dazzling perform-acne. The birds of paradise are also among the most impressive. All these species are polygamous, the males attracting females with toothier display grounds (leeks), where mating occurs the females then go off and lay the eggs, incubate them, and rear the young by themselves. Breeding in pairs almost all seabirds and birds of prey, for example. It is not understood why, but a very high proportion of the birds that are polygamous are vegetarians fruit- or leaf-eaters.
It has long been thought that the single pair feeding its young at the nest is the norm for birds. Studies in recent years, however, have shown that in many species it is notated all unusual for there to be several birds attending a single nest. The reason why this habit was overlooked originally is doubtless that most studies were conducted in temperate areas in Europe and North America. Such cooperative breeding is much more commoner in birds that live in warmer climates; in particular, it is very common among Australian species. The cooperative groups often include young from previous breeding seasons; in many species, these are mainly males. It appears that in these species the young females move from their natal territory and are accepted into other territories, young males staying at home.”
In temperate areas, birds are faced with wildly fluctuating food supplies often great abundances in summer and great shortages in winter. Numbers are cut by starvation in winter, so most of those that survive can find a place to breed in spring. By contrast, bird populations in places that are less seas-anal tend to be closer, year-round, to the limit of numbers that the habitat cans support. With fewer vacant territories becoming available. It seems to pay a young bird to remain longer within its parents’ territory though perhaps it is always on the lookout for a vacant territory nearby. Indeed it may inherit the territory when its parents die. Another factor is apparently affected by seasonal variation in clutch size. Birds tend to have larger clutches at high than at low latitudes. Again this is thought to be a reflection of the greater abundance of food available in summer in temperate areas.
Bird eggs take a considerable time to develop between about 12 and 6o days from laying to hatching. During this period, both incubating parents and eggs are very vulnerable to predation. Some birds protect their eggs and young by breeding in colonies. Others hide in the nest. In foliage. Or in a hole. Species that do not build a nest often lay eggs with camouflage markings. The simplest nest is a “scrape” made in the ground no added materials are used. Species that lay their eggs in scrapes include divers, and many game birds and waders. King and Emperor penguins do not even keep their eggs on the ground. But carry them about on the tops of their feet. Some birds, among them most petrels and Auks lay their eggs on the bare rock or the soil of a burrow. Many others, such as hornbills, some pigeons, many birds of prey, and owls, lay their eggs in holes in trees or cliffs without building a nest, though some (egg the woodpeckers, bee-eaters, and sand martins) excavate holes in trees or banks.
Other birds build highly complex nests, the most outstanding of which are the intricately woven nests of some weavers and New World blackbirds. They may have entrance tubes 1.5m (5ft) long. Other nest-building skills include those of the tailorbirds, which “sew” leaves together, and some swifts and swallows, which build saliva-and-mud nests attached to rock faces. The degree of development of the hatch-ling chick varies enormously. Some birds (generally ground nesters) hatch chicks that can fend for them straight away. Other newly hatched chicks, blind and helpless, may yet grow to full size within a fortnight. Still, others may leave the nest but remain with their parents for months.
See more: Bird Feeding Information