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The Evolution of Birds: An Overview of Characteristics

Paper Type: Free Essay Subject: Biology
Wordcount: 5140 words Published: 23rd May 2018

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Birds are best known for their ability to fly and are unrivaled in their environment such as the skies. They are a very diverse group of animals with amazing characteristics. ”Albatrosses glide long distances over the open sea, hummingbirds hover motionless in mid-air, and eagles swoop down to capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.”(Attenborough 1998; Sibley 2010)

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As we are aware, birds are members of the vertebrates or they are vertebrates, that means they possess a backbone; humans are in the same group ”They range in size from the minute Cuban Bee Hummingbird (Calypte Helena) to the grand Ostrich (Struthio camelus). Birds are endothermic and on average, maintain body temperatures in the range of 40°C-44°C (104°F-111°F), though this varies among species and depends on the activity level of the individual bird.” (Attenborough 1998; Sibley 2010)

In connection with their vertebrate family members, they are the only ones who have feathers, and the feathers are not only for flights but also have many other benefits,such as keeping warm when its cool and cool when it is hot. That characteristic is temperature regulation and coloration for camouflage purposes. ”Feathers are made of a protein called keratin, a protein that is also found in mammalian hair and reptilian scales.” (Attenborough 1998; Sibley 2010)

Humans and other animals have complicated digestive systems, but the bird’s digestive system is simple and more efficient; this means that birds are able to eat and pass food through their body and system quickly, so that they can minimize their extra weight of undigested food and the time it takes to extract energy from their food. ”Food travels through the parts of a bird’s digestive system in the following order before it is excreted:

  • Esophagus – narrow tube that carries food to the crop
  • Crop – a sack-like widening of the digestive tract where food can be stored temporarily
  • Proventriculus – the first chamber of a bird’s stomach where food is broken down by digestive enzymes
  • Gizzard – the second chamber of a bird’s stomach where food is ground up by muscular action and small stones or grit (ingested by the birds)
  • Intestines – tubes that continue to extract nutrients from food after it has passed through the gizzard”(Attenborough 1998; Sibley 2010)

On the other hand; it is important sometimes to understand the bird’s relationship and it is a great diverse assemblage and that order found among them. The taxonomists have found in the modern birds and developed systemic method for looking at the relationship of birds. Of course, those methods were based on their morphological feature in the past, while most of their work done on today by examining of their molecular similarities among them.

Nowadays there are many prominent features that stick out, and that has been used to differentiate them from one another; the palette and the ankle bones are the primary differentiators.

Their big divisions in birds are in the Neornithes: a separation of the Neognathae and the Paleognathae; the Paleognathae are represented by large (mostly) flightless birds such as the Moa, Emu, Cassowary, and other Ratites. These birds have a paleognathan palette and premolars that are connected to the brain case. The Neognathae represent the rest of the modern birds. Their palette is much smaller and their ankles have, instead of a process on the astragalus, it’s the calcaneum that has the process(Sibley 2010)

Birds are egg laying vertebrate animal feature with wing, bipedal and endothermic (class Aves) are winged, bipedal, endothermic (warm-blooded). Statistically they are above 10,000 species making them the most varied of tetra pod vertebrates, where 120 to 130 species wiped out after human interaction.

They inhabit ecosystems across the globe, from Arctic to Antarctic. They differ from the size of 5 cms (2 inch) around 2 grams, such as Bee Humming Bird to 9 feet 155kg of the Ostrich.

Modern birds are depicted by feathers, a beak with no teeth and with capability and skill to fly unlike other animals except Ratites and Penguins. Another characteristic is the laying of hard- casing eggs, as well as the metabolic rates and a four chambered heart. Of course they are different in types but they varies from one type to another have light weight but strong skeleton and wings except flightless Moa of New Zealand which have no wings. Their wings are evolved forelimbs.

Birds come in all kinds of colors; (red, blue, white, brown, pink, yellow, green, purple and even orange), they have an exceptional digestive and respiratory systems that highly adapted for flights; they are very intelligence such as the Parrots and corvids birds’. (Attenborough 1998)

They are peculiar species that carry out long distance annual migration. Many others play shorter irregular movements. Some of them are very social, by corresponding and using visual signals, such as songs; where they participate also in the social behavior like cooperative breeding, hunting, flocking and mobbing of predators. Their foods are nectar, plants seeds, insects; some of them eat the fish. Where hawk and eagle eats the meat of other species, including the birds.

Many of them are monogamous; one breeding season at a time. Sometimes for years but rarely for life, others are polygamous (many families) and rarely polyandrous (many males).

Eggs are kept in the nest and incubate and hatched by the parents. They also have an extended period of parental care after hatching.

Finally birds are very diverse group and they have concurred almost every habitat on earth.


Birds (class Aves) are winged, bipedal, endothermic (warm-blooded) egg-laying vertebrate animals. About 10,000 living species making them the most varied of tetra pod vertebrates globally. Around 120-130 species extinct after human interaction.

They inhabit ecosystems from the Arctic to Antarctic. Extinct birds range in size from 5cm (2 inch) -2g Bee Humming bird to (9fit) 155kg ostrich. The fossil record indicate that, birds evolved from therapod dinosaurs during the Jurassic period around 150-200 million years ago. The earliest known bird is the late Jurassic Archeopteryx.

Bird anatomy

Modern birds are characterized by feathers, a beak with no teeth and with ability to fly unlike other animals except Ratites and Penguins. They lay hard-shelled eggs, a high metabolic rate because of the long distances involved in several flights. During migration body proteins are converted into fat and carbohydrates to provide enough energy since no feeding takes place in the journey. Have a four chambered heart to supply blood and nutrients to other vital body systems. Also light weight but strong skeleton (see figure last page) and wings except flightless Moa of New Zealand have no wings. Wings are evolved forelimbs. Digestive system above includes, crop- stores food taken in through its bill, gizzard- contains coarse rough sand material for grinding food since have no teeth, stomach- for digesting food into smaller particles, small intestines effects absorption of broken down food for assimilation into the body and cloacae- for expelling waste products of metabolism. Respiratory system is unique adapted for flights, inhaled air 75% passes into the air sac, stored and some taken to bones before returning to lungs; 25% of air enters the lungs directly for oxygenation and into the body. Central nervous system has cerebellum for coordinating balancing movements during flights, cerebrum for intellectual, deciding in breeding, nesting and courtship. Come in all kinds of colors; red, blue, white, brown, pink, yellow, green, purple and mixture. Parrots and corvids are intelligent birds’ species; they are observed manufacturing and using tools.

Many species undertake long distance annual migration while others shorter irregular movements. Communicate through visual signals, calls and songs. Participate in social behavior like cooperative breeding, hunting, flocking and mobbing of predators. They eat nectar, plants seeds, insects, other birds and fish.


Archeopteryx from the Tithonian stage of the late Jurassic (150-145 million years ago) is the earliest known bird and one of the first transitional fossils to be found in Bayern, South Germany. It supported the theory of evolution in the 19th Century (1861). Had reptilian characteristics; teeth, long lizard like tail, clawed fingers and feathers. Hence name “archeopteryx” which indicates Old Feathers. Preserved its wings with flight feathers identical to those of modern bird. The bones were not hollow like the modern bird. It did not have breastbone. Was not a good flyer but used three claws on the wings for climbing. It is not considered a direct ancestor of modern birds but is the oldest and most primitive known member of Aves or Avialae.



The beak, bill or rostrum is an external anatomical structure of birds. It is used for: feeding, grooming, manipulating objects, killing prey, probing for food, courtship and feeding young ones.

Diagram: The bill of a scavenger-the Griffon Vulture.


Beaks vary in size and shape from species to species. Composed of an upper jaw (maxilla), and a lower jaw (mandible). The jaw is made up of bone typically hollow or porous to conserve weight for flying. The outside surface of the beak is covered by a thin horny sheath of keratin called rhamphotheca. Between the hard out layer and the bone is a vascular layer containing blood vessels and nerve endings. The beak has two holes called nares (nostrils). They connect to the hollow inner beak and hence to the respiratory system. The nares are usually at the base of the back, near the dorsal surface. KIWIS have the nostrils at the end of the beak. Some birds the tip of the beak is hard with dead tissues and used for heavy duty tusk like cracking nuts and killing prey. In ducks the tip of the bill is sensitive and contains nerves for locating things by touch. The beak is worn down by use. Therefore, it grows continuously throughout the bird’s life.


Did you ever wonder why there are so many types of birds’ bills? You can learn about birds’ behavior and what they eat by looking at the bill. The beak is one of the characteristics used to identify birds.

Nectar feeders the hummingbirds, sunbird, Lories and lorikeets have adapted brushy tounger with bills designed to fit co-adapted flowers.

A cone shaped bill (Finches and Grosbeaks birds) – Strong beaks for cracking nuts seeds.

Thin slender, pointed beaks (Warbler birds) – insect eaters.

Long tubular bills resembling straw (Hummingbirds) – Sipping nectar from flowers.

Sharp tooth like structure on the edge (Mergansers) – for holding fish tightly.

“Hooked” beaks (Hawks, Owls) – birds of prey to bite the skull or neck and to tear body into small pieces enough to swallow.

Flat and wide beaks at the base- catch insects.

Below is common bills shapes:


Birds have different shapes, sizes of feet and vary greatly depending on the species. Generally the legs, feet and claws are structured to allow a bird to take off, land, climb and grasp with them. Birds also use feet to hold food stuffs (e.g. Hawks) they are feeding on. Loons, diving ducks, penguin and pursue prey underwater using their feet for propulsion. Others use them to fight and defend their territories from invaders with aid of claws at the distal aspect of the feet. The claws can also be used by ground birds for searching insects, food particles on the ground grass and in the soil matter. Since birds spend most of their lives perching, the feet and legs are covered with a tough skin and scales than the skin on rest of the body. In most birds the feet are light to assist in taking flights, with air sac spaces in the bone periosteum. Like the shape of the bill, the anatomy of birds’ feet tells us about the ecology of different species of birds. Bellow is examples of birds’ feet:


Feathers structurally are epidermal growths that arise in specific tracts of skin called pterylae. The distribution pattern of these feathers tracts (pterylosis) is used in taxonomy and systematic. Hence are feature characteristic of birds. They are used to facilitate flights, provide insulation that aid in thermoregulation, in display by male counterpart during courtship, camouflage and signaling. By fluffing the feathers birds defend their territories. There are different types of feathers (see above) each serving its own set of purpose. The arrangement and appearance of feathers on the body, called plumage may vary within species by age, social status and sex.

Plumage is regularly molted. Standard plumage of a bird that has moulted after breeding is called the “non-breeding” plumage. Moulting is annual in most species; some may have two moults a year. Large birds of prey may moult only once every few years. In passerines, flight feathers are replaced one at a time. The innermost primary being the first. When the fifth of sixth primary is replaced, the outer most tertiaries begin to drop. The secondary’s starting from the innermost begins to drop as well. This proceeds to the outer feathers (Centrifugal moult).

Ducks and geese species lose all their flight feathers at once temporarily becoming flightless. Before nesting the females of most bird species gain a bare brood patch by losing feathers close to belly. The skin there is well supplied blood vessels and helps the bird in incubation.

Feathers need maintenance. Birds preen or groom them daily (see figure below). Spending an average of around 9% of their daily time on this. The bill is supposed to brush away foreign particles. Beak applies waxy secretions from the uropygial glands. These secretions protect the feathers “flexibility”. Also acts as antimicrobial agent, inhibiting the growth of feather degrading bacteria.


Birds have a body plan that show many unusual adaptations compared with other vertebrates mostly to facilitate flight. The skeleton (above) consists of very lightweight bones. They have large air-filled cavities called pneumatic cavities. The cavities connect with the respiratory system. The skull bones are fused and do not show sutures. The spine has cervical, thoracic, lumber and caudal regions. The numbers of cervical (neck) vertebrae are highly variable and especially flexible. Movement is reduced in the anterior thoracic vertebrae by cushioned synovial fluid and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened. The sternum is keeled for the attachment of flight muscle except in the flightless bird orders. The forelimbs are modified into wings.


The common cavity into which the large intestines, genitals and urinary parts opens in the birds. The open end of large intestine of a bird (cloacae). The cloacae are multipurpose opening. All waste is expelled through it. Birds mate by joining cloacae. The ostrich and ducks are exception; they have a penis for copulation. Females lay eggs from cloacae. Like reptiles, birds are primarily uric telic. Their kidneys extract nitrogenous wastes from the blood stream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestines. Hence birds do not urinate. They mix solids and liquids waste together. It comes out as white paste in the waste (uric acid) almost with no water at all. Dark substance in the droppings is solid wastes. The waste comes out through the cloacae. Hence birds do not have a urinary bladder or external urethral opening. The exception is the ostrich which do urinate. However, hummingbirds can be facultative producing ammonites. Excreting most of the nitrogenous waste as ammonia. They also excrete creatine rather than creatinine like mammals. In addition other than excreting waste many species of birds regurgitate pellets.


Birds are characterized by a pair of wings, feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart and a lightweight but strong skeleton system. This characteristic features of birds have made them different than other living creature, so they as classified in a special way in taxonomy.

Taxonomically, birds are classified under Animal kingdom. Birds are further categorized into several groups such as family, genus and species according to their physiological similarities and genetic make-ups. But classification of birds is a contentious issue and it is frequently debated and constantly revised by the scientist community. The most accepted way of classification of birds is as follows:

Kingdom: Animalia

Phylum: Chordate, as they have vertebral column, i.e. backbone or spine

Class: Aves

Order: There are 23 orders; Passeriformes is the most common order which includes half of all bird species, Piciformes and Galliforms are few common examples

Family: There are 142 families of Aves. Family is basically classified under different orders. For example order Passeriformes is further divided into several families such as Meliphagoidea, Corvoidea etc.

Genus: Each family is further divided into numbers of genus according to their genetic make-ups. There are around 2,057 genus of birds.

Species: This is the smallest unit of bird class. In some cases the species is further divided into subspecies, this often occurs because birds of the same species living in a different geographical area may differ slightly. There are 9,702 species of birds.

Each unit species of the kingdom is named according to their genus and species following a system called binominal nomenclature. In this system of naming each organism is indicated by two words, the genus and species name which are basically Latin words.

For example, Crow is called as Corvous corone, where Corvous is the genus and corone is the species of the bird in the classification.


The power of flight has enabled birds to overcome barriers to dispersal, making them available in almost all parts of the terrestrial earth. They live and breed in most terrestrial habitats and on all seven continents, from southern pole to northern Antarctica. The highest bird diversity occurs in tropical regions. According to the extent of hospitability of the habitat different species are found to be dispersed or concentrated in specific areas. The distribution of birds is constantly changing due to human activities or prolonged climate alteration. Basically the dispersal of the birds occurs due to following major reasons:

Hospitability of habitat:

Birds always choose habitats with abundance of food, safety, growth and breeding possibilities. They always migrate shorter or longer distance in search of these needs. Birds show adaptation and try to cope with the change in their habitat to some extent.


Migration in birds is the common phenomenon. Mostly to prevent themselves from extreme seasons they migrate to a new place and return back to the old place in the favorable season again. Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the fall to wintering grounds in warmer regions to the south.

Habitat modification:

If sudden shock appears to the natural habitat of bird they are forced to disperse to the new hospitable habitat. The shock can be related with availability of food, security and reproduction possibility. To prevent the change that happened to the ecosystem birds migrate to a new place in search of safe life. This can be permanent or temporary. They have apparently been aided in colonizing new habitats after they migrate from older habitat. Habitat modifications also include many of activities that can make habitats less attractive to birds. Thinning or cutting of vegetation to remove protective cover can discourage birds from roosting. Most deciduous trees can withstand removal of up to one-third of their limbs and leaf surface without causing problems.


Birds have evolved into creatures that use a variety of methods to meet the challenge of providing the next generation. Courtship and mating of birds is the first step of reproduction in birds. Courtship and mating practices are very common behaviors that all bird species show. The sequence and variety of courting behaviors vary widely among species, but they typically begin with territorial defense and song followed by mate-attraction displays, courtship feeding, and selection of a nest site.

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In most of bird species adult birds generally return to their nesting grounds each mating season. A male claims a territory by singing a distinctive song. He then sings a song that attracts a female. Birds have different courtship rituals. Some use song, while others display colorful plumage. The female finds her male partner. Now this follows the process of reproduction: pair comes together, mate, and builds a nest.

All birds are dioecious with internal fertilization. Dioecious are those species whose one member can produce only one type of gametes or reproductive cells. Birds are oviparous. Oviparous animals are animals that lay eggs, with little or no other embryonic development within the mother.

Birds Adaptation

To survive and reproduce, all living organisms must adjust to conditions imposed on them by their environments. An organism’s environment includes everything affecting upon it, as well as everything that is affected by that organism. Conformity between an organism and its environment constitutes what biologists call adaptation.

Birds, from these entire organisms, can adapt depending on the environment.Different species of birds have developed different types of wings, beaks, and feet to adapt to their lifestyles. These adaptations help birds live in their habitats and carry out their feeding methods in the most efficient way possible. In this section we presented three different environmental adaptations of Birds that are desert, forest and water.4

Desert Birds

Like the other creatures of the desert, birds come up with interesting ways to survive in the harsh climate. There are many birds that can live in the climate of the desert. Some birds will migrate and stay only awhile in the desert but others will remain year around as they are adapted to the harsh conditions of the desert. There can be intense heat and a lack of water in the desert so birds have adapted to these conditions (Starnes, Dorothy, 2002)5

Some birds are able to take protection from the intense desert heat as they can dig underground and have a place to be in a lower temperature (Starnes, Dorothy, 2002). Others will take over already inhabited nests to escape the heat. Others just do not have a home anywhere and do not migrate as they cannot fly successfully to leave. Birds will get in the shade if possible, under rocks or brush to escape the intense heat especially if they do not have a good capacity for being able to sweat. There are birds that just do not have to drink as often as others. Species of birds are able to obtain water from their diet. Some birds need to quench their thirst more than others. Since birds have this flying ability they are able to fly long distances to obtain water when necessary even in the desert.

Some examples of desert birds are:-

Insect hunting bird is the pale crag martin (formerly Hirundoobsoleta) lives in MiddleEast and Africa (From Wikipedia, the free encyclopedia) that survives well in the desert. When the wind blows insects are blown against brush and rocks and this bird is able to live from the insects.

The roadrunner (Geococcyxcalifornianus) is probably the most well-known desert bird. Roadrunners are so named because they prefer to run rather than fly. Lives in the desert of the North American southwest or Mexico and is a large, black-and-white ground bird with a head crest, long white-tipped tail and an oversized bill (Wikipedia, the free encyclopedia and Starnes, Dorothy, 2002)and their youngest depend on walking to find food and water.

(This is a roadrunner. During the day, roadrunners hide in the bushes to keep cool)

Forest Birds

Forests are home to many birds of prey like hawks, eagles, and vultures. Vultures are seen virtually everywhere in the tropics because they feed on the remains of other creatures. Vultures may seem ubiquitous, but many birds of prey are threatened by habitat destruction and hunting as pests. One of the best examples is the Mauritius kestrel, is the only bird of prey in Mauritius and still the rarest falcon in the world (Schirf, Diane L., 2000)6

(This is Mauritius kestrel, the rarest falcon in the world)

The golden eaglealso a good example of forest birds and one of Switzerland’s biggest birds of prey, with a wing span which can go up to a little more than 2 meters (over 7 feet). It feeds mainly off ground-living birds and mammals, especially hares, marmots and foxes. They have excellent eyesight: it has been shown that they can see a hare at distance of one kilometer (more than half a mile).

Water Birds

Aquatic environments provide critical habitat to a wide variety of bird species. Some aquatic birds divide their time between aquatic and terrestrial environments, while others spend most of their lives in water, returning to land only to breed. Many familiar bird groups are aquatic, including gulls and penguins as well as recreationally important species such as ducks and geese. Diving birds is one example of water birds. It describes a broad group of species that occupy waters deeper than wading species. These birds dive, plunge, or swim after fish. Wading birds occupy shallow-water habitats in both fresh-water and saltwater environments.3

The American widgeon is a common marsh duck which spends much of its time in deep water. It is nicknamed “bald pate” because the male has a white stripe on its head.

Mountain Birds

”The Mountain Bluebird (Sialia currucoides) is a medium-sized bird weighing about 2-5 ounce, with a length from 15-20 cm (6-8 in). They have light underbellies and black eyes. Adult males have thin bills are bright turquoise-blue and somewhat lighter beneath. Adult females have duller blue wings and tail,fake grey breast, grey crown, throat and back. The Mountain Bluebird is migratory. Their range varies from Mexico in the winter to as far north as Alaska, throughout the western U.S. and Canada. Northern birds migrate to the southern parts of the range; southern birds are often permanent residents. Some birds may move to lower elevations in winter. They inhabit open rangelands, meadows, generally at elevations above 5,000 feet.” (From Wikipedia, the free encyclopedia)


A habitat “is an ecological or environmental that is inhabited by a particular species of animal, plant or other type of organism. It is the natural environment in which an organism lives, or the physical environment that surrounds (influences and is utilized by) a species” (Wikipedia, Free encyclopedia). In this section we discussed about habitat fragmentation and habitat selection.2

Habitat fragmentation

As the name implies, it is the emergence of discontinuities (fragmentation) in an organism’s preferred environment. Habitat fragmentation can be caused by geological processes that slowly alter the layout of the physical environment or by human activity such as land conversion, which can alter the environment on a much faster time scale (Wikipedia, the free encyclopedia, the encyclopedia of Earth). Habitat fragmentation reduces the size of patches of forest, shrub land, wetlands and grasslands. A consequence of this is to reduce the total area of neighboring habitat available to birds and increases the isolation of the habitat. It also leads to an increase in “edge” habitat that is successfully exploited by a variety of predators that eat bird eggs and young (Campbell, Mike and Johns, Mark).1

Study in Europe and North America confirms that decreasing of reproductive success and food supply observed for the reason of interspecific nest-site competition in relation to wood patch structure. Parental time and energy budgets may also be adversely affected by increased exposure to poor weather conditions in small woods. “Birds in small English woods bred later than pairs in large woods, possibly due to microclimatic effects on vegetation development and invertebrate availability” (HINSLEY, S. A. et al., 2006).

Habitat selection

Habitat selection is the process or behavior that an animal uses to select or choose a habitat in which to live.Birds appear to choose habitats to which they are well adapted in terms of resource exploitation. The selection have many factors, such as landscape structure, can influence exactly how “ideal” and “free” animals are while moving through a landscape and selecting habitats, interference and water were found to be the most influencing factors.One of birds’habitat selection in relation to forest edges, with an emphasis on breeding populations,distinguishing between four main primary causal factors which operate at the local-scale: (1)species-specific differences in resource and patch use, (2) biotic interactions, (3) microclimaticmodification and (4) vegetation structu


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