Introduction to Birds

There are three groups of animals which can fly; bats, insects and birds.

Birds are homoithermic and their fore limbs are modified into wings and they have skin covered in feathers.

Birds have the power of flight and they are all oviparous i.e. they lay eggs. The skull is extended forward into mandibles which make a beak.

Like fish, amphibians and reptiles, birds have a nictitating membrane over their eyes.

Birds are similar to reptiles in having scales on their legs.

Like reptiles, birds' excretory product is the dry and solid uric acid crystals which conserves water in the bird's body.

Like mammals, birds have a four-chambered heart with two ventricles and two auricles.

Like reptiles, fish and amphibians, birds' red blood cells are ovoid and have a nucleus.

Shape of birds:

Birds have small and light bodies, which are an advantage in flying.

Some birds like the ostrich and the emu which have very large bodies have lost the ability to fly.

The body of birds is streamlined i.e. narrow at both ends and broadest in the middle. So birds move in the air with least effort.

All the feathers point backwards so that they are not ruffled by the wind during flight.

The ostrich can reach a height of 9 feet Pictured here are ostriches with young in Namibia Africa.

Feathers:

This is the single external feature, which distinguishes birds from other vertebrates.

Feathers are produced by the skin, which has no sweat glands.

Feathers insulate the body keeping the temperature constant and they also repel water.

The water-repelling properties of the feathers are got from the secretion from the oil gland at the hind end of the bird. This oily secretion is got by the bird preening itself, reaching the oil gland with its beak and smoothing the feathers down.

Having very little weight and a large surface area, the feathers adapt birds for flight.

The quill feather:

A quill feather has a central rod which starts as a quill and ends up as the rachis or shaft.

The rachis has a flat expanded surface called a vane, which is made of barbs, which are arranged, on either side of the rachis. Each barb has two rows of smaller branches called barbules.

Barbules of adjacent barbs overlap and hook into each other's dents or ridges.

This arrangement makes it possible for a bird in flight to re-arrange any ruffled feathers by merely preening itself with the beak.

The quill feathers attached to the carpo-metacarpus are called primaries and those attached to the ulna-radius are called secondaries.

Other types of feathers include:

Covert (contour) feathers that cover the body and are similar to the quill feathers.

Downy feathers that are found between the covert feathers and also on young birds of preccocial birds. Downy feathers have no interlocking barbs.

Filoplumes that are found near the body and have slender threadlike shafts. They are the most widespread feathers on a bird and their barbs are only found at the end of the shafts.

A Contour Feather

FLYING:

This may be by flapping wings or by gliding without flapping the wings.

A bony ridge along the breastbone blue

allows birds to remain stable as they fly.

Gliding flight occurs when the bird tilts the wing so that the lift forces acting on the bird are equal to its weight.

Flapping flight is achieved by the pectoral muscles which cause a downward stroke and upward stroke of the wing.

Down stroke of the wing:

The pectoralis major which is attached to the lower side of the humerus contracts.

The pectoralis minor which is attached to the upper side of the humerus relaxes.

The wing is pulled downwards.

There will be air resistance on the wing giving the wing an upthrust.

The upthrust lift will be transmitted from the wing through the coracoid bone to the whole body.

So the whole body will be lifted.

Upstroke of the wing:

The pectoralis minor contacts and the pectoralis major relaxes.

The wing is pulled upwards.

The joint between the humerus and the radius-ulna is bent to reduce air resistance on the wing.

Steering in the right direction is achieved by the bird applying unequal strokes for the wings.

Tilting of the wings and the tail acts as brakes for the bird.

One birds flight as seen in a tern species.

Features which adapt birds for flight:

The fore limbs are wings with a very wide surface area provided by the feathers.

They have large pectoral muscles for depressing or raising the wings.

They have a deep keel from the breast bone (sternum) for attachment of the pectoral muscles.

They have a coracoid bone for transmitting the lift of the wings to the whole body.

They have a rigid skeleton giving a firm framework where only few bones move in the body.

The skeleton has hollow bones which reduces the weight of the bird.

Their bodies are relatively small and light.

They have an efficient vascular and respiratory system where the lungs have extensive air sacs which store air when the bird is in flight.

A bird fluffs his feathers to stay warm keep from losing energy and to maintain homeostasis.

REPORDUCTION IN BIRDS:

Reproduction in birds follows a general pattern:

• Courtship and pairing
• Nest building
• Mating and egg laying
• Care of the eggs
• Care of the young.

Courtship and pairing:

Courtship is the means by which a male and female select each other for the task of reproduction and rearing young ones.

Many birds develop special colours and feathers in order to attract the other sex for example weaverbirds and turkeys.

Some birds rub one another's beaks and develop special dances and calls.

Nest building:

One of the pair or both of the birds construct a nest which may be built from grass, leaves, feathers, mud, sticks or it may even just be a hole scraped in the ground as the ostrich does.

In altricial birds the nest is built above the ground in trees. The young get no problem falling off the heights since they are born blind and the height gives them further protection from possible land predators.

In precocial birds the nest is constructed on the ground. The young are active and on the ground they do not stand the risk of falling off tree heights. But on the ground the young stand a risk of being preyed upon.

Mating and egg laying:

The male mounts the female and applies its cloaca on to hers and passes sperms into her oviduct where fertilization occurs i.e. internal fertilization.

Generally one egg is laid each day until a final batch is laid. In some birds only two eggs are laid.

Care of the eggs:

Eggs are kept warm by the adult birds sitting over them i.e. incubation.

In some birds only the female incubates for example in hens; while in others only the male and yet in others both parents share the task for example in doves.

When incubating eggs birds develop bare places on their undersides i.e. brood patches and so warm blood comes near these patches to keep the eggs warm.

Incubation is important because:

- It protects the eggs;

- It keeps the eggs warm;

- Reduces evaporation from the eggs.

The incubation period of a hen is 21 days, of a duck is 31 days and that of a weaver birds is 13 days.

Care of the young:

After hatching, the young are fed and looked after by the parents.

In altricial birds which lay eggs above the ground like weaver birds and pigeons, the hatched young ones are naked, blind, helpless and are fed by the parents regurgitating partly digested food into their beaks.

In precocial birds which lay eggs on the ground like the fowls, pheasants and quails, there are downy feathers on the young on hatching and the young can see, walk and pick seeds soon after hatching.

So there is parental care in birds; with young ones responding to calls of the parent by taking cover or seeking her out.

However, there is one bird where there is no parental care in itself - that is the cuckoo.

THE EGG:

The egg of birds is much larger than that of mammals because birds' eggs develop externally and have to contain enough food reserves for the developing embryo to last it till hatching when the young can feed.

Egg yolk

Functions of the parts of an egg:

The shell protects the egg from damage and is porous enough to allow gases diffuse through.

The shell membrane separates the shell from the egg membrane.

The egg membrane protects the albumen and the chalaza from being damaged.

The air space found at the blunter end of the egg supplies air to the germinal disc.

The chalaza is formed as a result of rotation of the egg as it passes down the oviduct causing a twisting of the albumen. It suspends the germinal disc inside the yolk in proper position.

The germinal disc becomes the embryo which develops into a young bird.

The albumen is the source of proteins and water for the developing embryo.

The egg yolk is a source of protein and fats for feeding the embryo until it hatches.

A one week incubated egg

The amnion is a fluid-filled sac for buoyancy of the embryo providing a shock absorber.

The allantois stores the embryo's excretory products.

The chorion carries out gaseous exchange by absorbing oxygen through the porous shell.

The umbilical stalk connects the embryo to the yolk sac and the allantois.

The embryo later develops an egg tooth which it uses to break the egg shell during hatching.

BEAKS AND FEET:

Beaks are shaped according to the way the bird feeds.

Omnivorous birds which eat anything like crows have strong and pointed beaks.

Flesh-eating birds like eagles and owls have hooked beaks.

Seed-eating birds like weaverbirds have strong, short and conical beaks.

Insect-eating birds like the swallows and swifts have short beaks with a very wide gape.

Nectar-eating birds like the sunbird have slender and slightly curved beaks.

Fish and worm-eating birds like the water heron have long and strong beaks for piercing into mud and searching for worms and fish.

In many birds the feet are also adapted to the way of feeding.

Fresh-eating birds like the eagle and the owl have strong, sharp claws or talons for catching and holding prey.

Birds that feed on food in water like water ducks have webs between the toes for swimming.

Breathing in birds:

In birds there is no diaphragm. Birds have large air spaces called air sacs joined to the lungs by tubes.

These air sacs are found in the thorax, abdomen and each of the humerus bones.

These air sacs send air to the lungs for gaseous exchange which takes place in the capillary bronchioles.

The air sacs are especially helpful during flight.

But gaseous exchange never takes place in these air sacs; only in the capillary bronchioles of the lungs.

Why Birds are Important

We are probably most familiar with birds as food. Around the world, people consume chicken, turkey, and even more exotic birds, like ostriches. Can you think of other ways that birds are important?

• In agriculture, humans harvest bird droppings for use as fertilizer.
• Chickens are also used as an early warning system of human diseases, such as West Nile virus. Mosquitoes carry the West Nile virus, bite young chickens and other birds, and infect them with the virus. When chickens or other birds become infected, this warns humans that they may also become infected in the near future.
• Nectar-feeding birds are important pollinators, meaning they move the pollen from flower to flower to help fertilize the sex cells and create new plants. Many fruit-eating birds help disperse seeds.
• Birds are often important to island ecology. In New Zealand, the Kereru and Kokako are important browsers, or animals that eat or nibble on leaves, tender young shoots, or other vegetation. Seabirds add nutrients to soil and to water with their production of guano.
• Birds have important cultural relationships with humans. Birds are common pets in the Western world. Sometimes, people act cooperatively with birds. For example, the Borana people in Africa use birds to guide them to honey that they use in food.
• Birds also play prominent and diverse roles in folklore, religion, and popular culture, and have been featured in art since prehistoric times, as in early cave paintings.

Lesson Summary

• Birds are warm-blooded.
• Adaptations for flight involve features that are lightweight, flexible, strong and that take advantage of air currents.
• Reproduction usually involves a courtship display, nest production, egg laying, incubation and parental care.
• With 10,000 bird species, there is a lot of diversity. Specialized structures are adapted for specific habitats or living requirements.
• Birds are important economically, ecologically, and in human culture.

Review Questions

Recall

1. List three traits which are important for flight.

2. Give an example of how a bird's breeding system is adapted to avoid predators.

Apply Concepts

3. Explain how the absence of land predators on islands results in flightlessness in birds.

4. A large bird that nests with other birds has pale eggs even though the environment is brown and the eggs stand out to predators. Why have these birds not evolved camouflaged eggs?

Critical Thinking

1. You detect the presence of antibodies to the West Nile Virus in young chickens. How did the chickens get the virus? What does this mean about human West Nile infections?