All about birds

Learning about birds is a leap into the fascinating lives of earth’s most magical wildlife — the only animals with feathers and the only ones, except for bats, that can fly! But, birds aren’t just for show — they’re critical to our planet, and to humans. Here’s why: 
More than 900 species in the world are important pollinators of wildflowers. Some birds are solely responsible for the propagation of valuable trees. Birds are relevant to farmers because they eat harmful crop pests. Scavenger birds eat massive amounts of roadkill, which slows the spread of disease and helps keep our highways cleaner.

Birds and their eggs are food for carnivorous wildlife, and for humans. And, the existence and fitness of birds in an ecosystem is related to the health of the ecosystem itself, an early-warning system if there’s trouble afoot.

Birds are an economic stimulus here and abroad, and especially to developing countries. Hotels, restaurants, national parks, and other entities important to birders depend on seasonal influxes of tourists. Birding also encourages sales of equipment, such as binoculars, maps, and guidebooks.

And, there’s more! Birds are good for the human soul. Their beauty and song, their cheery jauntiness, the freedom of their flight, and spirited activities combine to draw us gently into oneness with nature. Wherever we are — sitting on a porch, walking in the woods, or staring out an office window — we’re transported to a calmer state of mind. Literally. Studies show that watching birds reduces stress, depression, and anxiety, and increases cognitive ability.

So, from where did these extraordinary animals come? Would you believe…dinosaurs? A dinosaur fossil found in 2005 had two eggs inside it. The embryos show feathers and wings, elongated arms and a long list of other similarities to birds. In 2006, five well-preserved bird fossils, discovered in China, were found to be the oldest ever found. They date back at least 110 million years, to the age of dinosaurs. The fossils are missing their heads, but they’re physically adapted to diving and swimming and were probably duck-like.

Recently, there’s been more fossil evidence linking birds to dinosaurs. So, this would seem conclusive; however, some scientists strongly believe birds are related to reptiles, because of certain similarities there: both birds and reptiles have scales, lay eggs and have some similar anatomy.

Scientific classification

Birds are animals that belong to the scientific class Aves (A-veez). Aves contains about two dozen orders of birds. All birds that exist today belong to the sub-class Neornithes (nee-OR-nuh-theez). They differ from their ancient ancestors, in part, by lacking any teeth. There are about 165 families of birds, divided into two superorders: Palaeognathae (pay-lee-OGG-nuh-thee), which are mostly flightless, like ostriches and emus, and Neognathae, which includes all other birds.

The largest order of birds, Passeriformes (passer-uh-FOR-meez), represents more than half of all species. These are the perching birds, like warblers, wrens, cardinals, finches, woodpeckers, and others we enjoy seeing in our yards. There are about 10,000 species of birds in the world, most of them in the tropics.

Physical characteristics

Bird shapes and sizes are highly varied. They range from the tiny Bee Hummingbird (Mellisuga helenae), a native of Cuba, that’s about 2-1/2 inches (6.35 cm) long, way up to the Ostrich (Struthio camelus), which stands 9 feet (2.74 m) tall and can weigh up to 346 pounds (156.8 kg).

The smallest bird in N.A. is the Calliope Hummingbird (Selasphorus calliope), about 3 inches (7.6 cm) long. N.A.’s largest flying bird is the rare¹ California Condor (Gymnogyps californianus), which has a wingspan of 9 feet (2.74 m) and may weigh up to 20 pounds (9.1 kg). The largest N.A. waterfowl is the Trumpeter Swan (Cygnus buccinator), with a wingspan of 6.6 feet (2 m). They can weigh up to 28 pounds (12.7 kg).

Black and white drawing of three eggs side by side.

Comparison of the egg size of the smallest bird to the largest, left to right: hummingbird, hen, Ostrich. (The New Student’s Reference Work, 1914 / Wiki; PD)

Birds are similar to other animals in that they’re warm-blooded, have two eyes, two ears, and their bones are strong and well-muscled. But the features of some bird species differ from each other based on their lifestyle.

Some examples: Herons and swans have a long neck for dipping their head deep into water. Many shorebirds have long legs adapted for standing in water while they “fish.” A penguin’s body is tapered at both ends for streamlined swimming.

Eyes: A bird’s two eyes fit very tightly within their sockets. This restricts their ability to move up and down and back and forth as a human can do. Consequently, if a bird wants to look at something, say, to its right, it can’t just shift its eyes that way; it must turn its head. The placement of their eyes in a bird’s head varies, depending on the species. The eyes of some birds are located more toward the sides. Birds of prey and others that need binocular vision for judging distance have eyes situated in front.

Birds need excellent vision to survive — an eagle, for example, reportedly can see food on the ground from a mile high. Birds can also see colors, some even into the ultraviolet range. Owls and other nocturnal birds are an exception; they can see only a limited range of colors (hunting in the dark doesn’t require the need to see many colors).

Bill or beak: Bird bills (also called beaks) are highly varied. Their shape and sharpness are clues to what a bird eats. For example, a Cardinal’s heavy bill is designed for cracking seeds. Hummingbirds have a long, narrow bill for sipping nectar from tubular flowers. A woodpecker’s bill is strong and tapers to the tip, for chiseling trees. Mergansers have a serrated bill with a hooked tip, for grabbing and holding fish. A mallard’s bill has fringed edges for straining seeds and plants from mud and water.

Hearing: Most birds lack visible ears, but they have excellent hearing. A Barn Owl, for instance, reportedly can hear a rodent rustling about from half a mile away. Birds hear through a tube which leads from each side of the head to internal structures much like that of humans and other animals. The tubes are covered by tiny, unique feathers designed to cut down on wind noise while allowing sound waves to pass through. Some diving birds have strong feathers that cover their ear-holes to protect the inner ear from water pressure. The hearing range of birds is similar to ours, which is why they sing to each other in a range we can hear.

Some birds have tufts of feathers that give the impression of ears.

Ear tufts of a Eurasian Eagle-owl, Bubo bubo sibiricus, with ear tufts standing up.

Ear tufts of a Eurasian Eagle-owl, Bubo bubo sibiricus. (Carlos Delgado wiki cc by-sa 4.0)

Smell: Birds have two nostrils, called nares (NARE-eez; singular: naris), located at the base of the top bill (mandible). Nares are used for breathing and smelling. Until recently, it was thought most birds couldn’t smell very well. A 2007 study, however, shows that many if not most birds have a well-developed sense of smell. They use it to help navigate, find food, and identify individual birds.

Feet: Birds walk on their toes. Most have four, with the first one facing backward and the other three facing forward. The spacing of the forward arrangement and their length varies depending on the species. Some birds, like chickens, have a fifth toe that has evolved into a spur, used for defense. Waterfowl have webbed toes. This allows them to paddle in water with ease, like using an oar — they spread their toes wide apart for the backstroke and pull them together for the forward stroke. Some birds use their toes as fingers for holding their food.

Color illustration of birds' feet, comparing that of a coot, mallard, hawk, woodpecker, grouse, ostrich, parrot, jacana and crow.

Many different kinds of bird feet. (Uwe Gille / Wiki)

Have you ever wondered why perching birds don’t get blown off tree limbs by strong winds or fall off while they sleep? Two reasons: First, their opposable toes allow them to grasp their perch front and back. Second, a system of tendons in the legs makes the feet lock automatically onto a perch until the bird releases them. Here’s how it works: When a bird perches, it bends its legs. This movement automatically tightens flexor tendons that run down the bird’s leg to the toes and causes them to curl, thereby clinching it tightly to the perch.

Feathers: What would you guess is the most unique characteristic of birds? No, it isn’t their wings, as you might think — insects and bats also have those. It’s their feathers — no other animal in existence today has feathers.

Feathers are made of a protein called keratin. Flight feathers are strong, but light. They’re constructed to give lift and precise control to the bird. The center part, the shaft, is the stiff linear structure from which the feathers sprout. The calamus, hollow and featherless, is the part of the shaft closest to the bird’s body (this was used as a writing instrument in olden days and called a quill.) The feathery parts of feathers are called vanes. Each vane is made up of barbs and hooks called barbules, which hook together to create a strong, but flexible, structure.

Not all feathers are for flight. Some lack hooks and are located between other feathers. Some are hair-like and have sensory organs at the base. Down feathers are for keeping birds warm. They lack hooks, so the rigid structure we see in flight feathers dissolves into a soft, fluffy mass filled with air pockets that insulate the body. Many birds are featherless when they hatch, but some, such as ducklings, already have down covering their body.

Feathers are water-repellent. What makes them so? First, an oil secreted from the “preen gland” (technically, the uropygial gland), located near the base of the tail. Typically, a bird rubs its head or beak in this oil and then uses its head to spread it over the feathers. This action introduces a second factor, an electrostatic charge to the oil, which further insulates the feathers. This is so effective that ducks can float comfortably in near-freezing water.

Close up of the waxy-looking red tips of the gray and white feathers of a Bohemian Waxwing bird.

Bohemian Waxwings have waxy red tips on their wing feathers. (Amphis / Wiki; CC)

A bird’s feathers — the arrangement and color — taken all together, are called plumage (PLOO-mij). Feathers may be for showing off, or not: While some species sport bright colors and beautiful patterns, others dress for camouflage.

Often, bright coloration is seasonal, which is particularly noticeable with ducks. Male Mallards, for example, have bright plumage during the breeding season but change their appearance drastically afterward. They molt (shed) into dull colors that more resemble the females. The male American Goldfinch makes a noticeable transformation, too. In spring and summer, he’s a striking bright-yellow-and-black. Heading into fall, his body feathers molt into a dull color similar to that of the female

Almost all birds have a complete molt at least once a year.  Some molts are partial, with only some feathers replaced. Molting occurs for a couple of reasons: First, feathers wear out, especially those of birds that migrate. Second, breeding season calls for birds to put on their courtship plumage. 

Mandarin Duck, Aix galericulata, floating in water.

Mandarin Duck, Aix galericulata. (Katja / Pixabay; PD)

Coloration: Pigments and feather structure are responsible for giving birds their colors.

Pigments are chemical compounds within a feather that absorb all wavelengths of light except the color we see, the one that’s reflected back to our eyes.  Reflected colors are reds, oranges, yellows, and browns. Absorption of all color wavelengths produces black. Because of pigments, the Northern Cardinal is truly red.

Other colors are from the feather structure itself. When light strikes them, they refract into a scattered light, as a diamond does. Blues, greens, and violets are refracted colors. So, we see blue on a Blue Jay when, in fact, their feathers are actually brown. If you find one of their feathers, you can prove it to yourself by dunking it in water or crushing it to break up its structures. Or, cast a shadow on it, as this photographer has done:

Close up of a Blue Jay feather. Part of it is in the light and appears to be blue. A shadowed area is brown in color.

Blue Jay feather. Notice the blue disappears where there are shadows on it. (Steve Snodgrass / Flickr; cc by 2.0)

Iridescent colors, like on the throat of a Ruby-throated Hummingbird, are the result of feather structures, as well as films or air bubbles on the surface that cause reflected colors to cancel each other out.

Birds have color vision, and many depend on their own colorful plumage to catch the eye of a potential mate. Feather color, however, does more than just that. Their colors provide camouflage. For example, many ground-dwelling birds are a dull color. Nestlings are always a dull color, so they can stay well-hidden in their nest. Rock-dwelling birds are usually gray. With some species, the male is a bright color while the female is duller.

With hollow bones that are light, but strong, and powerful pectoral muscles to move their wings, birds are built for flight. Air sacs in their body help keep them lightweight and easy to lift into the air, and a high metabolism keeps their energy at peak levels.

Black-and-while illustration of a bird's wing structure with descriptive labels.

Bird’s wing structure. (Shyamal / Wiki; cc by-sa 2.5)

Their wings consist of many structures in common with humans, including a scapula, humerus, ulna, and radius. The outer part of the wing is composed of modified hand bones. 

Smaller birds take a little jump and push their wings downward. Some large birds have to jump from a tree branch, a cliff or run several steps to gain some speed before leaping into the air. 

When birds lift upward, the air above their wings compresses and pushes downward, but the air pressure below them lowers, which gives them lift. They move forward by essentially “rowing” through the air: The wings are wide open to catch as much air as possible on the down stroke, but partly folded on the upstroke to reduce resistance. The wings can change shape during flight and the feathers can be expanded to control airflow.

Birds use their tail feathers to provide control, much like an airplane’s rudder. Thermal air currents (warm air moving upwards) help birds to fly higher or to glide. See an animated description of how birds fly here

Unless they’re migrating, birds fly below 500 feet (152 m); to go higher makes a greater energy demand. Their typical speed is about 15 miles per hour (24 km/h), but they can go faster. Ducks can fly at 60 mph (96.5 km/h), and a Peregrine Falcon can reach speeds of 100 mph (161 km/h). When they’re migrating, birds generally fly at about 20 to 30 mph (32 to 48 km/h).

Exactly how migrating birds know where to go is still being researched. What scientists do know is that birds have an inborn sense of direction, fostered by the earth’s magnetic forces. They also learn from their parents. They can use their eyesight to follow geographical landmarks. And, some seem to use the sun or wind direction to help guide them.

Why are birds willing to put themselves through the hardships of migration? They do it for food. Many birds feed on insects, which aren’t around in the barren, cold months of winter. Some species also eat nectar and, of course, flowering plants are dormant. Birds may migrate for other reasons, too. Sometimes their food sources dwindle, say, during a drought, which forces them to move temporarily or permanently. They may also migrate in search of better nesting sites. For birds that migrate long distances in the spring and fall, it’s the length of days that tells them when to begin.

Migrating birds fatten up ahead of time to help carry them through their journey. They also begin to form into flocks. They wait for the right weather conditions, and up they go. Some fly at night (most songbirds do), some by day. Migrating birds usually fly at heights between 1,500 and 6,000 feet (457 and 1828 m). But, some fly higher: Bar-headed Geese have been recorded flying at more than 29,000 feet (5.5 miles; 8.85 km) high!

Some birds fly incredibly long distances, too. The Ruby-throated Hummingbird, weighing only 1/8-ounce (3.4 g), makes a 600-mile (965.6 km), non-stop flight across the Gulf of Mexico between the Yucatan Peninsula and the southern U.S. coast twice a year. The Arctic tern migrates over 18,000 miles round trip between the Arctic and Antarctic every year.

Not all birds are long-distance champs. Some only relocate a few miles farther south in winter, just far enough to find food. Migrating birds generally fly at 20 to 30 miles per hour (32 to 48 km/h). 

Migration isn’t without its hazards — enormous ones. Tall buildings, power lines, windmills, and automobiles are just some of the threats. Collisions with window glass alone are estimated to kill at least 98 million migrating birds every year. Some drown when they are overcome by exhaustion while flying over large bodies of water. All this, and predators to worry about, too, including hunters.

Migrating birds may change their pattern. For instance, Canada Geese, which formerly flew north in the summer to Canada, now stay in some southern areas year-round. Global warming may be a partial explanation,  but also humans are feeding the geese in winter. That and the proliferation of neighborhood ponds is providing these birds a resort-like atmosphere. It’s like, why bother to leave?!

U.S. birds typically fly one of four main routes because of geographical constructions that either help or impede their progress. These include mountains, coastlines, waterways, and deserts.

Not all birds of a migrating species will migrate; some remain year-round in one general area. Birds that forage in flocks are less likely to migrate because groups are more likely to find food than one individual. Backyard birdwatchers consider spring and fall migration to be best times of the year. They can look forward to watching many different species as they stop to rest for a day or two on their way to somewhere else.

Food Sources  
Depending on the species, birds eat seeds, nuts, fruit, nectar, insects, fish and other sea creatures, reptiles, amphibians, other birds, mammals, carrion. If it’s organic, one bird or another will eat it. Not in my tree! Woodpeckers that guard their stash.

Swallowing and digestive system 
Some birds use their bill to crush or break open seed shells, while others use it to tear food apart. Either way, birds don’t have teeth and don’t chew their food. 

Birds don’t swallow like other animals, because they lack a soft palate. The soft palate is a soft but muscular tissue located on the roof of the mouth toward the back, and it’s needed for swallowing. Many other animals, including humans, have one. But birds are forced to compensate. They do so by tilting their head up, so that food and water move into their throat to be swallowed. This may be most easily witnessed at a birdbath, where you can see them fill their bill with water and then tilt their head. A bird’s mouth produces a lot of saliva, which helps to glue some foods into a ball for swallowing.

You’ve probably heard of a bird’s “crop.” It’s a muscular pouch located at the end of the esophagus of some, but not all, birds. Here’s how it comes into play: 

To avoid predators, birds feeding in the open often eat very fast so they can dash back to a safe place. If they eat more food than their stomach can hold, the extra remains in the crop. The crop serves as a chamber for storing and softening food until the food already in the stomach moves on through the rest of the digestive system. The crop is developed most highly in birds that eat seeds or vegetation. (In pigeons only, the crop produces a milk-like product that the female uses to feed her new hatchlings. Pigeons are the only animals other than mammals that do this. The same hormone, prolactin, controls its production.)

Illustration of a bird's digestive system

Bird Digestive System (Erik Beyersdorf / Wiki; cc by-sa 3.0)

The crop leads to a two-chambered stomach. One chamber, the proventriculus, secretes stomach enzymes for breaking food down. Food passes from there into the other chamber, called the gizzard (ventriculus). The gizzard is very muscular and grinds up the food. If you’ve eaten a chicken’s gizzard, you know just how tough it is. Basically, the gizzard takes the place of teeth and is necessary for breaking down tough seeds and fibrous vegetable matter into a digestible form. Some species of birds swallow small stones, which helps in the grinding process as they scrape against the food. As you might guess, birds that eat softer foods have weaker gizzards.

Birds of prey, like hawks and owls, tear apart and eat the entire body of their prey. That includes everything: the meat, organs, bones, teeth, feathers, and fur. Sometimes they swallow their prey whole. Any leftover matter still in the gizzard after several hours is formed into a pellet and eventually expelled through their mouth.

The rest of the digestive tract is similar to mammals, except for the cloaca (klo-A-kuh) which lies at the end of the large intestine. The cloaca serves as a collecting chamber. All bodily waste products, as well as semen and eggs, discharge through it. Probably all of us have seen birds poop (often where we don’t want them to!), but we never see them urinate. That’s because a bird’s body is very conservative with water. Instead of combining water with waste material to form urine, as mammals do, birds process it into a whitish powder or paste called uric acid. That’s why bird poop is whitish.

Songs and calls
There’s a difference between a bird’s “call” and its song. A call is a few simple notes used for non-sexual communication. Calls are used to express territory ownership, to warn of predators, keep track of one another, to show aggression, and distress. Many birds have several alarm calls, at least one of which warns of flying predators. Young birds call for food, migrating birds make calls to keep the flock together. Birds can distinguish the songs of individual friends and family from that of outsiders.

Nearly half of the birds in the world don’t sing. All of those that do belong to the order Passeriformes. Their songs are an additional means of communication. They’re more complex than calls and often lovely, specially designed to entice a desirable mate. 

Singing is triggered by the approach of warmer weather and longer days. A series of hormonal reactions in their brain tells (mostly) males that it’s time to burst into song and to show off the best they’ve got. Singing is usually in the early morning and late afternoon. Birds’ songs have pretty much stopped by the time nesting season is over.

Birds have a unique vocal organ, called a syrinx (SEAR-inks), located just where the trachea branches into each of the lungs. Half of the syrinx sits at the top of each bronchus and is capable of producing sound waves as passing air vibrates them.

Each side can make its own sounds. Depending on the species, some birds produce rising notes with one side while simultaneously singing falling notes from the other side. Some birds sing high notes from one side and low notes from the other. Some birds alternate sides, one note from one side, then a note from the other.

Birds learn to sing, in the same way that human infants mimic adults and learn to form specific sounds. If, for instance, a Northern Cardinal grew up listening only to chickadees, it would sing chickadee songs. Baby birds refine their songs over the course of about a year. 

Many birds sing only one or two songs, while others have quite a repertoire. Some wrens have over a hundred different songs. Mockingbirds, which are well-named, can sing over two hundred songs, many of them tunes “stolen” from other songbirds. Mockingbirds aren’t the only mimes. Starlings, crows, Mynahs, and parrots are some of the many other birds that mimic birds, as well as human voices and other sounds.

You’ve probably noticed birds singing from a perch at the top of a tree. They commonly do this so their song will carry far and wide. Some birds sing while flying.

Body language

Birds communicate with each other and other animals in several ways: with sound, but also through coloration, posture and body movements. For example, birds may fluff their feathers in the presence of a predator to make themselves appear bigger and too tough to mess with. The Killdeer builds her nest on the ground and tries to lure predators away from it by acting as though she has a broken wing, drawing attention away from her eggs or nestlings.

Nests and cover

All birds lay eggs, but not all birds build a nest; some lay their eggs directly on the ground or in a hole. The Burrowing Owl nests in the old holes of Prairie Dogs, ground squirrels or the like. Woodpeckers chisel a hole in a tree limb or trunk, a process that may take a couple of weeks. Nightjars lay their eggs directly on the ground. The White Tern, a seabird, lays her eggs on a bare limb. (Imagine trying to keep it there, warm and safe, for five weeks!) The Emperor Penguin male incubates an egg by holding it between the top of his feet and his warm belly.

Four tiny hatchling birds, Eastern Bluebirds, huddled together in their small brown nest with eyes still closed.

Eastern Bluebird nestlings. (© Carolyn Russell)

Usually, the female does the nest-building, but males often help. Most birds build a new nest each time they mate, but a few use the same nest for years. Where the nest may be built varies, but always the goal is to camouflage it as well as possible for the safety of the eggs and babies. With some species, the female selects the site. With others the male does, or both sexes do. Nest building gets better with practice.

Nesting materials may include grass and other vegetation, twigs, yarn, feathers, cobwebs, insect parts, flowers, cotton, hair, mud and even miscellaneous human-made items that catch a bird’s fancy.

The number of eggs laid (called a clutch) varies. Mourning Doves, for instance, lay two. House wrens usually lay three. The Red-bellied Woodpecker usually lays four eggs. The Blue Jay lays up to seven. Mallards lay up to 13 eggs. Songbirds usually lay one egg a day, generally in the morning. Some larger birds lay them more erratically, like one every few days. 

Once all the eggs have been laid, incubation begins. The eggs must be kept warm at all times if they’re to survive. The female sits on them around the clock, taking only short food breaks. With some species, males help by taking turns. With others, the female incubates, and the male stands guard over the nest when she leaves it to eat. Some males bring food to the female. Sandpiper males incubate the eggs and take care of the nestlings. The same is true of ostriches and some other large birds.


Hormonal signals in the brain give males and females the itch to find a partner beginning in early spring.

Males tune up their most beautiful arias in hopes of tickling the fancy of lovely females. But that’s not all there is to do. They also must wear their most beautiful plumage and perform visual displays better than all the other males. Depending on the species, they might have to strut, jump up and down, inflate their chest, flap their wings or bow. Cranes perform elaborate dances for their females. Some males bring a gift of food. Others try to impress by building several nests for an intended to choose from.

Females are picky; they don’t put up with just any old yokel. There’s much at stake; they want partners that’ll father handsome, hearty, smart offspring. But once the deserving males have won their prize, it’s time to get down to family business. 

Pairs often show each other “tenderness” by touching bills, snuggling together or preening each other. Mating occurs any place the couple finds suitable — on the ground, in trees or shrubs or, as with some waterfowl, in water. Swifts and swallows mate in the air. Some birds mate for life, but most bond for only the breeding season or part of the season. With some species, pairs form long before the spring breeding season.

Mating: Birds don’t have external sex organs, except for male waterfowl and the large flightless birds. Females can lay unfertilized eggs, but that, of course, won’t result in offspring.

So, here’s how eggs get fertilized: Males produce sperm in their testes, just like all other male animals, but it’s stored in their cloaca (the opening at the end of their body). The male climbs onto the female’s back, she moves her tail to one side, and they briefly rub their cloacae together. 

Male White-winged dove perched on the back of a female, with the rear part of their bodies touching.

White-winged Doves mating. It only takes a second or two. (Patricia Pierce / Wiki; cc by 2.0)

You may have noticed birds mating, with the male forced to relinquish all dignity as he slips and slides and flaps his wings to keep from falling off the female, which he inevitably does. It seems like a futile effort. But all it takes is a second or two of what’s called the “cloacal kiss” for his sperm to transfer to the female. The sperm swim up a tube inside the female called the oviduct, where they penetrate the yellow yolk of her eggs, which are already moving down from her ovaries.

From there, the fertilized egg continues slowly down the oviduct. During its journey, the yolk, which is a soup of nutrient-rich food for the developing chick, becomes surrounded by egg white. This is a protective material made of albumen, a protein. 

The eggshell is made mostly of calcium carbonate and develops over a period of about 20 hours. There’s much variability in eggs, including the thickness of the shell and its color. Birds that lay their eggs in visible locations, such as ground-dwelling birds, have cryptically colored eggs to camouflage them, but some birds (like chickens, for instance) lay all-white eggs.

Incubation and hatching: The incubation period of most passerine birds is 12 to 14 days. Developing baby birds initially get oxygen through membranes inside the egg that channel air from pores in the shell to their body. When babies are ready to hatch, they start breathing with their lungs for the first time. This means they need an air supply while they undertake the laborious task of chiseling their way out of the shell. Conveniently, there happens to be an air pocket waiting for them at the round end of the egg, behind a membrane. They break a hole in it to release the air. (This air pocket is the hollow spot we see when we peel a hard-boiled egg.)

Chicken egg. (KDS4444 _ Wiki; cc by-sa 4.0)

Now they must break their way out of the shell before they run out of air. With the aid of a specialized, strong muscle on the back of their neck and a sharp, bony bump on the tip of their upper bill, called the “egg tooth,” they make their strenuous, time-consuming escape. It takes a long time, and the parents seldom help. If a baby can’t make it out, it’ll die. Once out, the tooth later drops off, and the muscle shrinks.

The young of most ground-feeding birds are covered with down when they hatch. As soon as they dry they start following their parents around and feeding on their own. These birds are referred to as precocial (pre-KO-shul). Birds that nest in trees or a hole have a shorter incubation period, and the hatchlings are blind and naked. They’re called altricial (al-TRISH-ul), and their parents feed them until they’re fully feathered and can fend for themselves. 

Altricial parents carry high-protein foods, nuts, and insects, to their babies, both while they’re still in the nest and for several days after. It’s a busy time for these parents (or parent, as the females of some species raise their young alone). They must feed themselves and two or more offspring several times a day. Babies leave the nest (fledge) usually in two or three weeks when their feathers and wing bones have grown a sufficient length for flight. Often, though, they hop out of the nest before they’re quite ready.

You might see fledglings hopping around on the ground in a fluffy coat of camouflage-colored feathers. Their anxious parents are nearby — even if you don’t see them, they see you. Some parents issue a stern warning to leave their fledglings alone by scolding, dive-bombing or pecking at intruders. (By the way, unless you’re sure a fledgling has been abandoned, it’s best to leave it alone. An exception would be to gently and quickly move it under the nearest shrub for protection from predators if you’ve found one fully exposed in the yard.) Proper rescue, care for injured, orphaned wildlife

The fledglings will follow their parents around, mouths wide open, wings fluttering madly, in a pitiful, begging pose. “Feed me, feed me, like you’ve always done before.” The parents do just that for several days while their offspring learn the ropes — how to find food, what to eat, what a predator looks like and techniques for survival. Eventually, sometimes after quite a long time, the parents begin to ignore them or even chase them away when they beg.

Birds live a long time if they survive predation. The USGS Patuxent Wildlife Research Center has records of banded wild birds that have later been caught again, and they’ve been able to track the minimum age of numerous species from the original date on the bands. (The bird is always at least a little bit older than the date it was banded.) You can see their list right here. There’s a Common Grackle listed at more than 23 years old, a 15-years-old Northern Cardinal² and 16-years-old Blue Jay, among many other perching birds. A Whistling Swan is listed at more than 25 years. The champ is a 66-years-old wild Laysan Albatross named Wilson. You can read about her here

Birds live in all kinds of places: Mountains, forests (pine only, deciduous only or mixed), prairies, grasslands, and farmland. Also, seasides or rocky, barren shores, marshes, lakes, ponds, and rivers. Some spend most of their time on the open ocean. Or in deserts, where the Cactus Wren lives on, and even nests within, cacti. Some birds are very urbanized, living in our backyards, and willing to be watched by us. Some are just travelers, passing through on their way to other places; we may never spot them when they use our yards as way stations. Some are shy and will do their best to hide from humans in any habitat they occupy.

On the ground or in trees, birds are prey for any carnivore that can catch them. In nests, the babies are prey for other birds, such as Blue Jays, as well as squirrels, raccoons, and others. Bird eggs are savored as food by snakes and other animals. Sometimes the eggs are deliberately broken by territorial birds or tossed out of the nest by Brown-head Cowbirds. Birds are also prey while flying, targeted by birds of prey and hunters. Each year, millions of birds die during migration, from flying into window glass, skyscrapers, towers, electric lines and wind turbines. Collisions with cars kill millions more.

*Source: 2016 U.S. Fish and Wildlife Service report 
¹As of 2016, 276 Condors live in the wild — in California, Arizona and Baja California, Mexico. One hundred seventy are held in a captive breeding program. Source: U.S. Fish & Wildlife Service Condor Count 
²Seven states have named the Northern Cardinal as their state bird. That’s more than any other species

More reading about birds:

Is your yard a flyby?  
Choose a birdhouse with these best features in mind   
Thes are the birds of  ‘The Twelve Days of Christmas’ song  
Bird gone bald!  
These hummingbirds weigh less than a nickel, migrate 1,000s of miles