Overall, the birds respond to changing daylength, but weather conditions, e.g. prevailing winds, can modify the timing; ultimately the bird’s hormones drive it to eat lots of food to accumulate enough body fat to migrate.

Most but not all bird species engage in north-south movements each spring and fall; grouse-like birds, owls, woodpeckers less likely to migrate; even within migratory species, some individuals stay north in winter, especially in mild winter with lots of food and cover.

This is very difficult to study; possibly a combination of highly developed visual abilities, sensory receptors in the skin to detect vibrations from neighbouring birds’ wings,  excellent motor coordinaton, and strong reflexes are responsible.

Hummingbirds are masters at it, but other small songbirds, and even large herons, can flutter backwards if pressed to do so.

Some species like geese, cormorants and pelicans gain lift from the rising eddies of air currents whirling off the bird immediately ahead of them. This saves energy in the long run.  Formation flying also prevents mid-air collisions among large birds.

For precocial birds, they are left in the nest; for altricial birds, they are removed or eaten by the parents for calcium or crushed by the young.

Most birds cannot recognize their own eggs.  Parasitic layers like cowbirds and European cuckoos count on this inability.   Gulls sometimes even choose large, fake eggs to incubate over their own.  Most birds accept any birds that hatch in their nests and raptors will accept fostered babies of another raptor species.  Strange-looking young are often rejected by pheasants and ducks.

Only 3 percent of all birds have a penis, flightless birds and waterfowl among them; most have a grooved papilla-like organ down which the semen flows.

Chickens without access to sperm do; occasionally but rarely, members of a wild pair of birds are not synchronized, leading to infertile eggs.

Allopreening or mutual grooming is a ritualized form of agonistic behaviour that bridges the gap between aggressive attacks and sexual behaviour.

Mockingbirds and nightingales are famous for this, especially with full moons or artificial light; sometimes unpaired birds do it to attract mates.

The dawn air results in the best transmission of songs to defend the territory and/or attract a mate; the poor light is also better for singing than feeding.

Most birds with woven structures do not use them for anything other than nesting.  Some hole-nesting birds such as owls and tits do use nesting cavities for roosting.

Aromatic compounds in the leaves facilitate nest cleaning and help reduce nest-dwelling parasites; green leaves could also indicate ownership of the nest.

They roll the egg onto their feet and a thick fold of skin drops down to keep it warm.

Some birds like nighthawks, whip-poor-wills and plovers  simply deposit their eggs on the ground.  Seabirds like auks and murres lay their eggs on cliffs without nesting material. Emperor penguins have no nests

The female plays the most important role in collecting the materials and weaving the nest, but in some species, the male helps too.  Males of crows, herons, and storks do most of the material collection

Socially about 90 percent of all bird species are monogamous, but recent studies show a high percentage of infidelity in many different kinds of birds.  Copulations with sneaky unmated males and neighbouring males can lead to as much as 50 percent of the young being sired by them

Most birds return to the territory, not to a specific mate.  Some larger species have very strong pair bonds that can last as long as both birds live, but a small portion do divorce one another

They remove them from the babies to keep the nests clean and drop them into water bodies so as to avoid attracting predators to the nest area

They are highly mobile and use vocal and visual cues

Yes and no; some studies have shown  increased reproductive performance with access to feeders; large numbers of birds combined with poor feeder hygiene can lead to disease outbreaks

Omnivorous corvids such as crows, ravens, magpies and jay kill and consume other birds, especially eggs and young; grackles kill sparrows and eat their brains

Birds of prey, as well as other meat-eaters, form pellets of indigestible material, e.g. feathers, fur, bones, chitinous insect parts, and regurgitate them within 24 hours of a meal

Some use it in their stomachs to grind food (larks, doves, etc.) or remove grease (falcons).

Some use stones as ballast for diving (penguins)

No, They have hard cornified bills; some grazing or fish-eating waterfowl have lamellae to tear grass or hold slippery fish, respectively; all baby birds have an egg tooth for opening the shell, which is later lost; falcons have a primitive tooth (previously thought to be used to sever cervical vertebrae of prey).

Many songbirds do so if water is accessible. If not, they will eat snow.

Most dip their bills and tilt their heads back so that the water trickles down the throat. Doves hold their bills in the water and suck up the water.

Sometimes it provides a convenient perch, but on most occasions, the birds are feeding on ticks and other insects in the fur.

While gulls drop clams onto rocks and crows drop nuts onto paved surfaces to be crushed by vehicles, New Caledonia crows fashion hooked tools from leaves and twigs to extract grubs and larvae from cavities. The woodpecker finch of the Galapagos uses twigs, thorns and cactus spines to flush prey from holes.

There is absolutely no truth to this old person’s tale. Raptors only attack humans to defend their nests.

Some songbirds do this to rid the plumageof ectoparasites.

Vultures do this to gather the sun’s rays for thermoregulation, but cormorants and similar water birds lift their wings to drain the water from their hollow wing feathers.

The dust helps to clean off parasites from their feathers.

Their feet have tendons that lock them in a gripping position

Most birds roost or sleep in dense cover or cavities in trees and buildings; water birds, sometimes in mixed groups,  roost in their element, but some in trees; chicken-like birds on the ground or in trees

Small songbirds of various types flock together to harass hawks and owls to drive them out of the area; crows especially mob and chase owls in the daytime to ensure their safety while roosting at night

A number of birds like mourning doves and goldeneye ducks make whistling nposes when they take off.  Woodcocks make twittering musical sounds with stiff outer wing feathers and common snipe make a winnowing sound with rigid outer tail feathers

Drumming is used to scare away intruding males and attract mates.  Metal objects can maximize its volume

Almost all do.  Adult brown pelicans and  magnificent frigatebirds are mute, while adult turkey vultures and double-crested cormorants only issue grunts. Virtually all baby birds can be noisy

Much better than we originally thought, but they have considerably  less numbers of  taste buds than mammals

They are thought to be able to perceive infrasounds, i.e. extremely low frequency sounds, produced by these phenomena

Their ears are located under small stiff feathers on each side of the head; average hearing capability ranges from 5,000 hz in songbirds to 30,000 hertz in some owls

All birds have nostrils or nares, usually on their bills; while it is not their strongest sense, all birds can smell to some degree; the best “noses” are found in turkey vultures, kiwis, and petrels

Being so brightly coloured, birds actually have four visual pigments, whereas humans only have three; owls can also perceive colour, but less so due to their nocturnal lifestyle

They have one upper and one lower, plus a third one called the nictitating membrane to cleanse and protect the eye.

Due to several adaptations, their visual acuity is around three to four times better than that of humans; they may have greater sensitivity to movement; owls can see just as well during the day.

Sea birds as well as species in arid environments avoid salt overload by ejecting excess salt from special salt glands near their beaks controlled by hormones.

Non-shivering internal heat generation from intake of high energy foods, behavioural modifications such as fluffing out feathers to trap heated air, avoiding wind chill by taking cover, huddling, etc., lowering body temperature and heart rate at night, slowing down blood flow to the extremities, and countercurrent heat transfer from arteries to veins are just some adaptations.

They have many adaptations including non-hollow bones, removal of trapped air beneath the body feathers, swallowing stones or filling hollow wing feathers with water.

It takes two inspirations and two expirations to oxygenate and remove carbon dioxide from the blood for each package of air; a combination of the lungs and several air sacs achieve this bellow-like action; there is no dead-end like in mammals’ lungs.

Actually most birds are quite intelligent with large brains relative to body size; they perform well in choice tests and some have incredible short-term memory capabilities to find stored food; cervids, e.g. crows, ravens, magpies and jays, and many parrots are the most intelligent.

A system of well-developed cervical vertebrae and neck muscles allow an owl to turn its head no more than 270 degrees.

Occasionally albinism, i.e. complete loss of pigmentation, can be seen in most melanistic species. Partial albinism or leucism resulting from partial loss of pigments is more common, being caused by poor diet, trauma to the affected area, or genetic defects.

The bare areas on the heads of some species become more brightly coloured for courtship purposes; vultures and condors lack head feathers to minimize bacterial infection while sticking their heads into bloody carcasses.

James Fisher, a British ornithologist, gave an estimate of 100 billion. More recently, ornithological textbooks make it to be 300 billion.  The number is likely somewhere in between.

This depends on what book and/or list one consults.  According to BirdLife International, there 9,797 species, of which 1,186 are threatened in some way.

They are listed taxonomically with the most primitive first, e.g. ostriches, loons, and the most developed last, e.g. sparrows

Some closely related species do interbreed, especially when geographical barriers are broken down by human activities.

Occasionally but rarely a new species is found in a remote region; more often a new species is created when sophisticated genetic techniques along with morphology and behavioural differences indicate that one species should be split into two or more.

Generally, the larger the species, the longer it lives.  Captive birds usually outlive wild ones.  Most small songbirds do not make it past 2 to 4 years of age.

Bird populations fluctuate over time due to weather patterns, disease outbreaks, increased predator pressure; local populations are affected by changing land uses; birds requiring specialized habitat are declining

Only birds have feathers which originate from the skin and once grown, become dead tissue; if a bird loses a grown feather, they will often grow a new one within weeks

While many colours are caused by pigments, some from heredity and some from diet, reflection and diffraction of light resulting from the structure of feathers is responsible for the colours in some birds

In most species, the male is brightly coloured to attract mates and defend the territory, but in a few species such as phalaropes, the males which incubate the eggs and raise the young are less brightly coloured.