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.

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.

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