When the nest has been prepared, the birds mate so that the eggs are fertilised and the chicks will start growing. Unlike mammals, birds only have one opening as the exit hole for body fluids, and for reproduction. The opening is called the cloaca. A female bird, called a hen, has two ovaries, of which the left one usually produces eggs.
Most male birds have no sex organs that can be seen. But inside the male are two testes which produce sperm which is stored in the cloaca. Birds mate by rubbing their cloacas together, although with some birds, particularly large water birds, the male has a sort of a penis inside the cloaca.
Once the hen has mated, she produces fertile eggs which have chicks growing inside them. She lays the eggs in the nest. There might be just one egg or a number of them, called a clutch. Emus might lay as many as fifteen huge dark green eggs in a clutch. After the eggs are laid, they are incubated, or kept warm so the chicks form inside. Most birds stay together for the whole nesting season, and one advantage is that the work is shared. Many birds take turns sitting on the eggs, so that each adult can feed.
This is not always the case. With emus, the male does all the sitting and all the baby-minding. With emperor penguins it is also the male that cares for the egg. There is only one egg, which he keeps on his feet and under his feathers, standing in a big group of males without feeding until the chick is hatched. While the eggs are hatching, the females are at sea, feeding, so that they can care for the chicks when they return.
Some birds put the eggs inside or on top of the mound of leaves and twigs. The mound acts like a compost heap. The decomposition of the rotting leaves causes the temperature to rise. This is heat released by the chemical action of bacterial and fungal respiration. It is the same reaction as that which keeps mammals and birds at a high temperature. The parents leave the mound. When the chicks hatch, they are able to feed themselves.
Many small birds take 2–4 weeks to hatch eggs. Albatrosses take 80 days. During this time the female loses a lot of her body weight.
The quickest hatching time is for the cuckoo. Some types of cuckoos take only 10 days. This means that when they hatch in the nest of their <nowiki>"foster parents"</nowiki>, the eggs that the parents have laid are not yet ready. Newborn cuckoos are naked, blind and ugly, but they are strong. They get under any eggs that are in the nest and throw them out before they hatch. That means that the cuckoo has the whole care of both parents. Baby cuckoos grow fast and often get bigger than the parents who feed them.
When baby birds hatch, in most types of birds, they are fed by both parents, and sometimes by older aunties as well. Their mouths are open all the time and are often very brightly coloured, which acts as a ''releaser<nowiki>"</nowiki>, a trigger which stimulates the parent to feed them. For birds that eat grain and fruit, the parents eat and partly digest the food for the babies. It is then vomited carefully into the baby's mouth.
Many birds, particularly those that mate for life, are very sociable and keep together in a family group which might be anything from 4 or 6 adult birds and their young to a very large flock.
As chicks grow they change the fluffy down that covers them as babies for real feathers. At this stage they are called fledglings. Other family members may help care for fledgling chicks, feeding them and protecting them from attack while parents are feeding. When the fledglings have their new feathers, they come out of the nest to learn to fly. In some types of birds, like pigeons, the parents watch over this and as the young ones get stronger, will give them flying lessons, teaching them how to glide, how to fly in spirals and how to land like an expert.
Most birds are social animals, at least part of the time. They communicate to each other using sounds and displays.
Almost all birds make sounds to communicate. The types of noises that vary greatly. Some birds can sing, and they are called songbirds or passerines. Examples are robins, larks, canaries, thrushes, nightingales. Corvids are passerines, but they do not sing. Birds that are not songbirds include: pigeons, seagulls, eagles, owls and ducks. Parrots are not songbirds, even though they can be taught to sing human songs.
All birds make noises (<nowiki>"bird vocalisation"</nowiki>), but not all sing. Songbirds are passerines, many of which have beautiful melodic songs. Songs have different functions. Danger cries are different from territorial songs and mating calls are a third type. Fledgling may also have different calls from adults. Recognition calls for partners are quite common.
As to where the song comes from, there are three kinds of species:
Most singing birds that are kept as pets, like canaries, have several tunes and some variations.
The same species of bird will sing different songs in different regions. A good example of this is the currawong. This is an Australia bird which is like a black and white crow. In the autumn, families get together in large flocks and do a lot of singing. Currawongs from some areas sing much more complex songs than others. Generally, currawongs from the Blue Mountains are the finest singers. The song of the currawong can be sung as a solo, but is often performed as a choir. One bird will take the lead and sing "Warble-warble-warble-warble!" All the other birds will join in and sing "Wooooooo!". When all the birds know the song, the choir will sing the "Warble" part and the soloist will sing the "Woo!". The song changes from year to year and from place to place.
The Austrian naturalist Konrad Lorenz studied the way in which birds communicate, or talk to each other. He found that each type of bird had a number of sounds which they made automatically, when ever they felt a certain way. Every sound had an action that went with it. So, if the bird was frightened, it acted frightened and made a frightened sound. This told the other birds around it that something frightening was happening.
If a flock of birds were flying over a field, they would be calling "Fly! Fly!" But a hungry bird, seeing something good to eat down below might start calling "Food! Food!" If other birds were also hungry, they would make the same call until more birds were calling "Food! Food!" than "Fly! Fly!". At this point, the mind of the flock would be changed. Some of the birds would start to yell "Fly downwards! Fly downwards!" as they sank from the sky, until the whole flock was all noisily calling the same thing.
These communication sounds are often short hard sounds like: chirps, squeaks, squawks and twitters. Sometimes the calls are longer and more musical. They include the "Rookety-coo" sound of a pigeon and the "Cockadoodledoo!" of a rooster. The bird cannot change these sounds. They always make them in the same way. The bird is locked into making each sound every time a particular idea comes into its head. The connection between how they feel and how they call is innate: they are born with it. Some calls in some species are learnt. Then, it is the tendency to learn which is inherited.
Konrad Lorenz noticed that when birds sing, they often use a lot of their regular calls as part of the song. Lorenz had a flock of jackdaws which were scattered during World War II. One day, an old bird returned. For many months she sat on the chimney singing her song, but in the song she kept making the call which Lorenz knew meant "Come home! Come home!" One day, to the great surprise of Lorenz, a male bird flew from a passing flock and joined her on the chimney. Lorenz was sure that it was her long-lost "husband" who had found his way home at last.