Nymphicus hollandicus, otherwise known as the Cockatiel, parents altricial young. Anotherwards, the babies are helpless and funny looking, whereas others, e.g. ducklings, can immediately follow their parents around hunting for food, and are miniature versions of the adult. Cockatiel babies, in the first few weeks, are mostly tiny pink blobs with skinny necks. But how do we get to that point? How do we start with two feathery pets and end up with a nest of down covered bubble gum wads? First, we need to look at mating behavior.
Cockatiels of at least 18 months can be bred together. Although they are sexually mature younger, it's a bad idea to breed animals as soon as they are able. Many things can go wrong, and it's just not worth it. Even dog owners don't breed their bitch in her first heat.
Assuming that the cockatiels are proper age, and have been introduced properly, you should begin to notice mating behavior nearing their breeding season, which is April thru December. A telltale sign of imminent breeding is the male plucking the back of the female's neck. A few feathers missing is okay, but watch your male and make sure he doesn't pluck her bald. If he becomes aggressive during this time and just plucks her constantly, you will need to separate them. In some cases, it's the opposite way around. The female will pluck the male. The birds are indicating that they are ready to begin breeding, and need a nest box. If you don't want them to breed, separate them.
Sitting close to one another and preening each other are also indications that they are fond of one another. You will notice the male frequently tilting his head down so that the female is obliged to scratch his head. The male also likes to "nuzzle" with the female: it is at this point you may hear a few shrieks as the male snatches a feather or two from his mate's head.
Now the entertainment begins. The male cockatiel gets a lot noisier during breeding season. There will be times when you remember the "good old days" when his out of tune bawling was much more pleasant than the racket he makes now! The male attempts to "impress" the female in a variety of ways. Loud whistling, singing, and plain shrieking are apparently all very attractive to the female cockatiel. The male will stretch out his wingspan so that it can be admired from every angle. He ruffles himself up and preens himself constantly, in addition to the female. He bangs his beak on perches, wires, toys, anything to show his girlfriend he's a birdly bird. He runs back and forth with his wings puffed out just a bit from his body. His crest reaches new heights of importance (particularly after mating takes place!). My own bird followed his girlfriend everywhere, and if he couldn't get there by foot, he flew there, and loudly shrieked in her ear at the audacity of her leaving him! The couple becomes inseperable, and become extremely disgruntled (by way of shrieks and whistles) when split apart.
So you're sitting quietly, reading a good book, when you hear a variety of unusual noises coming from your birds' house. Sounds like you need to go get a nestbox. The male climbs on top of the female, and carefully angles her tail off to the side. He then proceeds to rub their vents together. The female makes a constant chick-chick-chick sound, while the male chirps every few seconds. (My bird says "Hello", so it's pretty obvious when they're copulating) Then the male climbs down and they continue normally as if nothing happened. Birds can "mate" like this several times a day; mine mated four times a day up until the first egg was laid. A sturdy, appropriate sized perch is needed in the cage for this explicit purpose.
One "session" of copulation will fertilize the female's eggs for about a month. If this is the first time a pair has bred, don't expect fertilized eggs. It happens, but there are many reasons why eggs aren't fertile or don't hatch, far too many to go through on this page. Egg laying generally takes place 1-3 weeks after mating. During this time, you need to carefully watch the female. If she appears to be crouching on the cage floor most of the time, and if her vent area is distended, she may be egg bound. It is an extremely serious problem that must be dealt with by an avian vet as soon as possible.
The female will generally lay one egg every two days, until her clutch size is 3-6. These numbers and days can easily vary, because they are generalizations. She will start to sit on the eggs usually after the third one, which will cause the first three to be hatched usually a day apart. The eggs will be incubated for 18-21 days.