How to Tell the Difference Between a Cockatiel and a Cockatoo
To tell the difference between a cockatiel and a cockatoo, take note of the bird's coloring. If it's mostly all the same color, like black, white, pink, or gray, you're probably looking at a cockatoo, but if it's colorful with patches of green, yellow, red, or brown, it's likely a cockatiel. Additionally, look at the length of the bird's tail since cockatiels have long tails that are about half the length of their body. Cockatoos, on the other hand, have shorter tails. You can also tell the difference based on the bird's size since cockatoos are generally around 26 inches tall and cockatiels are usually only around 13 inches tall. However, a young cockatoo may be smaller than that, so avoid relying on size alone.
Look at size. First, consider the size of the birds. Cockatiels are generally much smaller than cockatoos. Cockatoos do range in size, as they come in a variety of types, but can grow up to 26 inches tall. Cocktails, conversely, are usually only 13 to 14 inches tall. If you're dealing with a smaller bird, it's probably a cockatiel. However, a young cockatoo may be smaller if he is not yet full grown. Therefore, do not rely on size alone. Pay attention to other aspects of the bird's appearance to determine if he's a cockatoo or a cockatiel. Pay attention to coloring. Cockatiels and cockatoos also differ in terms of color. Examine the bird's color patterns to help you tell if you're dealing with a cockatoo or a cockatiel. Cockatoo's colors do vary somewhat by species. However, nearly all cockatoos are mostly solid in color with a few small patches of different colors here and there. Most commonly, a cockatoo's base color will be black or white. A few breeds of cockatoo may have pink or gray as their base color. If you're dealing with a larger mostly even-colored bird, you're likely dealing with a cockatoo. Cockatiels are far more colorful in appearance. In the wild, cockatiels are gray with white patches on their wings and green and yellow patches on their tails. Cockatiels bred in captivity come in a wide variety of colors that do not occur in nature, having patches of reds, browns, and yellows throughout their bodies. A smaller, more colorful bird is likely a cockatiel. Examine the tail. Cockatiels also have a specific type of tail. If you're still unsure if a bird is a cockatiel or cockatoo, this may help you clarify. A cockatiel's tail is very long, making up about half of the bird's length. When a cockatiel flies, its tail will spread out like a fan. Listen to the sounds the bird makes. In terms of behavior, the noises a bird makes can help you identify a cockatiel versus a cockatoo. Pay attention to how your bird expresses itself vocally. Cockatoos are generally louder. They talk more and may imitate words you say frequently. Cockatiels have softer, squeakier voices. They tend to make bird-like sounds more often than they talk. When they do speak, their voices are often harder to understand than a cockatoo. Consider social needs. In terms of social needs, cockatiels and cockatoos are very different. Between the two birds, cockatoos are far more social. They require a lot of time out of the cage with their owners and may become stressed if left alone too long. A cockatoo will be openly affectionate of his owner, wanting to be physically close a lot. Cockatiels, while friendly with people, are less socially needy. They're content to be alone for longer periods. See if the bird imitates household sounds. Both cockatoos and cockatiels can imitate household sounds, like the noise of a car outside or the sound of a phone ringing. However, cockatiels may be slightly more likely to do so. However, as both birds do imitate noises, consider other factors first when trying to differentiate between a cockatoo and cockatiel. Give the bird puzzle toys. Cockatoos are known to enjoy puzzle toys. These are toys in which a bird has to dismantle some kind of contraption in order to get a toy or a treat. A cockatoo may be more interested in a puzzle toy than a cockatiel. However, like imitating household sounds, this is not the best means of identification. Consider a bird's appearance, voice, and social needs first. 
Look at size. Pay attention to coloring. Examine the tail. Listen to the sounds the bird makes. Consider social needs. See if the bird imitates household sounds. Give the bird puzzle toys. 