Cockatiels are known for the crest of feathers atop their heads. This is one sign that they're closely related to cockatoos. Along with giving them a distinctive look, the crests are good indicators of mood. Whether a cockatiel's crest is up, down or relaxed tells you how he's feeling.
Agitation or Fear
When a cockatiel's crest stands straight up on his head -- with the tips pointed slightly forward -- it's a sign of extreme agitation. Something has him very worked up or even afraid. His eyes may be extremely wide as if he's shocked, and he may look around nervously. Often, he will hold his body in a very erect posture at the same time, and he may scream with alarm. Having someone he doesn't know come too close too soon, seeing someone who appears to be threatening his favorite person or even unexpectedly loud sounds can cause a cockatiel to show his agitation with an alarmingly upright crest.
Perhaps a cockatiel's crest is erect, but his body language doesn't indicate fear, and no one or thing seems likely to be making him agitated. It may be that something has piqued his curiosity. Cockatiels are very intelligent, highly curious birds. They love to explore and, while doing so, may encounter surprising things they haven't seen before. This is why cockatiels must be supervised when they are out of their cages. Your presence and reassuring voice can keep his curiosity from turning to fear, as you calmly explain and show him what to do with the object of his curiosity.
Other Crest Positions
The cockatiel's expressive crest shows his other moods, too. A healthy, happy cockatiel's crest is slightly lowered into a more relaxed position. The tips are slightly pointed backward, and the crest itself leans back just a bit. It's relaxed, and so is he. Chances are he's eating, playing with familiar toys or preening. If his crest lowers too much, however -- flat or nearly flat against his head -- it's not a good sign. This usually signals anger, and often shows in his body language, too, as his hunches his whole body forward. The goal is to calm him down by taking a step back so he doesn't feel threatened, talk to him reassuringly and remove or stop whatever has him upset.
When a cockatiel's crest is straight up in agitated position or flattened in anger, he may also hiss to show his displeasure. Hissing is one of the cockatiel's fairly unusual characteristics among birds, and there's no mistaking its meaning. A hissing cockatiel may bite, lunge or fly away if he can. Hissing stops when his crest is back to its relaxed position, so the key is to find and remove whatever's upsetting him to return him to his happy, relaxed-crested self.