He's probably yawning.
Yawning is a perfectly normal action for a bird. What is not normal is if the cockatiel is constantly opening its mouth wide. It can be a sign of distress. It could be having difficulty breathing, or be in pain. If the bird is behaving differently from normal, it should be taken to a veterinarian.
A whale
It's a bottle with a wide opening for easy pouring. It is a canning jar, of pint, quart, half gallon, etc., size with an round, screw-top opening that is approximately 3 inches in diameter.
No, they have a very narrow opening for a mouth, as they eat plankton.
These are commonly known as Mason jars. Mason is a brand, but there are several others. It is a heat resistant glass jar that allows for a metal screw on ring to be used as a lid. The come with different sized opening. The wide mouth has a larger opening than the standard.
a wide mouth pitcher. what is your facorite color?
A wide-mouth bottle is a bottle with a larger-than-normal opening at the top. These bottles are used to store chunky liquids like stews.
With a smaller mouth opening the air moves faster and 'wind chill' cools your hand.
Alligators bite by opening their mouth and then closing it so fast that it will 80% kill you.
it is the snake it is the only animal to do that!
Chew gum or just keep opening your mouth really wide and close it.
The spaces between the bars are probably too wide for a cockatiel. It would be able to get through them.
A wide mouth earthenware jar is a large container made of clay that has a broad opening at the top. It is commonly used to store and preserve food, liquids, or other items due to its durability and ability to maintain a consistent temperature inside. The wide mouth makes it easy to access the contents and clean the jar.