Chick is the name for any baby bird.
Hatchling is a very young, new baby bird.
Nestling is a baby that hasn’t left the nest yet.
Fledgling is a baby that has left the nest but isn’t weaned yet.
Fledglings are also called a juvenile.
Gosling is a baby goose.
Cygnet or flapper is a baby swan.
A pullet is a young female chicken.
A cockrell is a young male chicken.
A baby pigeon/dove is a squab or squeaker.
A baby duck is a duckling or flapper.
A baby eagle is an eaglet.
A baby hawk is an eyas.
A baby owl is an owlet or howlet.
Poult is a baby turkey (also baby chicken).
Cheeper or squealer is a baby quail.
Bappie is the unofficial name that birdkeepers call baby birds.