Bird cage

A bird cage (sometimes birdcage) is a cage to hold a bird. Bird cages are used in the home to contain a pet bird, such as a parakeet or a canary. Bird cages used in the home are made of metal wire. A bird cage usually has seed and water dishes, a cuttle bone, and toys such as little bells and mirrors. However, the best ones available are shredding and foraging toys, which are more expensive than the "classic" bird accessories. These keep the birds engaged and prevent boredom. There are perches for the bird to sit on. A cage may be hung from a ceiling or a floor stand. The floor of the cage is usually lined with some sort of paper to absorb the bird's droppings, water spills, etc.
When a bird cage is round, tall and narrow or made of galvanized metal it should be avoided. Round cages confuse the birds and cause behavioral issues due to their shape, narrow and tall cages don't let the bird fly properly and any type of galvanized metal can potentially cause a lethal heavy metal intoxication.
Size.
Bird cages should be at least 18x18x18 inches when it comes to small birds. For bigger species an aviary can be the best option.
Breeding birds will need a very big cage and a nest box. 
If a bird is going to be caged many hours everyday it will need at least a small aviary with a lot of shredding and foraging toys to not develop behavioral issues such as feather plucking.