There are many things to consider when buying a cage for your bird.
How big is the bird? How wide is its wingspan? How tall is the bird? Is it a very active bird or a more sedate bird? Will the bird be spending all its time in the cage or does it get a lot of 'out' time?
A large bird who is allowed plenty of out' time to play will not mind a smaller cage, so long as there is plenty of headroom and lots of stretching room. The bird's head must not touch the top of the cage. When the wings are spread, be sure to allow for a few inches on either side so that the wing feathers are not crushed against the bars. The bars must be close enough together so that the bird can't get its head through and become stuck. There should be space for at least two perches. The perches should be staggered so that the bird has a comfortable choice. Don't position one perch directly over the other or the lower one will become soiled.
If the bird will be spending a major part of its time in the cage, or if it is never allowed out to play, then the cage needs to be larger. Provide as roomy a cage as possible. Think about it-- you wouldn't want to spend all your time in an area no larger than a closet! Don't make your bird live in a cage that is too small either. Make sure there is plenty of room for the bird to climb around, stretch, hang upside down, flap its wings, and play. You will need to be sure there are lots of places to attach toys and that the food and water cups are easily accessible.
If you have small birds, such as canaries or finches, keep in mind that these little birds are extremely active. They love to hop around, play, swing, and bathe. Most birds of these types don't get much time outside for exercise so their cages must be large enough to allow them to expend their energy. Make sure the bars aren't spaced so far apart that they can escape. Provide a shallow bathing container for them--these birds love to play in the water! Also fasten swings to the top bars, toys and treats to the sides, and give them a comfortable nest basket for sleeping. An indoor aviary is a wonderful choice for these birds and can be an attractive addition to your home.
Parakeets, conures, lovebirds, and cockatiels are all smaller versions of the large parrots. They have hookbills just like the big guys and enjoy chewing and playing. Provide them with chew sticks or toys they can throw around. Their cages need to be roomy enough to allow for lots of play space. If you are keeping more than one bird per cage, make sure the cage is large enough for each bird to have some private' space. The same rules for bar spacing in the big bird cages apply to the smaller birds. Supply plenty of perches, water and food cups, and toys.
Before you purchase that pretty, pagoda-shaped cage, keep your bird's comfort in mind. Does the cage allow lots of free space for flying around and comfortable perch spacing? Round cages are not good choices--there are no corners for the birds to escape to when stressed. And cages with slide-out bottoms are easier to clean than ones with sloping sides around the bottoms. Outside access panels for water and seed cups are convenient, too, especially if your bird is a biter.
Keep your bird's personality and needs in mind when shopping. If in doubt about a certain cage, ask someone who owns a cage-like it what they think. How does their bird-like the cage? Is it easy to clean? What, if anything, about the cage, do they dislike? By asking lots of questions and knowing your bird's habits, bringing home a new cage can be a rewarding experience for you, and most especially for your bird!