Purchase the largest bird cage, suitable for the species of bird you own, that you can afford and comfortably accommodate in your home. A pet bird needs enough room to fully extend and flap her wings inside the cage. Add some square footage for bird toys, extra dishes, several perches, a swing perhaps and maybe even a sleeping "tent.” Assemble a large cage in the room where it will be used, as it may not fit through the door otherwise!
Any bird cage should be user friendly. Consider some horizontal bars to facilitate climbing. Analyze the shape to accommodate long tails or, in the case of small, flighted birds, room to fly to and fro. Outside access feeders are optimal for large birds, especially when bird sitters or timid family members are temporarily in charge.
There's no such thing as a self-cleaning bird cage.
Sorry to dash your hopes on this one! Although mess management features like angled cage aprons help direct cage fallout into the bottom tray, you'll still need to wipe droppings and flung food from perches, cage bars and often from the surrounding area. Many chores, such as washing dishes and replacing tray paper must be done daily, but other tasks, like washing the bottom grating, can be done weekly. Develop a cleaning routine that works for you. Be consistent. Eliminate small messes before they become big ones. Keep necessary items, like an enzyme based poop cleaner and a roll of paper towels handy for quick clean-ups.
Most bird cages don't last forever, but a quality cage will last a really long time!
Check for smooth welds and sturdy construction. Painted or "powdercoat” finishes are quite durable, but stainless steel is the ultimate "lifetime” material. Consider the bottom line: Removable gratings make maintenance easier and keep birds out of discarded food and other debris. Plastic bottom trays won't deteriorate from moisture and are usually not as heavy as metal trays. On the other hand, plastic may crack if the tray is dropped.
Your bird won't really be traumatized by moving to a new bird cage.
Most birds adapt readily to new habitats. If you've purchased a new cage, appropriate for the size and strength of your pet, and the bird seems reluctant to claim it, don't push. Instead, set up the cage near the existing one so that the bird can get used to the sight of it. Stock it with a familiar toy or two along with food and water. Place the bird on top of the cage for a period of time each day, leaving the door open so that it can go inside at will. In most cases, the bird will overcome its trepidation very shortly.
Replacement parts really are available (for cages made by major manufacturers) ...
… but they may be difficult to find. Purchase replacement perches and dishes when you buy a new cage, especially if a unique size or shape is required. Although retailers do not routinely stock a full line of replacement parts like trays, gratings and seed guards, they can often order them for you. Delivery may take some time, as distributors don't always have the needed parts in stock. If you're frustrated in your attempts to obtain replacement parts, contact the manufacturer directly.
Some people find the word "cage" offensive, noting that "jail" is often listed as a synonym. One couple refers to their bird's home as its "room" or "apartment." Some other euphemisms for cage are: habitat, environment, nest, enclosure, space, furniture or coop.
Want to learn more about bird cages?
History of Pet Bird Cages
Set Up Your New Pet Bird Cage