The amount of space should be large enough for the birds to fully stretch their wings, and allow for some flight, but also for your birds to hop, climb, feed, socialize, and exhibit other natural behaviors. Long-tailed species, such as parakeets, cockatiels, and macaws, will need housing that will accommodate their tails as they move around. If a cage is too small for these long-tailed species, the tails may become damaged. Adequate-sized housing is important not just for physical health, but birds in cages that are too small can develop bad behavioral habits, such as screaming, biting, and feather plucking. Even small birds need enough space to get the exercise they require.
There are certain "rules of thumb" for measuring minimum cage sizes for parrots. One rule is that the width of the cage should be at least double the wingspan length, and tall enough to accommodate long tails. For birds that spend much of their day outside the cage, the minimum dimensions could suffice. Birds that spend much of their day inside a cage should get one that is larger than the minimum recommended dimensions.
A popular option for people with more than one bird is an aviary, or an enclosed space where birds can fly freely and safely but still be contained. Most aviaries are located outside the home, but some people build indoor aviaries as well. Similarly, flight cages will allow your bird to get its daily exercise. They are small enough to fit inside a home but big enough to allow flight.
A habitat is a good option, too — it tends to mimic a parrot’s natural environment. Having a habitat will stimulate your parrot physically and psychologically. A habitat is very easy to create, and many people who own more than one bird prefer to house them in this manner, if space is available.