Parakeets are highly social birds and require a companion or two so that they are not lonely or bored when the owner is not available. In general, parakeets will get along with many birds, but it is important to avoid putting a parakeet with a territorial bird because the other bird might attack the parakeet, especially in the case of lovebirds. Before buying another bird as a companion to your parakeet, it is important to find out which birds get along with the parakeet best.
In general, the best bird to live with a parakeet is another parakeet. Since parakeets are highly social naturally, they will form a group without difficulty when put together. Just make sure the parakeets have enough space to spread their wings without bumping each other or touching for an appropriate space.
Cockatiels and parakeets traditionally get along well, though they require time to get used to each other. Since cockatiels are generally a gentle bird and are also sociable, the birds can get along well. Parakeets are somewhat more assertive than cockatiels, but the balance of a larger size will usually prevent major problems.
Finches are another small bird that can get along with a parakeet. The finches are smaller than parakeets and generally more active depending on the specific finch species, so they require more space when put in a cage. For concerns about the smaller finch, especially for more assertive parakeets, put them in separate cages at night or when you are unable to supervise the birds until they get used to each other. Finches require different foods, so take care to separate the birds for feeding or have separate cages so they can have separate foods.
Canaries are gentle song birds and far more delicate than the hardy parakeets, but the gentle nature makes them an appropriate companion for parakeets. They are slightly smaller than the typical parakeet, but are also social birds. Keep in mind that feeding for canaries and parakeets differ, so having a separate cage for the canary during feeding is ideal. In general, keeping the canary separate during times when you cannot supervise is best due to the gentle nature and relative fragility of the bird.