While they may seem lower maintenance than a dog or cat due to their much smaller size, all species of parrots require a lot of time and attention.
The most common reason pet parrots get sick is often due to a husbandry error, especially caging requirements, environment, and diet.
Parrots require a cage wide enough to spread their wings and tall enough that their head and tail don't hit the top and bottom of the cage. Caging must also be made out of parrot-safe materials. Many kinds of wire mesh contain zinc and other substances that are extremely toxic to birds. Many parrots tend to climb around the cage bars with their beak, so anything toxic on the bars has a high probability of being ingested. Any wood on the cage should not be treated with chemicals either (if the parrots can reach the wood, many will chew on it).
Due to their small size and sensitive respiratory systems, parrots are also highly susceptible to their environment. Everyday things we use like air fresheners and non-stick cookware can kill a bird. Non-stick cookware releases toxic chemicals when heated. Bug spray, cigarette smoke, and even some candles can also quickly kill a bird.
Many parrot owners purchase seed mixes for their parrots to eat. While part of a parrot's diet should include seeds, much of their diet should come from fresh vegetables and fruits. A pelleted formula is also recommended to fill in any nutritional gaps. Left to decide on their own, many birds will eat mostly seeds, so it is important to make sure they eat their veggies too (some owners find it easier to give their parrots meals throughout the day, having fresh veggies available throughout the day but not giving the parrots access to seeds all day). Dark leafy greens (e.g. broccoli, kale, cilantro) and carrots are very good for most parrot species. Fruits are also great, but they should make up a smaller portion due to their higher sugar content.
Dairy products containing lactose should be limited or excluded. Birds are not able to digest lactose.
In general, supplements are not necessary if the diet is adequate. A cuttlebone and mineral block, however, should be available to a pet parrot.
There are dietary differences between some species, so what is good for one may not be acceptable for another. Hyacinth macaws, for example, can live almost exclusively on various nuts, and cockatiels should be limited in the amount of sunflower seeds they are given because many will eat them to the exclusion of everything else (sunflower seeds are high in fat and low in nutrition).
In closing, here is a list of things to NEVER feed your parrot:
Sugary, fatty foods
This article does not cover every aspect of parrot care. Always research the specific requirements of the parrot you are interested in acquiring to ensure its individual needs are met. Avian veterinarians and reputable breeders are both excellent sources.