Supplements... Calcium is a must!
There are very few supplements that I recommend. As I mentioned in the diet page, if you are feeding a pelleted diet there is no need (in non-breeding birds) to supplement. If you are feeding only seeds, you simply cannot make up for all the imbalances by adding supplements.
You do not need grit. These are not poultry that need coarse materials to break up whole pieces of corn and whole seeds. Their hard beaks easily break apart the hull of seeds and crumble pellets for easier digestion.
Be careful if you add supplements to the water. If a bird does not like what you have done, it can take only 3 days and you will have a dead bird and no indication that it was dehydration that did it. If you have a heavy mineral content to your water you should consider using bottled water. There is no need to use distilled water.
However, there is one thing that should always been available as a supplement for your birds. Calcium. It is the building blocks for bones, and helps keep the heart beating. Iin birds it is the main component of eggshells.
The most common way to offer calcium is a cuttlebone. This "bone" is an internal structure found in cuttlefish. Other uses for it are for jewelry making, in toothpaste, as an antiacid and an absorbent. In caged birds it is a great source of calcium. There is a soft easily crumbled soft side and a hard rough side. Cuttlebone is usually attached in some way, you can buy holders for it, to the side of the cage. Make sure when attaching it, the soft side is facing inward towards the birds to encourage use. In may seem like it never gets used except as a perch. If it gets soiled, replace it. You can buy bags of it cheap online, I recommend Herman Brother's store. But no matter if your bird is just a pet and never used for breeding, you should always have it available. In addition or as a substitute, you can use a Manu calcium block. The small ones are shaped like roses and is a solid soft block of a calcium. Some people prefer it to cuttlebones.
There are alternative, or additional, options to offer calcium. Usually these methods require the calcium supplement be offerred in a dish so expect to have to change it frequently as it gets soiled.
An easy and cheap source of alternative calcium is oyster shell. You can get a 5 pound bag for next to nothing at the local feed store. It will likely last you years! If you only have a few birds you may check with a friend that raises chickens and see if you can get a cup from them. (Just think of all the calcium a chicken needs for all those eggs!) Put a small amount out in a dish and your birds will pick through it. I keep a small dish with a 1/2 teaspoon of shell in each cage along with a cuttlebone.
There are powdered calcium options as well. I have seen 2 available specifically made for birds. I got a big container for a reasonable price online called Avian Calcium. Its not flavored, its just a fine white powder. I dust my seed (yes I offer seed to my breeding birds in small quantities) several times a week. There is a flavored variety but I thought it was rather pricey for the small amount you get.
In a pinch you can use reptile calcium powder. However be VERY careful if you are going to try this. Make sure to read the label very carefully. It should have ONLY calcium, NOTHING else in it. Many reptile calcium supplements have additional ingredients especially Phosphorus. This is bad for birds as it can cause a major imbalance. But calcium is calcium no matter who it is packaged for, so it can be used for birds.
You can even crush up a TUMS for a source of flavored calcium. Just 1 is enough at a time, don't go overboard. Like the above options, just because its flavored doesn't mean your bird will suddenly dig in, they will only eat it if they need it.
Lastly, there are liquid calcium supplements. Most of these are to be added to water supplies. The one I recommend is Cali-Boost. You can get it in a premixed liquid form or a powder. There are 2 powders actually, 1 you can use either for water OR food and 1 for food ONLY. I use the powder for food and water. I find its more economical and versatile since you can easily increase the amount for an egg bound hen. But follow the directions carefully, you only need to use it a few times a week in non breeding birds. In breeding birds you only increase the amount when they are LAYING eggs and then reduce afterwards. NOTE : the powder makes the water yellow (I don't know what the liquid form looks like). It doesn't seem to affect the taste, my birds drink it just fine.
For hens with egg related problems due to low calcium levels, an avian veterinarian may prescribe a highly concentrated liquid calcium. But this should only be used under the supervision of a veterinarian as directed - too much can kill your bird. Do NOT use it on other birds as a supplement. Nor try to treat other birds on your own with it. Just because another bird is showing "the same signs" does not mean it is the same thing. If you think there is a problem with a bird, take it to the vet first, you don't have to pay for more medication if you have some already but you could do serious harm treating for the wrong thing.
Any supplement can be overdone. You can give too much calcium. In people it can accumulate in joints causing arthritis type pain. Stopping the supplement allows the body to then use the build up. However in birds, because they only take what they need, it is hard to overdose on an over-the-counter supplement. It is better to offer plenty of options than to offer nothing at all.
But all the Calcium supplements available are not going to help if you don't have Vitamin D - see my page on Sunshine for more information.