Obviously what you feed your bird will depend on the type of bird you own - whilst the majority of birds kept as pets are primarily seed eaters, some birds require live foods or nectar mixes. You should make sure you know what your particular birds requirements are before you bring it home.
The most common bird food fed to birds is bird seed, and there are a huge variey available, with mixes designed for each bird type. However, even birds that are seed eaters should not be fed only seed - this is an incomplete diet, and some estimates indicate a seed only diet could cut your bird's lifespan by as much as half. All birds should be given green foods in addition to seed.
Buy a good quality seed mix designed for your bird type. Some seed has been sitting in the store for a long time and is stale, having lost a lot of its nutrient value. To see if your seed is fresh, try soaking some in water overnight, then drain and leave for a day or two to see if it will sprout. If the seed will not sprout it is stale, and you should seek a better supplier.
Many birds will have a preference for the large oily seeds, such as sunflower seeds, and given the opportunity will eat these to the exclusion of other foods, resulting in an unbalanced diet. If your birds pick through the seeds like this you might like to give a mix without these seeds, and only give them small amounts as a treat. Alternatively, feed your birds daily rather than giving unlimited seed, so that they have to finish the complete mix. If doing this, make sure not to starve the birds, making sure they have plenty of green food, and pellets if you are using these.
Make sure you check your birds seed supplies regularly - birds will drop the husks back into the feed container, and so even though a container looks full, it may in fact only contain husks.
Some seed mixes also contain aditional vitamins and minerals, and other dietary supplements. These can help to make the diet more complete, but should not replace the provision of green foods. You may also like to add your own supplements to seed (see below for more information)
Millet sprays are very popular with most birds - these are the millet seed heads, and some people believe that they are healthier for the birds than the millet alone, due to extra vitamins and minerals being available in the husk.
Birds also appreciate other types of seed heads - these can often be picked fresh from the garden or roadside - just make sure they are washed, particularly if taken from a roadside where they have been exposed to exhaust fumes.
Soaked seed & sprouts
Birds can be fed part of their seed soaked or sprouted. This will increase the level of protein available from the seed. To make soaked seed, soak some of the birds' standard seed in warm water overnight, then drain. Rinse the seed thoroughly before feeding, and do not leave uneaten soaked seed in the cage longer than 24 hours, as the moisture could create moulds and toxins. Sprouted seed is soaked seed that has been left a little longer and has begun to sprout - rinse the seed throroughly 3-4 times a day for a few days. You could buy a seed sprouter to make this easier if feeding a lot of soaked or sprouted seed. Millet sprays can also be soaked before feeding to birds.
Seed bells and sticks can be a great way of presenting seed to your birds, as it is more interesting for them & makes them work a bit harder for their meal. Try to make sure the bells or sticks are held together with natural ingredients - some are held together with wood glue.
Shell grit & cuttlefish
Birds should have a constant supply of grit available, as this aids in the digestive process - the grit is what the birds us to grind up the seeds in the crop. Cuttlefish should also be available, as this provides a good source of calcium for your birds.
Green foods & fruit
All birds should have a regular supply of fresh food. Green foods such as broccoli, spinach or kale are good choices. Make sure all food given to birds is well washed. Birds should also be given foods high in carotene, such as carrot, sweet potato or red pepper - these should be grated or finely chopped. Other vegetables such as turnips, corn or tomatoes can also be given. Many birds will also appreciate the provision of fruit, such as apple, banana or grapes. you should not feed the pips or core to the birds.
Your birds may not take to these foods initially, particularly if they are used to being fed only seed. It is worth persevering with them, as they will have a major impact on your birds health - keep giving the food, even if they are not eating. It may take some months before they get the idea. You might like to speed up the process by taking the seed out of the cage for a while, as the birds will be more interested in trying new foods if they are a little hungry. If you are doing this, make sure that you do give the birds access to their normal foods every day to prevent starvation. You can also try mixing the seed in with the fresh food, so they have to pick through it to get to the seed.
Pellets are a processed, complete food for birds. They provide a complete balanced diet and so are healthier than seeds, but they are much less popular with the birds. Birds also need to be taught to eat pellets. This can be done by mixing a dry mix of pellets and seeds, gradually reducing the proportion of seeds, or by mixing the pellets with water and mixing the seeds into the resulting "porridge".
Once the birds are used to eating the pellets they can be provided freely, and can be useful to ensure the birds have enough to eat if you are restricting the amount of seed you are feeding, or trying to encourage the birds to eat fresh foods.
There are various vitamin and mineral supplements available for birds, and some seed mixes have these added already. These can be useful to ensure your birds are getting a balanced diet, but should not be used to replace fresh foods. You should be careful not to give too much of these supplements, as it is easy to give an overdose of vitamins, and you should not use an additional supplement if you are using enriched seed. You may like to add a wheat germ oil and cod liver oil mix to your birds feed, as this provides the most commonly deficient vitamins in a less concentrated, and therefore safer, form than the vitamin liquids or powders.
All birds need constant access to clean water. Check water daily - it is very easy for water to become contaminated by droppings or food pieces, which then can cause a spread of disease. Closed water containers are better than open dishes, as they help reduce the amount of contamination in the water. You can also buy water additives to add to the water which will prevent bacterial growth and keep water clean longer, but you should still monitor the water daily.