Sunshine... Its not just pretty, its necessary
This topic and the pictures are courtesy of Lucinda from Wings Down Under Aviary.
Vitamin D allows the body to absorb and use the calcium that is provided. A great source of Vitamin D is your own body - it is produced when you are exposed to natural sunlight. Natural sunlight is exposure to direct sunlight unfiltered by window glass or other means of UV protection.
What happens without Vitamin D? For the individual bird, they can become hypocalcemic. This means low calcium levels. Signs can be weakness, tremors and even seizures.
If your bird experiences any of the above signs take it to the avian vet ASAP - there could be other reasons for these signs, not just low calcium levels.
For egg laying hens low calcium levels can cause weak shells and "egg binding" where a soft egg gets stuck inside. Hens that are chronic egg layers are especially prone to this problem. Speak to your avian veterinarian about how to stop/prevent chronic egg laying. If you see a hen straining or notice a visible bulging of her vent -seek veterinary help ASAP!
So what to do? Not everyone lives in the perfect place where its balmy and sunny all day, every day. If you can easily and safely put your bird outside in the sun, even for a few hours every week in warm months, that is the ideal answer.
Always remember to make sure there is some shaded areas in the cage to prevent overheating/overexposure. Even if you have to create your own shade with a towel over a portion of the cage. And make sure there is a source of water so the bird can drink and not get dehydrated.
Lucinda says she takes her birds out and puts them in an outdoor cage to sun a few times a week for an hour at a time during the spring and summer, when it is warm at her house. She has never had problems with deficiencies. You can see by her photos how much her birds enjoy the time!
If it isn't possible to take your bird outside - try moving your cage by a sunny window (again leaving some areas shaded) and opening the window so the sun can shine through the screen in warm weather.
If its a really nice day out, no heavy winds to chill your bird - try offering a shallow dish of water. Make sure its stable enough not to tip over and not so tall your bird can't climb out of it. Let your birdie enjoy a bath and laying out to dry.
If you have a bird with feather picking issues and has exposed patches of bare skin, be careful - limit the sun exposure to avoid a sunburn! But sometimes a water bath and some sunbathing can actually help these birds get better.
You can partially supplement birds with Vitamin D3 supplements. You can add full spectrum "daylight" producing lightbulbs over cages to help simulate sunlight and stimulate Vitamin D production. I bought 18" undercabinet lighting fixtures and substituted the lightbulbs with Phillips daylight lightbulbs from the Home Depot. They need to be replaced every 6 months for best effect. But there is just no replacement for the real thing.
Chicks can be affected by lack of Vitamin D in hens. Some problems you can see are splayed legs (the legs splay out to the sides so the baby can't stand), stunted or abnormal growth sometimes of the entire bird or sometimes of just a body part, even broken wings/legs from brittle/weak bones.
Splayed legs can happen for a variety of reasons in chicks including poor bedding/footing. If seen by a vet at an early stage something may be done to help correct the problem. However some cannot be saved.
I am not saying natural sunlight (Vitamin D) and calcium will correct for all problems, but they are the underlying problem in a lot of cases.