This is the ability of animals, including humans, to produce and utilise vitamin D3 in the skin after exposure to unfiltered, natural sunlight. Vitamin D3, among many other uses allows the assimilation of calcium.
Without vitamin D3 in the correct levels calcium simply cannot be absorbed into the animals system properly. Vitamin D3 is produced more effectively when light is emitted at 297nm. A lack of D3 and calcium can cause feather plucking, painful and even fatal conditions in birds and reptiles.
With access to the right vitamins and hormones birds will not only feel better in themselves but they will be able to assimilate calcium in good quantities. This will help with ensuring good bone density, feather production and egg viability. There is also a feeling now that a critical lack of D3 can cause birds to become miserable. This can lead to plucking and bad behaviour. Fortunately there does not seem to be a point of no return! As soon as UVB is provided the subject seems to pick up very quickly.
Exposure to the right levels of UV will also increase useful preening. It seems also that the main preening gland is affected in a positive way by exposure to UVB and also contains ingestible D3. Birds can assimilate UVB through their feet, sear and any other bare patches of skin. They are also actively basking and seeking out exposure when preening in open sunlight.
As with captive reptiles, birds require a gradient of light. This is very easy to achieve. We recommend fitting your bird lighting over roughly a quarter to one third of the total living space of wire parrot cages. This becomes the “basking zone”. The rest of the enclosure will then have a gradual gradient into shade. Birds are very able to ascertain just how much exposure that they require at any given moment and they will move around the enclosure to regulate the exposure that they require.