This will probably be the longest article I’ll ever write because the internet is full of so many inaccurate statements. I’ll go through each myth one by one.
Myth #1: Any ultraviolet light can cause a bird to synthesize Vitamin D
There are actually two relevant types of UV light: UVA and UVB. Most articles about birds and lighting don’t specify with type and just say “ultraviolet” light.
So, first let’s establish that it’s UVB light that is required for Vitamin D synthesis.
During exposure to sunlight 7-dehydrocholesterol in the skin absorbs UVB radiation and is converted to previtamin D3 which in turn isomerizes into vitamin D3.Sunlight and Vitamin D: A Global Perspective on Health—Dermato-Endocrinology—’Jan 2013
UV light causes the body to produce vitamin D (specifically, UVB), which is essential for life.Ultraviolet—Wikipedia
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In humans, both UVA and UVB are responsible for secondary effects on the skin such as erythema (sunburn) or tanning, but UVB is the only range associated with the photobiochemical synthesis of vitamin D3.