An authoritative eBird writeup which appeared in 2013 contained the  
following:
"Unfortunately, identifying the age and sex of Snowy Owls is more  
difficult than is generally realized. Darkest birds are usually young  
females and the whitest birds usually adult males, it’s especially  
hard to determine the age and sex of intermediate birds. (Note to  
eBirders: When possible, take photos and embed them in your checklist  
(instructions here) to document the owls you see. With visual  
evidence, you later may be able to get help  determining the age and  
sex of your owl."

The original source is http://ebird.org/content/ebird/news/gotsnowies2013/

George Clark
Norwich, VT
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