I've found that IDying Hairy/Downy by tail is extremely difficult as very rarely do you get to see the tail, though if you have pictures then the tail can be helpful and often times at feeders you can see it clearly, but what do you do if you have a worn Downy that has no bars (or only very faint) on the tail? I've found that bill size is the way to go no matter what. What I do is mentally "remove" the bill and place it beside the birds head, Hairys will have a huge bill, approximately the length of the head, where as Downy will barely cover half. Always remember, Downy is dinky (and have dinky bills) and Hairy is huge (and have huge bills).
Hope this helps!
featherbrain
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