I stumbled upon a way to calculate the subject distance for birds. I have tried to condense the explanation as much as possible. You can go to the “Quick & Simple Calculator section” below; and comeback to the explanation or the link to Wayne’s page if you need more...
I found this wonderful Marsh & Willow Tit article, that I thought I would share. Here in Hokkaido there are Marsh, Willow, Varied, Japanese, and Long-tailed Tits. This attached article focuses on Willow and Marsh Tits.
Here is a very useful PDF about the differences between Cormorants. This article details in depth; areas in which one can tell the difference between the Temminck’s (Japanese) Cormorant, and the Great Cormorant more easily. This helpful article, is with greatest thanks written by Nick Lethaby & Nial Moores.