The followings are study result of specimens of Red-backed Shrike, Brown Shrike and Isabelline Shrike collected by Yamashina Institute for Ornithology.
Japanese version:
http://www002.upp.so-net.ne.jp/holi/LaniusJ.pdf
English version:
http://www002.upp.so-net.ne.jp/holi/LaniusE.pdf