ハイイロミズナギドリとハシボソミズナギドリ, Sooty and Short-tailed shearwaters

• ハイイロミズナギドリとハシボソミズナギドリは大変良く似ていて区別が難しい。後者は前者よりくちばしが短く細いという特徴があるので、 BHI (Bill Head Index)を定義して数値判定を行った。BHIは鳥の頭部の写真があれば算出できるので確実性が高いと思われる。 それに対し、翼裏面の色や頭部の形状は天候や見る方向によって異なって見えるので判定基準にしにくい。 また、鳥を捕獲しなければ長さの絶対値を測ることはできない。
• Sooty shearwater and Short-tailed shearwater resemble closely very much and are difficult to distinguish. Defining BHI (Bill Head Index), I tried original numerical determination by focusing the characteristic that the latter has shorter bill than the former. BHI is calculated by a photograph of a bird head and then it was considered to have high certainty On the contrary, it is hard to choose the under-wing color and the shape of the head to be the criteria because they look different depending on weather and a direction to look at. In addition, the absolute value of length can be measured only by catching the bird.

ハイイロミズナギドリとハシボソミズナギドリの数値判定, Numerical determination of Sooty and Short-tailed shearwaters.

• BHIを定義して、ハイイロミズナギドリとハシボソミズナギドリを区別する方法を提案した。
• BHI was defined and suggested how to distinguish Short-tailed shearwater from Sooty shearwater.
• BHIの定義, Definition of BHI (Bill Head Index = Bill length / Head diameter ×100 [%])
• ハイイロミズナギドリ, Sooty shearwater, BHI was around 70% (BHI=32.8/49.4×100=66.4%, BHI=29.2/39.5×100=73.9%)
• ハシボソミズナギドリ, Short-tailed shearwater, BHI was less than 60% (BHI=20.7/35.1×100=59.0%, BHI=19.8/34.0×100=58.2%)
• 試験ミズナギドリ, shearwater under test, BHI was less than 60% (BHI=19.8/35.7×100=55.5%, BHI=19.2/34.8×100=55.2%)
• 試験ミズナギドリのBHIは60%以下であることから、ハシボソミズナギドリと判定された。
• Shearwater under test was determined to be Short-tailed shearwater because the BHI was less than 60%.