All day long these whining cries were heard, and it was plain that a new system had been adopted by the parent birds at this stage, which was to keep their young on short commons, instead of supplying them with more food than they could consume.（The Book of a Naturalist by W. H. Hudson）注：タカの子育ての様子です。commons =食料
英文を前から読んでいくために、語句のかたまりを作ります。こんな具合に課題文をスラッシュ（斜め線）で区切ります（スラッシュリーディング）。語句のかたまりを意識すると、リスニングで英語の意味をつかむのが楽になります。
All day long these whining cries were heard,／（接続詞の前）and it was plain／（接続詞の前）that a new system had been adopted by the parent birds at this stage,／（関係代名詞の前）which was to keep their young on short commons,／（コンマの後）instead of supplying them with more food than they could consume.
語句のかたまりを前から訳していきます。
All day long these whining cries were heard,
一日中、こうした哀れな鳴き声が聞こえていた
and it was plain
そして、〜は明らかだった
that a new system had been adopted by the parent birds at this stage,
この段階で親鳥たちが新しいやり方を採用したこと
which was to keep their young on short commons,
それは子どもの鳥たちにえさを十分に与えないことだった
instead of supplying them with more food than they could consume.
食べきれないほどのえさを与えるのではなく
All day long these whining cries were heard, and it was plain that a new system had been adopted by the parent birds at this stage, which was to keep their young on short commons, instead of supplying them with more food than they could consume.
一日中、こうした哀れな鳴き声が聞こえていた。この段階で親鳥たちが新しいやり方を採用したのは明らかだった。それは、子どもの鳥たちに食べきれないほどのえさを与えるのではなく、十分に与えないというやり方だ。
All day long these whining cries were heard, and it was plain that a new system had been adopted by the parent birds at this stage, which was to keep their young on short commons, instead of supplying them with more food than they could consume.
which was to keep their young on short commons〜 は関係代名詞節ですが、先行詞は何でしょう？ 直前にある this stage を関係代名詞の部分に代入しても意味が通じません。この場合は、少し離れたところにある a new system が先行詞になります。
which に先行詞の a new system を代入すると→ a new system was to keep their young on short commons〜 →「新しいやり方は子どもの鳥たちに十分なえさを与えないというものだ」
動画の後半部分でリスニング用音声が流れます。
