The birds, I knew, had brought off their young successfully in this same wood in the three previous years, and I came to the conclusion that the same nest had been used every time and had grown to its present size by the addition of fresh materials each season.（The Book of a Naturalist by W. H. Hudson）
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。
The birds,／（コンマの後）I knew,／（コンマの後）had brought off their young successfully in this same wood／（前置詞の前）in the three previous years,／（接続詞の前）and I came to the conclusion／（接続詞の前）that the same nest had been used every time／（接続詞の前）and had grown to its present size／（前置詞の前）by the addition of fresh materials each season.
語句のかたまりを前から訳していきます。
The birds,
その鳥たちは
I knew,
私は知っていた
had brought off their young successfully in this same wood
ひなどりをこの同じ森でうまく育て上げた
in the three previous years,
過去３年間にわたって
and I came to the conclusion
そして私は結論に達した
that the same nest had been used every time
毎回、同じ巣が使われたという
and had grown to its present size
そして、今の大きさの巣になったという
by the addition of fresh materials each season.
毎年、新しい巣の材料が加えられることによって
The birds, I knew, had brought off their young successfully in this same wood in the three previous years, and I came to the conclusion that the same nest had been used every time and had grown to its present size by the addition of fresh materials each season.
その鳥たちが過去３年間にわたって、ひなどりをこの同じ森でうまく育て上げたことを私は知っていた。そして私は、いつも同じ巣が使われ、毎年、新しい巣の材料が加えられたことによって今の大きさの巣になったという結論に達した。
The birds, I knew, had brought off their young successfully in this same wood in the three previous years, and I came to the conclusion that the same nest had been used every time and had grown to its present size by the addition of fresh materials each season.
今回の課題文では、「同格のthat」が使われています。「同格のthat」は前の名詞の内容を示す節をつくるもので、「〜という（前の名詞）」と訳せます。先行詞を後ろから説明するための関係代名詞の that とまちがえやすいので注意が必要です・
the conclusion ← that the same nest had been used every time〜
that節が前にある the conclusion（「結論」）の内容を示しています。「どんな結論」かが記されているわけです。
動画の後半部分でリスニング用音声が流れます。
