渡り鳥 and 群れ: migratory birds, flocks and guided sheep
Compare 渡り鳥 returning or being searched for with 群れ in birds flying south and sheep guided by a sheepdog. Read the group members and guiding agent separately.
渡り鳥 returning and being searched for
渡り鳥 (わたりどり) means migratory bird. 「春になると湖に渡り鳥が戻ってくる。」 means “Migratory birds return to the lake in spring.” The sentence describes birds returning to the lake in spring.
「観察会では双眼鏡で渡り鳥の群れを探した。」 means “During the observation event, we searched for flocks of migratory birds with binoculars.” 探した reports searching with binoculars. It does not itself add a completed discovery of the flock.
群れ in birds and sheep
群れ (むれ) means a group or flock. 「空を鳥の群れが南へ飛んでいった。」 means “A flock of birds flew south across the sky.” The birds are not expressly identified as the migratory birds sought during the observation event.
「羊の群れを牧羊犬が静かに導いている。」 means “A sheepdog is quietly guiding the flock of sheep.” Sheep are the flock's members, and the sheepdog is the guiding agent. 導いている describes the ongoing guidance.
The bird name and the group relationship
渡り鳥の群れ combines a migratory-bird name with a group relationship. 群れ also occurs with sheep, so the word alone does not determine its members' kind.
In 羊の群れを牧羊犬が, the sheep flock before を is being guided, and the sheepdog before が guides it. Keep the birds' past flight distinct from this ongoing guidance.
Who are the members and guiding agent in 羊の群れを牧羊犬が導いている, and must every 鳥の群れ be migratory birds?
Show the answer and why
The members are sheep, and the sheepdog guides them. 群れ names the group relationship, so a flock of birds alone does not establish migratory-bird status. Searching for a flock at the event also does not establish that it was found.