HSK 7-9
成群结队
chéngqún-jiéduì(idiom) making up a group; forming a troupe; in large numbers; as a large crowd
秋天候鸟成群结队向南飞。
Qiū tiān hòu niǎo chéng qún jié duì xiàng nán fēi.
Migratory birds fly south in large groups during autumn.
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Appears across levels (7-9) in the HSK 3.0 word list.