The forest was populated by rare and colorful birds.

was 是 be 动词的过去式，表示"是"或"存在"的状态。 populated 是动词 populate 的过去分词形式，populate 的意思是"居住于、栖息于"，通常指动物或人类在某地生活。过去分词形式在这里与 was 构成被动语态，表示"被...栖息"的状态。 was populated 合起来表示"被...栖息着"，描述森林作为栖息地的状态。