The forest was populated by rare and colourful birds.

was 是 be 动词的过去式，表示"存在"或"处于某种状态"。 populated 是动词 populate 的过去分词形式，populate 意思是"居住于、栖息于"，这里用被动语态表示"被...栖息"。 was populated 合起来表示"被...栖息/居住"的状态。