We were absorbed in listening to the birds singing and didn't notice them at first.

were 是 be 动词的过去式复数形式，表示过去的状态。 absorbed 是动词 absorb 的过去分词形式，absorb 本意是"吸收"，引申为"全神贯注"，这里表示"被吸引、专注于"的状态。 in 是介词，表示"在...状态中"，与 absorbed 搭配构成固定短语。 were absorbed in 是一个固定搭配，表示"全神贯注于...、沉浸于..."的状态。