In the poem "sympathy", the caged bird's song is a song of joy.

No it's not it's describing terrible experience of being an bird stuck into a cage.

postards have pictures on them, these pictures are things that may never be seen again. the pictures are a part of our history that shows things. because of this many collectors would pay a lot of money for them.

i read on google they "offer a glimpse into the past"

hope this : )

you need to know what you have learned: )