To clarify and deepen my understanding of the Letter, I pondered the following comparisons with similar and/or opposing cards. Maybe my comparisons will inspire you to make your own!
Type of communication
Letter:
Communication in any form, and its manifold manifestations. Communication with quite a bit of depth and/or content.
Birds:
Communication in the form of idle chit-chat, gossip, or oversharing. Quick,
short, (and often unnecessary,) communication like text
messages, or twitter.
Communication vs. no communication
Letter: To communicate; to express things; shared information.
Mountain: To be silent; things unspoken; the elephant in the room.
Information / facts
Letter: Information. Some content is being shared, passed on, expressed - while this content could be factual it doesn't have to be; the information shared could be false, too.