Conversation so far:
what was the birdsong in relation to Olivier Messiaen?
Olivier Messiaen had long been fascinated by birdsong, and birds had made appearances in several of his earlier works.
Did Olivier Messiaen make any music pieces based on birds?
Olivier Messiaen found birdsong fascinating, notating bird songs worldwide and incorporating birdsong transcriptions into his music.

Question: Which one of them was most known?
Self-contained rewrite: Which one of Olivier Messiaen's music pieces based on birds was most known?