In a poem named 「生きる」 by 谷川俊太郎, the poet described "to live" with many instances.
In the instances put forward in the penultimate stanza, the poet put 「が」 after the subjects, such as in 「犬が吠える」, 「地球が廻っている」, etc.
However, in the final stanza, the poet started to use the topic marker 「は」, such as in 「鳥ははばたく」 and 「海はとどろく」.
I wonder what made the poet choose to use 「が」 in one stanza but 「は」 in another, and what effect that has brought about.
は can be used to describe the general feature of something, while が does not have such a nuance.
Let's take a look at the third stanza. See that いま is always used with が? Here, the poet is not trying to explain the general facts about something. He is saying a certain soldier is wounded somewhere in this moment, a certain dog is now barking somewhere, the earth is now rotating, etc. 「いま兵士は傷つく」 sounds odd because he has not introduced any soldier into "the universe of discourse" before that line, and a soldier is not something that is always wounded.
In the final stanza, the poet is presenting 鳥ははばたく, 人は愛する, etc., as the general facts which are always true.
In other words, the third stanza is saying "Many things are now randomly happening in this world, and that is what 生きる means." The final stanza is saying something more abstract and philosophical... "Every truth in this world is 生きる" or something like that.
