The speaker stands in a forest, listening to the animals around them. But what holds their attention isn't the calls themselves. It's the gaps between them, a "stillness of breath" where the sound drops away and everything seems to pause, as if the whole forest is holding its breath.
The last line shifts the poem somewhere stranger: "As I watch the sky fall." It's hard to say whether this is literal or figurative. It could describe dusk settling over the trees, or the sky seen through the branches as the light fades. It could also point to something heavier, a feeling that something is ending or giving way. The poem doesn't say, and that uncertainty is part of its effect.
The mood is quiet, hushed, and a little uneasy. The speaker seems calm and watchful rather than frightened, but the closing image leaves a sense of awe, or of something slipping away, that lingers after the poem ends.