The first stanza of "I have a Bird in spring" tells the story of a joy cut short. In spring, the speaker says, they had a little bird that sung to them. But "the spring decoys": that is, that happy season only deceives the speaker. As spring turns to summer, their bird up and flies away.
The poem's very shape suggests that the speaker's time with their bird was painfully brief. The speaker and bird only share the poem's two first lines before the summer comes and "Robin is gone."
Worse, the speaker is left all alone in the summer, when the "Rose appears," a symbolic time of blossoming joy. The deceptive, decoying spring feels all the more wrong-footing because it should lead into a happy summer—and doesn't.
Even the poem's rhythms jar. Listen to the meter in the first lines:
I have | a Bird | in spring
Which for | myself | doth sing—
The spring | decoys.
That short third line cuts off a pattern of neat iambic trimeter (that is, lines of three iambs, metrical feet with a da-DUM rhythm) at the knees. Though the poem won't stick to those iambic feet all the way through, it will keep moving back and forth between longer lines of three beats and shorter lines of only one or two beats, a halting rhythm that evokes a struggling, faltering voice.
Everything about this first stanza, then, suggests that the bird's departure broke the speaker's heart when they were least expecting it. This will be a poem about how on earth one can cope with the shock of separation from a beloved. For now, all the stunned speaker can say is that "Robin is gone"—a line whose soft /au/ assonance sounds like a quiet moan.