Samuel Hoffenstein’s little verse plays with a very relatable kind of crankiness. The speaker begins by declaring that he loves hearing a bird sing in the morning—but then he adds one condition: the bird had better not be the reason he is awake. If it interrupts his sleep, the bird loses all charm. The final couplet says it all: a bird that wakes a fellow up should have been a buttercup. In other words, it should have been something beautiful but silent.
The mood is wry and comic rather than genuinely angry. The rhyme and short lines make the complaint feel light and teasing. It is the sort of mock-grumpy thought a person has while lying in bed at dawn, half-annoyed and half-amused by nature.