She expresses happiness as fullness and as a well-watered tree that sustains life by harboring a nest in its branches and a tree ready for harvest. A raised platform made of down and silk speaks of warmth and comfort. She wants it lined with fur and purple as in royalty. Doves, pomegranates and peacocks add to this royal theme. She feels special and royal on her birthday because she has found love. The verbs "hang," "carve," "work," are commands that an official might daily use.
The doves represent purity and innocence. While at the same time, they are common, just like love. Pomegranates bring out the red and show multiplicity. The peacocks show royalty with blue and green feathers. The spots on the tail symbolize her desire that the whole world to see that she has a lover.
Ms. Rossetti lived between 1830 and 1894. The poem appeared in Macmillan's Magazine in April of 1861. It uses lots of medieval terms to convey the message of her feelings about that special birthday. Ms. Rossetti wrote mostly devotional and children's poems in her later years after she experimented with forms such as sonnets, ballads, and hymns finally settling on devotional and children's poetry.
Ms. Rossetti had three lovers any of which could describe the one in A Birthday. She never married however; though, she had these three suitors. She always refused to marry because of religious reasons. She, her mother and sister devoted themselves to the Anglo-Catholic movement from the time she was fourteen until her death. Two of the three suitors were painters, so maybe a lot of her vivid descriptions come from watching them work.