In this poem by William Henry Davies, the poet addresses the kingfisher, his subject, directly, theorizing that it "was the Rainbow gave thee birth." Here he personifies the Rainbow, capitalizing its name and envisaging it as a mother, capable of carrying the kingfisher and thus passing on her own physical characteristics to it as offspring. Davies imagines that it was through this process that the Rainbow "left thee all her lovely hues." He suggests that the kingfisher's brightly colored feathers have been handed down to it from its mother. The metaphor is a reference to the fact that the kingfisher seems to contain all the colors of the rainbow, so beautifully arranged that it seems to have been by design that a rainbow became embodied in the bird's plumage.
Davies goes on to praise the fact that the bird, while incredibly beautiful and brightly colored, is not "vain." Like the rainbow, it is simply in the nature of the kingfisher to be so brightly colored.