Which of the following is not a unique characteristic of a bird?

The evolution of molecular biology has made possible to establish a new classification of all organisms according to their DNA, and it's called phylogenetic classification. This classification group living beings according to their kinship it is established according to anatomical, and especially genetic, based on the similarity of genes between species.

This made it possible to discover kinship ties between species of which there is no suspicion of any morphological link between them, something which the old classification (the traditional classification) was incapable of doing (and this proves the importance of the DNA and genes in organisms classification.