This data set describes the phylogenetic relationships of the families
  of birds as reported by Sibley and Ahlquist (1990). Sibley and
  Ahlquist inferred this phylogeny from an extensive number of DNA/DNA
  hybridization experiments. The ``tapestry'' reported by these two
  authors (more than 1000 species out of the ca. 9000 extant bird
  species) generated a lot of debates.
The present tree is based on the relationships among families. A few
  families were not included in the figures in Sibley and Ahlquist, and
  thus are not included here as well. The branch lengths were calculated
  from the values of \(\Delta T_{50}H\) as found in Sibley
  and Ahlquist (1990, figs. 354, 355, 356, and 369).