by 1987, both ahlquist and sibley had left yale. in 1988, ahlquist and sibley were awarded the daniel giraud elliot medal. in january 1991 (often listed as 1990), charles sibley and ahlquist published "phylogeny and classification of birds", which presented a new phylogeny for birds based on the dna. in 1999, ahlquist was retired (ahlquist, 1999).