question: Sibley and Ahlquist ' Phylogeny and Classification of Birds is a landmark work on the classification of what animals ?
answer: birds
turker_answer: Sibley and Ahlquist ` Phylogeny and Classification of Birds is a landmark work on the classification of birds .
rule-based: Sibley and Ahlquist ' Phylogeny and Classification of Birds is a landmark work on the classification of birds .