Aside from passerine vs non passerine, which should probably always be the first question, surely subsequent questions should be directed towards dividing the remainder into smaller sets, which are as close to 50/50 splits as possible? For a non passerine, perhaps the second question is: "Does the species lie within Mute Swan and (say) American Coot" on the British List?
http://www.bou.org.uk/thebritishlist/British-List.pdf
I think using this rather dull system would enable you to get to any species on the British List in about 10-11 moves? 600-300-150-76-38-19-10-5-3-
cheers, alan
For WP expand as appropriate!