I will try a rather "formal" definition of BIRD, NOTBIRD and UNKNOWN as I understood. Maybe it helps.
X = (hx,wx) is a animal
where hx: height, wx: weight
F = Infos from biologists
R = (Hmin,Hmax,Wmin,Wmax) is the unknown range of Birds.
X is a BIRD
<=> For all (possible) R's that satisfy F, X is (always) in R.
X is a NOTBIRD
<=> For all R's that satisty F, X is not in R.
X is UNKNOWN
<=> otherwise (i.e. You can find two ranges R1 and R2 satisfying F, so that X is in R1 (BIRD) but not in R2)
@Snip3r: I'm looking for a test case which fails...