a1. Axillars and under wing-coverts white; wing about 140 mm.b1. Larger; culmen more than 23 mm. geoffroyi (p. 106)b2. Smaller; culmen less than 20 mm. mongolus (p. 107)
a2. Axillars and under wing-coverts smoky brown; wing about 165 mm. veredus (p. 108)
