Distinguishing Characteristics

Medium-sized, narrow-bodied bird, all blackish above and white below with white tail; basal half of bill bright red, distal half black. Strikingly dimorphic in size. Total length 40–50 cm; bill 50 mm (female) to 60 mm (male); mass averages about 265 g (female) to 365 g (male). Uniquely shaped bill long and laterally compressed (knife-like), with the lower mandible extending 2–3 cm beyond the upper, which is hinged and can be freely elevated and clamped shut. Many males have a narrow visible slit between the mandibles. Wings long and narrow, averaging about 350 mm (female) to 390 mm (male); feet webbed, bright red-orange; tail shortish, square or slightly forked. Juveniles buffy brown, streaked with black above, whitish below. At hatching, the 2 mandibles are equal in length, but by fledging at 4 wk, the lower mandible is already nearly 1 cm longer than the upper.

Relatively large pupil which constricts to a narrow vertical slit, a unique feature among birds; possibly an adaptation to protect the retina from bright light or to enhance vision for nocturnal feeding (Wetmore 1919, Zusi and Bridge 1981). The eye, surrounded by black feathering, is often invisible.