For most birds, females are stuck with child care, but not so for the South America's greater rhea (above, chicks nestle into their dad's back feathers at Washington D.C.'s National Zoo).

Females mate with several males during the breeding season, and several birds will lay their eggs in a nest created by a male. The male then incubates up to 50 eggs for six weeks and cares for the newly hatched young. The dads aggressively guard the babies, charging at any animal—even a female rhea—that approaches.

The male rhea at the National Zoo, a second-time father, keeps his chicks from meandering too far away with a rapid clacking of his bill, according to zoo officials.

Because the females put such a "heavy investment"—energy and resources—into producing large eggs, it makes sense for males to pick up the responsibility of caring for the offspring, Forbes said.