At least one species of large flightless (ratite) bird inhabits each of the major southern continents: the ostrich in Africa, the emu in Australia, and the Rhea in South America. All probably followed a parallel evolutionary path of flightlessness and gigantism from their flighted ancestors, once their continents had broken away from the southern supercontinent of Gondwanaland during the Cretaceous period.
Evolution of flightless (ratite) birds, from MappingIgnorance.org
While traveling through the Cerrado of Brazil, we spotted Rheas many times, as they foraged in agricultural fields and grassy, open areas of the Cerrado.
Question: How do flightless birds get over a fence?
Answer: they go under it!
Rheas stand over 5 feet tall, supporting their elongated neck and football-shaped body on long legs. They are not particularly attractive, with their loosely feathered plumage that usually looks disarrayed and scraggly.
Three large, wide-spaced toes and long, sturdy legs support this 80 pound, giant omnivorous bird.
They do have pretty brown eyes, though.
Rheas spend a lot of their time, head down, walking slowly through open grassland or crop fields hunting for tidbits of grain, grass, insects, perhaps small nestling birds, or anything else edible to pick up and toss back into their gullet.
During the non-breeding season, they may be shy, and run from perceived threats. This bird is moving at a fast trot, but Rheas can run almost 40 mph, using their outstretched wings as rudders to steer right and left.
Other than being large and flightless, Rheas seem mostly unremarkable. But not when it comes to their love life.
Males engage in chest-bumping and neck thrashing to establish their nesting territory.
Two male Rheas are duking it out, pushing against each other breast to breast while beaking each other.
Once they have established who is top dog in a particular area, a male actively courts and copulates with a number of females. Then he builds a nest by trampling and scraping out a grassy disk, pushing dirt away from the center until he has built up a mound three feet across and 30 inches deep and invites his courted harem of females to lay their eggs within. Before he begins incubating his clutch of eggs, there may be somewhere between a dozen to 50 eggs in his nest mound. Incubation takes about a month, and the eggs hatch within hours of each other. The chicks are protected by the male for up to six months. So each male is making about a 7 month investment in his offspring.
Meanwhile, in a complete reversal of the usual female role in nurturing their offspring, the females have wandered off, looking for other, likely males with whom to mate, and deposit more of their eggs in another nest, leaving that male to incubate and rear their chicks. They will have nothing further to do with the rearing of their chicks.
This complicated “romantic” breeding strategy has the fancy name polygynandric, which simply means males breed with more than one female (like Red-winged Blackbirds) and females breed with more than one male (like many shorebird species).
Footlose, and fancy free, a female Rhea, can just lay her eggs, and then walk off.
So, does this peculiar breeding strategy work? As you might imagine, incubating such a large number of eggs might be disadvantageous, and in fact, only about 3/4 of such large clutches actually hatch. Worse yet, territorial males sometimes give up incubating duties to another male so that they can go collect another harem at another nest site. And 65% of the males were found to desert their nests in the middle of incubation, either because of disturbance from predators, livestock, or humans, or for no apparent reason. In one study of 34 Rhea nests, only 20% hatched any chicks at all.
With this low reproductive success, Rheas in South America are a near-threatened species, especially with more of their Cerrado habitat being converted to agriculture.