Carriageoose 

The carriagoose is a large, ostrich-like descendant of the chariot that has become flightless, no longer needing flight as it is a very fast runner - it has lost the claw on its inner toe, and the outer toe is reduced in size, demonstrating specialization toward very highly cursorial locomotion with most of the weight carried on the central toe alone. Thanks to these adaptations, this species can now run at 45 miles per hour.

Common across southern Serinarcta's savannah woodlands, these large sparrowgulls are most closely related to the much smaller mowerbirds and share their preferred diet of grass and leafy plants. They differ from their relatives in how they reproduce; they are excellent parents, but they don't make nests. Adapted to a nomadic lifestyle on the grasslands, carriageese incubate their eggs and raise their chicks all on the go. One to two eggs are laid per female, and are held beneath her wings, up against a hot, featherless patch of skin. The wings of the carriagoose, no longer needed for flight, have evolved a locking tendon that holds them in place when at rest, keeping the egg from falling even as the parent sleeps. When the chick hatches, the male takes over and the female leaves him to finish rearing their young, her job complete. The male puts the chicks on his back, where raised, stiff contour feathers on his upper arm now serve as a little fence that keeps the clumsy youngster secure and helps keep it from tumbling onto the ground. The carriagoose has evolved a more flexible, mitten-like hand with a soft, padded thumb originating from the alula, and a short, mobile finger made of cartilage, which allows them to pick up their chicks in their wings with great dexterity to set them on the ground so they can look for food on their own, beginning around 3 weeks of age, and to snatch them up again and run off with them if a predator threatens.  Very young chicks are fed entirely by the father, who regurgitates food for the young in the manner of almost all sparrowgulls, but they become less dependent within a few weeks and can start feeding themselves before they are a month old. Nonetheless, dad watches them closely for six months before they are self-sufficient. They may continue to ride on his back, at least occasionally, until they are 25% adult size, or around 3 months old. Males usually have several partners, and thus raise more than one chick - sometimes, they may be left with five or more, in which case they are forcibly evicted from his back much younger than if he had just one to carry. Chicks politely avoid soiling their parent by maneuvering their hindquarters to the edge of the wing nest, and expelling their droppings some distance away. 

The carriagoose is a social bird with a complicated social structure. Groups are large, 20-40 animals, and composed of equal numbers of both sexes. They are distinct units, composed of individuals that know each other, rather than random assemblages, and new individuals are rarely allowed to join except at specific times. Males are the less aggressive sex, and have few quarrels, but are not subordinate to females. Rather, separate female hierarchies exist to determine which females get to mate with the biggest, most experienced, and thus most desirable males. Because mate choice is entirely to the female, males have no reason to fight each other, and their competition to get a partner is limited to flashy displays and loud calls.  Groups stay together for a long time as chicks are born and grow up, if only for safety in numbers, but the polygamous mating system that favors only a few fit males and a few dominant females reproducing means most flock members are unlikely to breed at any given time. This means that after a couple of seasons, younger members of the group are growing restless, and begin wanting to disperse. Carriageese have an interesting solution when their groups become too large, and they have many younger or otherwise low-ranking members present. Two different flocks will meet up and effectively trade these surplus members. Only during such exchanges do flocks accept new members readily, with still one caveat: only males can join a new flock. Males which have had little or no breeding success leave their flocks willingly with no encouragement needed, for they stand a better chance of mating in a new group, for they will be unrelated to those females, and thus more likely to stand out as a good option to pair with compared to the males born in that group. Females, however, are not welcome in another established group. As the males leave willingly and join the second flock, younger females are chased out by dominant females with aggressive posturing. They are expected to join the females expelled from the other group at the same time, and thus form an entirely new flock. Some young males from both groups will ultimately go with this new flock too, as it will have no older resident males to compete with. A preference for different looking, and thus less related, partners means that most of the time, the new pairs which form in this arrangement will include a partner from each of the two previous flocks, ensuring strong genetic diversity in the species. In several years, when the new flock has reproduced and grown larger, it will then meet with another group and trade members again, continuing the process, and maintaining a strong variety of genes in each group.