The spear and the shield

The hedgeback (Spinapteryx septentrionalis) has chosen the offensive strategy: in the course of evolution, this insectivorous rail has drastically changed its feather structure, losing most of its barbs and reinforcing the rachis. Now its dorsal plumage is filled with spikes that work very well to repel both flying and terrestrial predators. When threatened, hedgebacks can dangerously charge their opponent, often causing lethal wounds: its quill-like feathers are so sharp that can pierce the skin, damaging muscles and internal organs. Few predators have the gut to attack these aggressive birds. Feather quills can easily detach from the skin and remain hooked to the body of the aggressor, becoming an even greater nuisance.
Despite being useful for defensive purposes, dorsal quills become a very big and spiky problem when talking about mating. To avoid spikes, hedgebacks mate face to face, since their abdomen does not possess quills. It would be the second known bird to evolve this type of mating (the first one is the holocenic hihi).
This porcupine rail occupies several types of tree associations, including the Aurora forest, where they mainly feed on invertebrates. During winter, when this type of food is scarce, they usually scavenge on small carrion or eat berries. 

Hedgebacks from the Belgrano forest are sympatric with another armored rail, that has chosen a more defensive strategy: the armadrail (Scutipteryx belgranoensis). This robust rail has long legs, being a decent runner; however, when running is not enough, they can defend themselves thanks to their highly derived feathers, which look like scales: in this case, rachis has slowly expanded laterally, creating a sort of imbricate armor around the entire body of the animal, similarly to pangolins. Predators need strong teeth and claws to successfully reach the skin of this tank-like bird. In addition to their armored body, armadrails have also a very bad temperament: aggressors are often countered with powerful pecking focused on the head. If you will ever see a Jackal biwolf without an eye, you know what caused it!
They are the essence of omnivory, feeding on nearly any type of food. Fruits and seeds become their major diet during autumn, while during winter they can often feed on ducktails' feces, which are rich in vitamins and partially digested fibers.