The life of a kiwi bird on Apterra during these first few tens of thousands of years is one of constant persecution by opportunistic rats. The vast majority of eggs and chicks were picked off during the first generations after Corvolea stopped providing assistance, and at one point the population was in such a decline that it risked spiraling towards extinction. Kiwis have since adapted a few simple tricks to avoid this fate. At this early stage in their evolution on Apterra, when major biological changes have yet to arise and take hold, simple mutations with outsized results are the key to survival.
Extreme vigilance and parental care was an obvious first step. Each pair of kiwis only produces one egg per year, and the less diligent among them quickly found their genes removed from the population, while those with protective instincts surpassing those of their peers saw great success. Even the toughest brown rat, easily capable of ravaging an unattended nest, would find itself outmatched by an aggressive, fully-grown kiwi. The bird’s long bill allows it to keep the assailant at a distance, and its larger size means it is capable of overpowering the egg thief if it doesn’t flee immediately.
These physical traits were the next characteristics to be shaped over the following generations. Kiwis grew larger, gained stronger, sharper beaks, and grew long claws on their toes for fighting off their rodent neighbors. The rats, in turn, became better thieves and even active predators, and so the arms race continued for millennia until it reached its current state. Now, several distinct subspecies of kiwi have each adapted unique strategies for keeping their young safe.
One group, living around the marshy estuaries of northern Abeli, nests on small islands surrounded by shallow water, which are ferociously defended by both parents against any intruder. Another arms race is taking place here, with rats gradually becoming better swimmers to reach these nests. In turn, the semi-aquatic kiwis enhance their own swimming capabilities through webbed toes, streamlined bodies, and increased buoyancy. This population numbers around 5,000 and is growing slowly with each passing year.
The water provides safety to another type of kiwi. To find them, we must travel immediately to the west of the estuarine kiwis’ home, to a small island a few miles off the shore of Abeli. By chance, a group of kiwis rafted here just a few decades Post-Abandonment, finding one of the only rat-free territories left, save for the far-flung islands that no land organism would reach for many generations yet. Here they expanded their population, living off grains and isopods as the years went by. The environment here, like all of Apterra at this point, was poorly suited for a creature like a kiwi. They made do, however, with whatever they found. The most opportunistic omnivores among them found success after incorporating washed-up algae into their diets as they wandered the shoreline, and the island population swelled to tens of thousands in the absence of predators.
Around the same time, digging behavior was emerging on the mainland. These groups faced the harshest pressure from the rats that swarmed across the fertile grasslands, without the benefit of isolated nesting sites to protect their eggs. Apteryx parents here are now the most fierce out of all the birds of Apterra; they're more than willing to skewer any rat in sight of the nest entrance. The nest itself is a simple but well-defended underground fortress, built by both parents well in advance of laying their egg. It is placed out of sight behind a rock or within a particularly dense patch of grass. The egg is cautiously rolled inside by the female - herself too large to fit through the nest opening - after which her smaller mate squeezes inside to incubate it. He won’t leave until after the chick hatches. Nearly half his own size at birth, this baby will be fully independent in a matter of days. It and its father will be fed by the female during this time, who spends nearly her whole day picking off pill bugs to feed her family. She patrols the surrounding territory, fending off predators and competitors alike. The male also actively defends the nest when necessary, popping out of the burrow and using his sharp bill to stab and poke at invading rats.
Another group faces fewer predator threats than the other mainland populations, but it must contend against a fierce environment and scarce resources. The mountain kiwi dwells on the slopes of Abeli’s long, sinuous mountain range. Here, few grasses grow, few isopods scuttle across the soil, and few rats endanger the birds' all-important eggs. As a result, the steep hills are a refuge for any bird that can survive their harsh conditions. Every winter, thousands make the trek uphill. This loose herd of migrating birds spends the warm months down on the plains, living alongside their burrowing cousins. But now they leave behind the warm prairie and ascend, traversing miles of foothills until they reach a predator-free cliff or peak. Here the female will lay her egg, which has become slightly conical and taken on a bumpy texture to avoid rolling away. Any intrepid rodent that makes its way up the mountains is summarily tossed off a cliff through a joint effort by both parents. The mother may periodically return to the more bountiful lowlands to find food for her family during this time, while her mate stays; as in most kiwis, it is he who does the incubating.
These are far from the only kiwi factions living on Apterra at this time - not even the only ones on Abeli, in fact. Other lineages will become relevant soon; this early radiation will continue for quite some time still. The world around them has begun to change too, though. Soon, the days of buffalograss monoculture will be over.