Mowerbird Ecology

Mowerbirds are a genus of several very closely related but physically distinctive sparrowgull birds, usually green but with varying colorful adornments, which are found across Serinarcta's soglands and upland environments. They are evolved from the scampering meadowbird, one of many plover-like shorebirds which have existed for tens of millions of years - the most primitive group of sparrowgulls. By numbers, the mowerbirds are now the most numerous birds on Serina 280 million years PE, with the most-widespread horned mowerbird's population swelling after nesting to more than 2 billion, and that of multiple other species not far behind. 

The canary-sized mowerbirds, somewhat unusually for such small flying birds, are now almost exclusive herbivores that feed mostly on green vegetation. They evolved about 4 million years ago, becoming specialized to feed in the wake of the immense thorngrazer herds over Serinarcta's upland plain and the margins of the soglands, flying in their swarms and descending upon recently grazed land like a cloak of feathers. 

Above: horned mowerbirds, one of about 8 endemic species native to Serinarcta and one of the most distinctive; males are slightly more colorful than females, with toothpick-like paired red feather crests and blue near their bills while juveniles prior to their adult molt are distinctly faded and more tan than adults. Both sexes have crests, which serve as a signal to identify their own species.

Mowerbirds pick at the new shoots which emerge within days of heavy feeding by large herbivores and to eat the immature green seeds growing close to the ground, which have escaped the larger animal's appetites but now are left revealed with the cover of taller foliage stripped away. Their beaks are now robust and serrated to cut stems with ease, and they are extraordinarily mobile too, with both the upper and mandible having a wide and independent range of motion, and so mowerbirds can chew their food with a circular movement of their beaks to more efficiently break down and digest it. Today they also associate with soggobblers, and in their case also find additional food in the grass seeds deposited in their loose droppings, which are much less processed than those of thorngrazers, which leave pellet-like waste with almost all usable nutrition extracted. Their diet overall is relatively poor in nutrients, but is effectively in unlimited supply, and so mowerbirds feed for many hours each day. The mowerbirds pass each mouthful in under an hour into their own watery, green droppings with only the most immediately digestible components absorbed, for they are too small to store food longer for more thorough digestion. This green manure, a rich fertilizer for plant life, also feeds a wide variety of tiny insects, worms and arthropods that would not otherwise be able to feed on living grass directly, and so supports a richer food chain than if the bird did not exist. 

Birds have followed thorngrazer herds to pick at shoots and scattered bugs for as long as thorngrazer herds have existed, but the mowerbirds' immense populations arose only recently. It has coincided with the evolution of the cementrees and subsequent spread of the spire forests, for it is only here that such immense colonies of birds can make their nests relatively safe from ground-based enemies. No longer restricted to single nests hidden in the grass, vulnerable to all sorts of enemies, mowerbird ancestors could now aggregate into humongous gatherings to raise their young with safety in numbers and away from terrestrial predators, and only then to truly make full use of the abundant food resource provided by the sprawling northern grasslands.

Such an abundant bird species has provided a stable food source for predators to capitalize on. Herds of billions, each turning grass into a bite-sized piece of meat, mean that mowerbirds have many predators indeed, including specialists which feed on little else. They serve to cycle nutrients throughout the broader ecosystem, transforming grass into rich fertilizer and into flesh that can feed a wide range of predatory animals too small to prey on either thorngrazers or upon gantuans which do the same on a greater scale.

Above: a grizzled guzzle preys on black-throated mowerbirds, T. flavucaputus nigergulus.

One of the most important predators of mowerbirds are guzzles, a genus of flapsnappers which have grown to large sizes with the explosive population increase of the mowerbirds, now their primary prey. Not dramatically changed from pre-ice age ancestors that just barely scraped by through the mid-Ultimocene by preying on surviving smaller sparrowgulls and tribbats near the sea coast, guzzles are much bigger  - up to seven and a half pounds in the largest species, the grizzled guzzle - and have longer jaws. They have evolved an expansive throat pouch able to hold over five pounds of food - a remarkable two-thirds of their own body weight - and put it to use by striking at the incredible flocks of their prey at dusk and dawn as they return to and leave their night roosts within the branches of upland cementrees. Like whales filtering krill from seawater, these great tribbats swoop directly into the flock without targeting any individuals, instead seeking to collect as many of the birds as possible in its mouth and swallow them in the several minutes per day they fly in such dense aggregations to allow it. Each feeding guzzle can collect upwards of forty birds in one sitting and not impact the flock's numbers substantially, so great is the population. The guzzle can then return to its own roost in the spire forest after only a few minutes of exertion, sustaining off its very large meal for as long as an entire week. The tribbat does digest the small bones of its prey, however feathers and beaks of are not digestible to it and so are regurgitated as a series of colorful ovular pellets, each about six inches long and three across, which come to litter the ground below their weekly roosting site.

 Carrying this much weight in food is difficult for the guzzle, and is only possible where prey is caught almost directly nearby suitable trees to land and roost afterward. Mowerbirds will not roost again in places where they have recently been threatened, instead moving on to new areas to sleep almost every day as a result. This means that the guzzle too must be nomadic, flying longer distances when they need to hunt to locate prey. They may wait in a likely place for flock to settle in the evening, and then ambush the birds as they land. Hunting is easier in the morning though, as the guzzle can seek out the birds themselves in the dark hours before they can see well enough to escape, and this is when most feeding occurs. The great tribbats, each with wingspans of up to five and a half feet, fly into the flock just as they lift off and sometimes creates chaos as the birds crash into each other trying to avoid its toothy maw. Some mowerbirds attempt to escape by closing their wings and dropping at the last moment, taking advantage of their light weight to land mostly unharmed on cushioning tall grass below where they are unable to be caught - guzzles never intentionally land on the ground, as they have difficulty taking off without the benefit of a high perch to drop from, even without a full pouch. Many other predators - aeracudas, sparrowgulls and skewers - frequently join in to take advantage of the mayhem produced by feeding guzzles to pick off individual mowerbirds while they are so distracted. Such competitors can also take advantage of the guzzler more directly by waiting until it goes to roost, already fed, and harassing it so that in order to flee it regurgitates some of its catch for the pirate to take.

Above: a white-tipped skydart pursues a single yellow-headed mowerbird, T. flavucaputus flavucaputus, the nominate subspecies of a very wide-ranging form that includes the black-throated mowerbird. The two subspecies hybridize extensively along the boundaries of their broadly east and west distributed ranges across Serinarcta.

Frequently taking advantage of the guzzle's disturbance of mowerbird flocks are the highly aerodynamic birds known as white-tipped skydarts, aerial hunters which descend upon the swarms of grazing songbirds from great heights as they scatter and single out a target. Using gravity to their advantage in long, stooping dives, these raptorial descendants of the skewer strike their victims at some 200 miles per hour, skewering them upon a lance-like upper mandible and killing them virtually instantly. Their bodies have become expertly-adapted to high-impact hunting, with the bones of their necks fused into a compact missile-like position and likewise their upper mandible stiff and immobile, no longer able to be folded downwards at rest. These birds feed on the wing, using their long radula-like scraping tongue to pull apart their kills as they are held securely upon the length of the barbed bill. Indeed they virtually do not land at all, but spend their adult lives flying high on thermals and even sleeping in the air, and so have nearly lost their feet all together. Just a single toe on each leg, barely visible, protrudes from their abdomens. These birds are social as adults, often flying in close formation together and demonstrating coordinated maneuvers. When not following guzzles, they hunt cooperatively, seeking to break smaller flocks of their prey off from the swarms so that they are more easily captured, and flock-mates will even share kills, flying delicately in tandem so that one may take bites from a bird held on another's beak. 

Descended from species with fossorial burrowing larvae, which used their long rostrums to pierce plant roots and feed on sap, white-tipped skydart young feed on a more specialized diet - mowerbird droppings. Adults mate in flight, with females then seeking out nesting colonies of their prey in which to lay their eggs. They deposit them in clutches of several hundred, each one no bigger than a grain of rice and with only a thin membrane protecting it from the air, after digging a scrape with their beaks into the accumulated waste piles of the mowerbirds that build up below their nesting sites. This is the only time the adult will ever land, and the female takes just a minute or so to finish and take flight again. There within the droppings their larvae grow and develop, making use of the mowerbird's rapid and ineffective digestion to find plenty of nutrients still usable for its own growth. Pupation occurs in just under one year, long after the noisy breeding flocks have dispersed, and the juveniles emerge from their cocoons about the same size as their eventual prey - some eight inches nose to tail. They feed mainly on insects and other types of metamorph birds for their first year, including other butterbirds, but interestingly can also solicit meals from all adults of their species even though their own parents, being nomadic, may never see their young. Adults are instinctively altruistic and responsive to the distinctive calls of juveniles and most will respond by actively sharing food. Sharing prey is central to the adults' social structures, and juveniles will remain near helpful adults as they grow, returning the favor later in life.

White-tipped skydarts are just one species of a worldwide family which cover a similar size range to falcons. All species are beautifully well-adapted to continuous flight, with long, tapered wings and athletic bodies that resemble fighter jets in profile - ideally shaped to fly through the air with the least wind resistance. Different forms have variable larval habits and diets that differ from their ancestors and from one another, and may be scavengers, herbivores, or even predators. 

Because mowerbirds feed on the ground, an animal doesn't even need to fly to make use of them as a food source. The springheel is a descendant of the viridescent sawjaw, and is now one of the birds' most specialized ground-dwelling enemies. Many of these intelligent predators followed their thorngrazer prey out of the soglands and onto drier ground over the last few million years but here found increased competition with other carnivores, leading some populations to switch to alternative diets. The springheel is a plains sawjaw that has now become specialized to hunt much smaller animals - the vast swarms of the mowerbirds, which it catches with an extraordinary tail that terminates in a grasping set of hawk-like talons. It's green coat hides it in the grass, while its long ears let it listen for movement - they are even tufted at their tip with white hair, and can resemble puffgrass seed-heads swaying in the wind. Lying in wait in tall grass, this hunter still works in small packs to orchestrate attacks on these tiny prey, able to live in groups because the birds, while small, are so incredibly numerous. Hunts are orchestrated by breaking the group up into teams, one which lies in wait ahead of the flock as it feeds on the ground and the other which stalks from behind and scares them into taking flight in range of the others, which then leap up and snatch as many as they can out of the air. Before the great flock scatters, each individual may be able to knock a dozen out of the air. The catch is then distributed among the whole clan.

Though it hunts much smaller prey, the springheel is slightly larger than the viridescent sawjaw, mainly because of its greatly elongated tail and limbs. The latter are able to power unmatched vertical leaps of up to ten feet from a standstill, letting them snag fleeing birds with ease. A strong tendon in the leg stores energy with each leap, making this an energy-efficient way to move, too - springheels are the only sawjaws that hop instead of run. Their jaws though are much weaker than their ancestor, and yet are still over-engineered for a diet of almost nothing except birds weighing less than 2 ounces each. The face has shortened and the size of the teeth is less than half their former dimension. Their jaws are now most useful for self-defense against other predators, as are their small but still fully functional sickle-like wrist claws. These claws are now put to another use too - climbing cementrees to flee enemies - not unlike how they originally evolved in the Mid-Ultimocene in their chiseler ancestors before they came down to the ground.

Because their prey is now so small, springheel cubs can begin hunting quite early in life without being in any danger and will accompany the adults virtually from their birth, clinging to their backs. As they grow more coordinated, mothers will encourage their young practice their skill by letting them cling to their tails and from this perch lifting them up up to try and catch a bird of their own. This game is seemingly quite fun, and the cubs will continue it long after all of the birds have flown off, sometimes simply swinging from their parent's tail and using it as a jungle gym. 

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Despite heavy predation pressure, mowerbirds manage to maintain great numbers, though populations do rise and fall cyclically, because breeding in the mowerbird occurs less often than most birds of similar size and usually at two year intervals. It is never the same year for all birds in the population however, with those living in different regions sticking to their own timetables even if they associate with others which breed at different times. Which years to breed may be genetically determined and one way that mowerbirds readily separate into highly different species, most of which coexist. When nesting season is imminent there is a frantic change of pace for the birds, as they take to the skies and blot out the sun, with the groups taking hours, sometimes days to pass by. They select large and contiguous tracts of spire forests and there, within the higher branches, begin to construct their woven purse-shaped nests made of long grass strips that they carefully cut with their scissor-like beaks. Soon every tree is festooned with hundreds or even thousands of little grass balls until their branches hang limp. Pairs of these birds hatch out up to ten chicks each within each nest, making sometimes very long forays out of the forests to collect food for them, which is different from the adult's preferred diet and consists of some 70% insects; the rest is mostly green seeds. Bugs are often collected from the shores of freshwater ponds. Tiny chicks hatch in just 20 days and leave the nest at 3 weeks, and by a month old they will have left the forests altogether with their parents and returned to the plains, beginning to feed on the grass with the adults. Predators feast at this time, for the fledgelings take weeks to become strong fliers and learn to avoid their enemies, and between their breeding seasons the population of these strange little birds may fall by more than half, only to swell again massively again later, in a constant swinging cycle. Their relatively low breeding rate serves as their major population limiter, not predation; if they bred every single year, as do so many other birds, they could exhaust their entire food supply and cause widespread ecological collapse. Likewise, though the plant diets of the adults may be difficult to exhaust, the insects on which their chicks depend may not be; it takes more than one season for their populations to recover in the vicinity of breeding colonies, as hundreds of millions of parents snatch up everything they find and leave the surrounding area almost barren of invertebrate life while feeding nestlings.

The cementrees are left severely beaten and bedraggled by the time the two-month-long breeding season of the mowerbirds is complete. Branches lie bent and broken, with many fallen to the ground. It may seem mysterious why their ant colonies tolerate this incursion, to the apparent detriment of their host trees. Yet cementrees are incredibly hardy; most of their mass is protected within the cement spires, and branches quickly re-grow. Fertilizer in the form of up to 18 inches of deposited guano from the nesting birds provides an immense nutritional boom to the forest, allowing it to ultimately grow back even better than before. And as the mowerbirds are true nomads, with a given individual rarely ever ending up back in the same place twice, flocks are unlikely to choose the same tracts of trees to nest again for many years, sometimes decades, giving them a long time to recover.

Mowerbird predators have different strategies to reproduce, some of which are independent of their prey's breeding cycles and others which align. The guzzle breeds irregularly, similarly to its prey, and may not do so every year.  As mowerbird populations rise and fall cyclically, they only breed when their food supply is at its greatest. The trigger is usually the nesting of the mowerbirds themselves, and colonies of guzzlers form in the same forests, though often a mile or two away to avoid scaring off the birds. The tribbats mate as the birds begin to court, but have a longer gestation so that the young are born just as the mowerbird chicks start leaving the nest. They are clumsy and uncoordinated, and there is no easier time to hunt, letting the parent guzzlers easily collect enough prey for their usual two pups, which fledge themselves in a little over three weeks, but are not independent for over six months. The female leaves shortly after the pups leave the nest they are kept in before they can fly, but the young enjoy a prolonged adolescence flying with their father which demonstrates how to hunt and provides food until they are competent predators in their own right. Guzzles have few predators and live a long time, usually more than twenty years, but potentially twice as long. They are thus in no immediate rush to breed, and sexual maturity doesn't occur for at least four years, and sometimes longer. In contrast the springheel and the white-tipped skydart breed every year; the former has just one pup at a time, and spends a long time caring for it, meaning populations grow slowly and mostly remain at a stable size independent of temporary booms in food supply. The latter are highly mobile, able to travel hundreds or sometimes thousands of miles on the wing to locate large flocks of prey and so remain well-fed even as localized populations of mowerbirds may temporarily fall. Their mobility also lets them seek out widely-separated mowerbird nesting colonies to lay their eggs every year, even when any single colony only assembles less frequently.