Other sizeable birds on islands included: the giant swans of Malta (Cygnus falconeri); the moa-nalo, consisting of four species of ducks in the Hawaiian Islands; the Viti Levu giant pigeon (Natunaornis gigoura), which was on one of the Fiji islands; and the giant cursorial owl (Ornimegalonyx sp.) of Cuba. All of these birds were flightless, and as one might guess from the adjective “giant” applied to their common names, they were significantly larger than any of their living relatives; for instance, imagine a 1.2 m (4 ft) tall owl prowling the forests of Cuba. All of these birds had something else in common, which was their rapid extinction soon after humans came in contact with them, found them delicious, introduced egg predators, and changed their habitats. Fortunately, traces again have helped fill in a few details about the behaviors of these vanished birds. For instance, moa-nola diets are known through their coprolites, which showed these were important grazers in Hawaiian ecosystems that lacked mammals as herbivores.
Although all of these recent birds were impressively sized, none were potential predators of people. So perhaps the closest situation comparable to Jurassic Park, in which theropods could have preyed on people only half their size on a remote island, was during the Pleistocene Epoch and on the island of Flores, Indonesia. Bones from about 12,000 to 100,000 years ago there include remains of both the largest known stork (Leptoptilos robustus) and a diminutive species of hominin (human relative), Homo floresiensis, nicknamed the “hobbit.” The size disparity between these two species was like that between Utahraptor and a modern Homo sapiens. The storks were just shy of 2 m (6.6 ft) tall, whereas adult H. floresiensis stood only a little more than 1 m (3.3 ft) off the ground, and stork tracks were probably twice as long as the hominin tracks.
Also, storks, despite being associated with heart-warming fables of delivering babies, are voracious carnivores. Hence, these big storks, rather than fulfilling fables, would have been more interested in consuming warm hearts. So far, though, no one has noted any trace fossil evidence of storks preying on or scavenging
H. floresiensis, such as beak marks on bones or coprolites holding the remains of furry feet. More likely menu items for these storks would have been rodents of unusual size, such as the Flores giant rat (Papagomys armandvillei), which was more than double the length of the biggest urban rats.
However, Pleistocene trace fossils from South Africa do show evidence of a bird attacking a small hominin. These consist of holes in the skull of the “Taung child,” which belonged to a juvenile Australopithecus africanus. At first interpreted as leopard toothmarks, these holes are now regarded as beak and talon marks from an eagle, neatly matching traces left on modern monkey bones by African crowned eagles (Stephanoaetus coronatus). Some paleoanthropologists even speculate that frequent eagle attacks would have selected for more cooperative behavior and larger body size in hominins, thus deterring these predators and contributing to greater stature.
As one might have figured out by now, islands do funny things with the evolution of birds, especially related to body size. Paleontologists and biologists have noted two seemingly opposite trends: smaller animals that colonize islands tend to get larger over time, whereas larger animals get smaller. Both can be summarized as the “island rule,” although this can be split into “island gigantism” and “island dwarfism,” respectively. Given the right ecological conditions and enough time, natural selection on an island can drive an animal’s lineage down either path. For instance, if what we would consider today as “normal”-sized swans, ducks, pigeons, or owls had flown to islands during the Pleistocene, were genetically isolated from others of their species over several hundred generations, had no natural predators, and plenty to eat, then their descendants might have become much larger. However, with great size came great responsibility: to gravity, that is. Part of this natural selection toward gigantism in birds would have reduced any energetically expensive traits, such as wings. Consequently, wings shrank as weight and height went up in these island birds, rendering most of them flightless.
Speaking of islands, if you’ve ever fantasized about ones where dinosaurs continued to evolve after the Cretaceous, then look no further than New Zealand. Back in the Cretaceous, plate-tectonic shifting caused the main landmasses of New Zealand to split from eastern Australia, taking its dinosaurs with it. However, very few mammals came along for the ride, so geographic isolation combined with the end-Cretaceous extinctions to favor the survival and evolution of birds over non-avian dinosaurs and mammals. As a result, New Zealand experienced a grand evolutionary experiment, answering the question: What would have happened to dinosaurs over the last 65 million years if there had been no pesky mammals competing with them? More important, as an ichnologist I might also ask what sorts of unique traces might have been made as a consequence of such evolutionary innovations?
The variety of birds and the ecological niches they filled in New Zealand were remarkable. For example, before humans showed up about eight hundred to a thousand years ago, Haast’s eagles (Harpagornis moorei) were the top carnivores in New Zealand, predatory theropods that delivered death from above to other large theropods. Like some other birds, female eagles were larger than males, weighing about 12 to 15 kg (26–33 lbs) and with wingspans of 2.5 to 3 m (8–10 ft). Although the biggest eagles today have comparable wingspans, they are all less than half the weight of Haast’s eagle. And just what other large theropods were these eagles hunting in New Zealand? Moas, which were among the most famous and diverse of recent avian dinosaurs that remind us of non-avian dinosaurs.
Fortuitously enough, paleontologist Sir Richard Owen—who coined the word “dinosaur”—first studied these fabulous flightless birds in the early 19th century after British explorers brought moa bones to the U.K. Since then, paleontologists figure there were about nine species, which ranged from 1 to 3 m (3.3–10 ft) tall. Moas were grazing and browsing herbivores, basically filling the ecological role of ornithomimids, ornithopods, small ceratopsians, and other mid-sized Cretaceous dinosaurs in New Zealand, yet were alive when Geoffrey Chaucer wrote Canterbury Tales. Considering their sizes, abundance, and wide geographic ranges, moas must have worn deep trails throughout the landscapes of New Zealand and left easily visible browse lines along forest borders. Thanks to their coprolites, we even know what plants they ate and in which ecosystems, described in a 2013 study done by Jamie Wood and other scientists. Like the elephant birds in Madagascar, though, they fell victim to the ways of humans, and were extinct by the end of the 15th century.
Luckily, New Zealand still holds some examples of unique avian legacies, including some of the strangest birds in the world and some unusual avian traces. Among these birds are the flightless kiwis, consisting of five species of Apteryx. These birds are nocturnal foragers, eating a wide variety of plant materials, invertebrates, and small vertebrates, which they locate with nostrils on the ends of their beaks; no other bird has such an unusual adaptation. Female kiwis also stand out from other avians by having two ovaries, sharing nesting burrows with males for as long as twenty years, and laying a single egg that can take up one-third of their body volume and one-fourth of their weight.
Two other New Zealand birds, the kea (Nestor notabilis) and the kakapo (Strigops habroptilus), are examples of parrots that evolved in unexpected ways. Although we normally think of parrots as subtropical–tropical birds flitting about in rainforests, keas live in alpine environments—hopping along easily on icy glaciers—and kakapos are flightless, nocturnal, and the largest of all parrots. Kakapos also have odd mating rituals that involve traces. Male kakapos dig a series of half-meter (20 in) wide bowl-like depressions that they use like megaphones to project their mating calls. They further link these depressions by making tens-of-meters-long trails between them—made by their compulsive need to clean out their “amplifiers,” which prompts them to walk constantly between them.
The point of this all-too-short stroll through the evolutionary history of birds and a glimpse at some of their ichnology is to
emphasize how birds, despite the extinction of their theropod relatives 65 million years ago, continued to be dinosaurs. Accordingly, they also made traces that overlapped in size with those of Mesozoic dinosaurs, such as the tracks and nests of theropods and ornithopods. Moreover, these bird traces reflect myriad behaviors that also may have been shared by their non-avian predecessors, giving us search images for trace fossils that might be used to interpret behaviors currently unknown in dinosaurs. For instance, imagine the incredible coolness of finding a Cretaceous trace fossil consisting of a series of depressions linked by a network of trails and realizing that it might be analogous to a kakapo wooing trace, perhaps providing a key to understanding a dinosaur’s mating behavior.
Thus, knowing about this evolutionary history of avians helps us observers of present-day traces better appreciate that every bird track, nest, probe, splatter, or other mark reflects a rich evolutionary history linked to a dinosaurian past that is minimally 160 million years old. Yes, birds have certainly changed and diversified enormously since the Jurassic, as well as spread throughout the world, occupying land, sea, and air. Yet modern birds also still hold insights to their Mesozoic origins, and the behaviors and traces we observe today might help us to better understand dinosaurs of the Mesozoic.
Tracking Birds, on the Ground and in the Air
As mentioned before, tracking is a longtime practical activity connected to hunting, and as a science has great applications toward understanding animal presence and behavior. Despite all of this, tracking is normally applied to mammals, not birds. Just to put this in perspective, watch the difference in people’s reactions if you say “I’m going to go track a deer!” versus “I’m going to go track a robin!” I predict the latter will result in arched eyebrows, double-takes, and quizzical laughter. After all, the conventional wisdom is that most birds are too small to track, they fly so often that they do not leave many tracks, and their tracks all look alike.
Wrong, wrong, and wrong. Granted, the tiniest of birds—such as the bee hummingbird (Mellisuga helenae) of Cuba, which weighs only about 2 grams (0.07 oz)—would be very difficult to track. But nearly all other flighted birds, from wrens to condors, come to earth and make abundant and easily identifiable tracks. Of course, some flightless birds—such as rheas, emus, ostriches, and cassowaries—make the largest amount of tracks, and the largest tracks period. But many flighted birds regularly come in contact with the ground as part of their everyday lives and are thus capable of leaving tracks, too.
Bird tracks can be divided into four main categories based on overall form, all of which are derived variations of theropod dinosaur feet: anisodactyl, palmate, totipalmate, and zygodactyl. Anisodactyl is the easiest of these to link to Mesozoic dinosaurs, as it consists of three forward-pointing toes and sometimes a more backwardly pointing one (digit I). In some birds, though, this toe also may be reduced in size or absent. All songbirds have anisodactyl feet, as do chickens, turkeys, vultures, and raptors, as well as most wading birds and shorebirds, such as herons and plovers respectively. Palmate is a condition in which a basic anisodactyl foot has webbing between its main three digits, like on the feet of ducks, geese, and gulls. Totipalmate feet take this form a little further with webbing between all four toes, making for impressively broad tracks, such as those left by pelicans. Zygodactyl feet have their digits arranged with two pairs of toes and each bunched together to form either a K or X pattern. These feet are typical of owls, roadrunners, and woodpeckers.
Each bird-foot form reflects adaptations selected over thousands of generations of birds, meaning that each track made by a bird also reflects its evolutionary history and adaptations. For example, the extremely long, widely spaced, and thin toes of some wading birds, such as moorhens, are well suited for walking on lily pads in freshwater ponds. The three forward-pointing and single backward-pointing toes of songbirds are excellent for grasping and perching on branches. The webbed feet of ducks, geese, and pelicans aid in their surface swimming and diving in either freshwater or marine environments. Woodpecker feet work quite nicely for moving rapidly up and down vertical tree trunks. Raptor and owl feet are very good at grabbing and pinning down struggling prey. As a result, many bird species can be identified from their tracks, which can be further used to interpret their probable lifestyles.
Figuring out which bird made which track, however, does require careful measurements of footprint lengths and widths, toe lengths and widths, angles between toes, as well as basic knowledge about where birds normally live and when certain birds might be in the neighborhood for a visit. For one, I will not be adding secretary bird (Sagittarius serpentarius) tracks to my list of possible track-makers while doing field work in North America, as this bird is restricted to Africa. Yet migratory birds, such as sandhill cranes (Grus canadensis), might stop by for brief cameo appearances in places where they typically do not hang out. Hence, people who track birds also should be aware of which birds are migrating and when.
Bird trackway patterns fall into five behavioral groupings: diagonal walking (or running), hopping, skipping, standing, and flying. Of course, birds are not necessarily locked into making just one type of trackway while doing their business. Robins, for example, are great little runners when on the ground, zipping from one place to another while looking for earthworms or other invertebrate treats. However, they can also switch from running to skipping, in which both of their feet leave and hit the ground at nearly the same time, but with one foot slightly ahead of the other. (Hopping differs from skipping by being done with both feet together, side-by-side.) Also, just before changing from running to skipping, robins might stop with their feet next to one another. Moreover, robins are very good at taking off from a standing start; so paired tracks might be the last ones seen in a trackway. Conversely, paired tracks might be the first in a robin trackway, telling exactly where a robin landed. Many a time I have followed a bird trackway and along its length seen such shifts in its behavior, both understated and overt, recorded all throughout: a script narrating a scene that is normally much more detailed than if I had actually watched the bird make the tracks.
Diagonal walking is an easy pattern to understand because it is so similar to our own walking pattern: right, left, right, and so on, in which a diagonal line can be drawn from one track to the next. I have seen this trackway pattern made by birds as small as sanderlings (Calidris alba) to as large as cassowaries and emus, and by birds with anisodactyl, palmate, totipalmate, or zygodactyl feet. However, the biggest difference between bird and human diagonal-walking pattern is how bird trackways typically have very narrow straddles, as if they are walking on a tightrope. For example, if some pranksters decided to create fake moa tracks in New Zealand by wearing big three-toed “feet” and wanted to make the trackway look more convincing to experts, they would need to swing their hips, placing one foot almost directly in front of the other. On the other hand (or foot, rather), walking normally would cause a wider-straddle trackway, which would be a dead giveaway that someone wearing oversized three-toed shoes had fabricated them. (However, the fact that a few idealistic people would happily accept such tracks as evidence that 3-m-tall flightless birds are somehow winning a centuries-long game of “hide-and-seek” is another matter.)
The most important variation on the diagonal-walking trackway pattern is running, in which the footprints are farther apart from one another, reflecting increased stride length. With this increase in speed, trackway straddles become even tighter, and individual tracks may show signs of claws digging in deeper, and sand or mud having been pushed behind where the feet registered. In this respect, nearly every non-avian theropod and most ornithopod trackways bear similar basic patterns or features like those made by equivalently sized modern birds. Thus, despite having anatomies that differ from Mesozoic dinosaurs, it is no wonder that dinosaur ichnologists still turn to birds—especially large flightless ones—as their default models for how bipedal dinosaurs moved and made tracks.
H
as a bipedal dinosaur trackway—whether from a theropod or ornithopod—ever shown hopping and skipping patterns like those made by some modern birds? Not yet, and no one is holding their breath in anticipation of finding these in the fossil record. Even so, non-avian feathered theropods might have been capable of short flights, such as the Late Jurassic Anchiornis or Early Cretaceous Microraptor, and thus could have made hopping or skipping tracks to get themselves aloft.
In a 2013 study, paleontologists using CT scans of non-avian and avian theropod skulls (including that of Archaeopteryx) suggested that some non-avian dinosaurs had brains well suited for the complexities of flight, with a few better than Archaeopteryx. Based on other anatomical traits, like long fingers with claws, other small feathered theropods, such as the Late Jurassic Scansoriopteryx (Epidendrosaurus) and Epidexipteryx of China, look like they were better adapted for climbing trees and hanging on branches than being on the ground. This means a better place to look for their trace fossils might be on petrified logs in same-aged strata, or at least sedimentary rocks that originally formed near forests. However, a few feathered theropods were far too big to have either flown or climbed trees, such as the Early Cretaceous Yutyrannus huali, which was close to 9 m (30 ft) long and weighed more than a ton. For such weighty theropods, there was no hopping, skipping, jumping, or tree climbing, unless they did these as small tykes.
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