Dinosaurs Without Bones (45 page)

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.

Has 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.

Thus it might behoove paleontologists who are interested in learning more about the origins of bird flight to pay attention to flying tracks associated with modern birds. After all, a continuing controversy in dinosaur paleontology—a real one, not a fake one like the old “Was
Tyrannosaurus
a predator or scavenger?” argument—was how self-powered flight evolved in non-avian theropods. Granted, nobody denies that self-powered flight provided some great advantages for those dinosaurs. For one, it took them to far more places than running, swimming, or gliding, but while using less energy than those means of transportation. Moreover, those places may have offered more choices in food, mates, nest sites, and habitats for raising offspring. This ability especially came in handy for migrating, in which these dinosaurs could more easily switch locations with seasonal changes or severe alterations of local climates. These are big questions that might be helped by looking at little tracks.

Flying tracks are by far my favorite of all bird tracks to find, and many other people must share this feeling, as the more frequently forwarded photos I receive from ichnologically inclined fans are of these. One of these photos shows a snowy vista punctuated by repeating sets of four mouse tracks, which end abruptly as they coincide with the feathered outline of an owl. The tale is all there in the tracks: a small mouse galloping across the snow, knowing that it is risking its life by being out in the open; an owl spotting it from its roost, then taking off with a whispery flap; a glide down with talons extending and grasping the mouse just as it landed on the snow; a beat of the owl’s wings against the snow surface to continue its forward momentum and become airborne again, but this time carrying a little treat. Sadly for ichnologists, though, such detailed and evocative traces are more likely to be made in snow, which, once warmed, has a rude habit of melting and thus erasing all of the evidence.

In my experience, the best places to look for flying tracks are in soft mud or sand along a seashore, lake margin, or river floodplain.
While examining these tracks, watch for the paired ones, and for gaps in their trackway patterns. Nothing quite says “flight” like a right–left pair of bird tracks with no other tracks in front of or behind them. Do you see no tracks behind them, followed by a normal trackway? These are landing tracks. Do you see a normal trackway that ends with two tracks, and nary a track after that? These are take-off tracks. Other details to note in such tracks are linked to whether a bird was trying to control its descent or begin its ascent. For instance, in landing tracks, birds with rearward-pointing toes (digit I) on each foot direct these forward, leaving long claw-marks while “putting on the brakes.” As the other forward-pointing digits contact the ground, these will push against the mud or sand, forming mounds in front of those digits as the entire foot comes to a halt. For take-off tracks, these reveal whether a bird left the ground instantly—with a burst of wing-driven power, it is aloft—or needed a little more forward momentum, such as a running, skipping, or hopping start, aided by much flapping. In such trackways, distances between the sets of tracks become greater, reflecting how lift forces gradually took the bird farther upward into the wild blue (or mild gray) yonder.